Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/ftp/documents/ghrs-handbook/ghrs-handbook_all.ps Äàòà èçìåíåíèÿ: Tue May 10 21:32:17 1994 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 19:34:33 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: aldebaran

Instrument Handbook for the
Goddard High Resolution
Spectrograph (GHRS)
Version 5.0
For the post­COSTAR observatory in Cycle 5 of the HST science mission
May, 1994
The STScI GHRS Team
D.R. Soderblom
S.J. Hulbert
C. Leitherer
L.E. Sherbert
Space Telescope Science Institute
Baltimore, Maryland

This is version 5.0 of the Instrument Handbook for the Goddard High Resolution Spectrograph (GHRS) of
the Hubble Space Telescope.
Editor and principal author is David R. Soderblom.
Date of issue is May 31, 1994.
Date of last revision was May 2, 1994
This GHRS Instrument Handbook supersedes all previous ver­
sions. If a conflict exists between this document and another, the
document with the latest date of issue should be accepted.
STScI Contacts for Assistance
Function Contact Name Title Phone e­mail
General instru­
ment questions
David R.Soderblom Lead GHRS Instrument Scientist 410­338­4543 soderblom@stsci.edu
Stephen J. Hulbert GHRS Instrument Scientist 410­338­4911 hulbert@stsci.edu
Claus Leitherer GHRS Instrument Scientist 410­338­4425 leitherer@stsci.edu
Lisa Sherbert Technical Assistant 410­338­5036 lisa@stsci.edu
Proposal­specific
questions
User Support
Branch
Technical Assistants usb@stsci.edu
GHRS Instrument Handbook Revision History
Version Date of Issue Authors
1.0 1985 October D. Ebbets
2.0 1989 May D. Duncan and D. Ebbets
2.1 1990 March D. Duncan
3.0 1992 January D. Duncan
4.0 1993 January D. Soderblom
4.1 1993 March D. Soderblom
5.0 1994 May D. Soderblom

GHRS Instrument Handbook 5.0 3
Part I: Introduction to the GHRS and this Handbook
Chapter 1 Preliminaries and Essential Reading
.......................................5
1.1 This Handbook 7
1.2 Changes since the previous version 8
1.3 Where to find additional information, changes, errata, etc. 10
Chapter 2 Instrument Summary --- Why Use the GHRS?
...................11
2.1 Fundamental properties of the instrument 12
2.2 A Brief Description of the Instrument and Its Operation 13
2.3 A Little About HowHST and GHRS Work 15
2.4 GHRS Modes of Operation 17
Part II: Writing GHRS Proposals
Chapter 3 Phase I: What the T
AC Sees...................................................21
3.1 Essential questions 22
3.2 Creating an Observation Scenario 23
3.3 Specifying Target Acquisition 24
3.4 Calculating the Exposure Time 26
Chapter 4 Your Phase II Pr
oposal...........................................................33
4.1 Acquisitions 35
4.2 Image Mode 42
4.3 Accumulation Mode 43
4.4 Rapid Readout Mode 45
4.5 The Precision and Accuracy of Standard Calibrations 46
4.6 Other Considerations 47
Chapter 5 Phase II Pr
oposal Examples...................................................49

4 GHRS Instrument Handbook 5.0
Part III: GHRS Reference Information
Chapter 6 Design and Construction of the GHRS
.................................55
6.1 The HST Focal Plane and the GHRS Apertures 56
6.2 Gratings and Optical Elements 60
6.3 The Digicon Detectors 62
Chapter 7 Target Acquisition Refer
ence Information.............................67
7.1 Predicting Target Acquisition Count Rates for Stars 68
7.2 Constraints on the Value of the STEP­TIME Parameter 71
7.3 Acquisition Count Rates for Extended Objects 76
7.4 Other Acquisition Information 78
Chapter 8 Reference T
ables for Instrument Performance.......................81
8.1 The Effect of COSTAR on the GHRS 82
8.2 Properties of the First­Order Gratings 83
8.3 Properties of the Echelle Gratings 87
8.4 Standard Patterns for Substepping and Background Measurement 92
8.5 The Effects of Reddening in the Ultraviolet 93
8.6 Instrumental Properties 94
Chapter 9 GHRS Bibliography.............................................................101
9.1 Ultraviolet Extinction 102
9.2 GHRS­Related Technical Papers 102
9.3 GHRS Scientific Papers 104
9.4 Acknowledgments 109
Part IV
Glossary of T
erms and Abbreviations..................................111
Index.....................................................................................115

GHRS Instrument Handbook 5.0 5
Chapter 1 Preliminaries and
Essential Reading
1.1 This Handbook 7
1.2 Changes since the previous version 8
1.2.1 This Handbook 8
1.2.2 Side 1 Availability 8
1.2.3 Acquiring Extended Objects with the GHRS 8
1.2.4 Acquiring Faint Objects with the GHRS or FOS 8
1.2.5 Noise Rejection for Very Faint Objects 9
1.2.6 The Proposal Process for Cycle 5 9
1.2.7 Updated Instrument Parameters 9
1.2.8 GHRS Sensitivity 9
1.2.9 GHRS Aperture Nomenclature 9
1.3 Where to find additional information, changes, errata, etc. 10

Preliminaries and Essential Reading
6 GHRS Instrument Handbook 5.0
There are two reasons you may be referring to thisHandbook. First, you may be consid­
ering using the GHRS to observe some celestial object, and you would like to know
what the instrument can do and how long it might take. Better yet, your proposal to use
the Hubble Space Telescope with the GHRS has been successful and you now need to
supply all the details of your observations that are needed in Phase II of proposal pro­
cessing.
Both tasks may seem daunting at first because any versatile instrument has many
options. But for what most people want to do most of the time there are defaults that
apply, and the GHRS is, in fact, a very easy­to­use device. Once you know that what
you want to do falls within the bounds of conventional uses of the spectrograph, you can
proceed with some confidence that your observations will be successfully obtained in
the form you originally desired. Or you can at least get a sense that what you are propos­
ing is truly unusual and may push the limits of the instrument.
This Handbook exists as a basic reference manual for the Goddard High Resolution
Spectrograph (GHRS), and describes its properties and operation. ThisHandbook is
revised and reissued approximately once each year. This version is written for observers
wishing to propose to use the GHRS inHST's Cycle 5, and it supersedes all previous
versions of the Instrument Handbook. No HST document stands alone in providing
complete information because each fills a particular need. TheCall for Proposals, for
example, describes the proposal submission process and provides a summary of the
observatory and its instruments. The Phase II Proposal Instructions give detailed
instructions for providing STScI with the specifications that translate your program into
commands that HST executes. The instrument handbooks supplement both documents
by providing the technical details of instrument performance and operation. TheHST
Data Handbook describes how software takes the raw data from the telescope and trans­
forms it into a reduced and calibrated form for your further analysis and interpretation,
and how you can duplicate those steps.
HST is now a fairly mature spacecraft and so we can predict what many observers will
need. This Handbook is designed around the needs of the majority of users, so that
essential information is concentrated in a few sections. Full details must also be given,
of course, and they are provided in a reference section. We demonstrate the ease of use
of the GHRS by providing examples for some of the different situations that a user
might encounter. We have also tried to provide the information you need to decide when
your observations deviate from the ``normal'' and involve special aspects.
No handbook of this kind can be complete and error­free until the instrument itself is
obsolete. We have, of course, edited it thoroughly, but if significant revisions are called
for they will be announced via STEIS 1 , as with otherHST news items. Do not be afraid
to consult with us if you have questions; the means of contact are provided just after the
title page.
As we said, this particular version of theGHRS Instrument Handbook is being written
for Cycle 5 of theHST science program. That means that we are providing information
on how the instrument works after the Servicing Mission installed COSTAR and a
GHRS Repair Kit. COSTAR changed the image scale ofHST at the entrance apertures
1. This and other terms are defined and described in a Glossary at the end of thisHandbook.

GHRS Instrument Handbook 5.0 7
Preliminaries and Essential Reading
of the GHRS; it also produces diffraction­limited images, but does not alter GHRS oper­
ations in any fundamental manner. In other words, COSTAR alters some of the parame­
ters used in various calculations for pointing and throughput, but the instrument still
operates in the same way.
The GHRS Repair Kit enabled the reactivation of Side 1 of the GHRS, once again per­
mitting short­wavelength echelle spectra to be obtained, as well as spectra with the
G140L and G140M gratings. At the time this is written Side 1 reactivation has been
completed and a few observations have been made. They indicate that Side 1 is per­
forming with essentially the same science capabilities that it had at the time ofHST's
launch.
1.1 This Handbook
If you have looked through the Table of Contents, you will have seen that thisHand­
book is divided into four parts:
Part I is a summary in which thisHandbook is described and suggestions are made
on how to use this document and how it is related to otherHST­related documents.
Part I also describes howHST proposals are processed for review by the Telescope
Allocation Committee (TAC) and how successful proposals get turned into com­
mands that the spacecraft can execute; and it describes how the installation of COS­
TAR affects the GHRS and what the basic properties of the instrument are.
Part II elaborates on the writing of proposals to use the GHRS, both for Phase I and
Phase II. The Phase I proposal is what the TAC sees, and it describes the observa­
tions to be made in broad terms. The most important technical decisions to be made
regard the target acquisition strategy to adopt and the amount of exposure time to
request. The Phase II proposal encompasses all the details that the planning and
scheduling systems need to turn your program into commands for the spacecraft.
Part III is a reference section, and it includes the details that aHandbook should
without cluttering the introductory explanations.
Part IV is a short glossary of GHRS terms and abbreviations and an index.
Data reduction and analysis are not covered in thisHandbook because they are treated
in detail in the HST Data Handbook, although we anticipate merging portions of that
document into this in the future.
External references have been included where appropriate in order not to duplicate what
is available elsewhere, but we have tried to include almost everything you need to know
about the GHRS when writing a proposal. If you find that you need information that is
not in here, please consult with us.
This document follows the usual STScI convention in which terms, words, and phrases
which are to be entered by the user in a literal way on a form are shown in a typewriter
font (e.g., BRIGHT=RETURN,EARLY ACQ).

Preliminaries and Essential Reading
8 GHRS Instrument Handbook 5.0
1.2 Changes since the previous version
1.2.1 This Handbook
This version of the GHRS Instrument Handbook has been only modestly rewritten. It
incorporates a few updates since version 4.0 that were issued electronically. Please
bring errors to the attention of a GHRS Instrument Scientist (see back of title page). As
before, wavelength units in thisHandbook are in ångstroms (å), in keeping with astro­
nomical tradition. A transition to SI units may take place in a future edition.
1.2.2 Side 1 Availability
At the time this is written, the capabilities of Side 1 of the GHRS have been fully
restored, permitting one to obtain high­, medium­, or low­resolution spectra with a
cesium iodide photocathode. We encourage you to consider its use. The low­resolution
mode on Side 1 using grating G140L particularly makes the GHRS useful for observing
faint objects in the far ultraviolet. The next two paragraphs discuss issues related to
acquiring such targets.
1.2.3 Acquiring Extended Objects with the GHRS
An EXTENDED option for ACQ/PEAKUP was described in earlier versions of this
Handbook. It will be tested in Cycle 4 and should be available for routine use in
Cycle 5. It was written with the Gallilean satellites of Jupiter in mind, but should be
applicable to other extended objects. See Section4.1.5.4 on page 39 for more details.
We have also provided some guidance for users who may wish to observe extended
objects so that acquisition count rates can be estimated; see Section7.3 on page 76.
1.2.4 Acquiring Faint Objects with the GHRS or FOS
In some cases the G140L grating on Side 1 may provide an efficient means of obtaining
a low­resolution spectrum of a source, but acquiring that object can be difficult or
impossible with the GHRS' Side 1 because of the limited response of mirror N1 and the
maximum permissible STEP­TIME of 12.75 seconds. There are two ways to overcome
this problem:
. Acquire the object with Side 2 of the GHRS (mirror N2), then observe with Side 1.
Using this technique will add about 40 minutes of overhead time involved in switch­
ing Sides, but often that can occur when the spacecraft is behind the earth anyway.
. Acquire the object with the Faint Object Spectrograph and then move it to the LSA
of the GHRS. The positions of the COSTAR mirrors for the FOS and GHRS are
quite close, so that the movement of an object from an FOS aperture to the LSA of
the GHRS is only about 1 arcmin, a small enough motion to ensure that the object
will fall within the LSA because one set of guide stars will suffice. This method
should not be used for SSA observations.
At the time this is being written this method of cross­spectrograph acquisitions is being
evaluated and tested. Please consult with us if you wish to consider using it. Also please

GHRS Instrument Handbook 5.0 9
Preliminaries and Essential Reading
note that the opposite sense will also work, namely acquiring a ``bright'' object with the
GHRS in order to observe it with the FOS.
1.2.5 Noise Rejection for Very Faint Objects
A special commanding option called FLYLIM can be invoked to reject noise in the
GHRS when the object observed is significantly fainter than the level of the background
noise. Although only applicable in special situations, it can be very effective. Please see
Section 8.6.3 on page 97.
1.2.6 The Proposal Process for Cycle 5
The basic methodology of proposal writing forHST has been changed. You should rely
on the proposal instructions being issued with thisHandbook for guidance if there are
any conflicts, but the essential information needed (sequence of operations and expo­
sure times) has not changed.
1.2.7 Updated Instrument Parameters
Because of the Servicing Mission, the GHRS is a brand­new instrument in many
respects. We have undertaken to measure many basic instrumental quantities, such as
sensitivities, ab initio so that observations obtained in Cycle 4 and beyond can be cali­
brated to the best possible level. Many of those observations were not been fully ana­
lyzed at the time this Handbook was last revised before issuance. The numeric values
herein, particularly sensitivities, are therefore not ``final'' values that will be in place for
data reduction, but they do reflect our knowledge of the post­Servicing Mission GHRS
and will therefore lead to reliable exposure estimates. Updated information will be pro­
vided on STEIS as it becomes available.
1.2.8 GHRS Sensitivity
The sensitivity of the GHRS has turned out to be more wavelength­dependent than was
anticipated prior to the Servicing Mission; see Section8.1 on page 82. The reasons for
this are not known, but users should use the measured sensitivity values in Section8.2.3
on page 85 and Section 8.3.1 on page 87 when calculating exposure times and should
definitely not scale exposures from older proposals.
1.2.9 GHRS Aperture Nomenclature
The plate scale at the entrance apertures of the GHRS is altered by COSTAR such that
the angular scale per unit of physical length is reduced by a factor of 0.87. (COSTAR's
mirrors also introduce anamorphic magnification, but its effect is so small -- about 0.5%
-- that it is ignored here.) That means that the aperture sizes change in the following
way:
Aperture Nomenclature Pre­COSTAR Size New Size
Large Science Aperture (LSA) 2.0 2.00 arcsec 1.74 arcsec
Small Science Aperture (SSA) 0.25 0.25 arcsec 0.22 arcsec

Preliminaries and Essential Reading
10 GHRS Instrument Handbook 5.0
Note that the names used to designate the apertures of the GHRS have not changed even
though their angular sizes did. To lessen confusion somewhat, we will use ``LSA'' and
``SSA'' to refer to the large and small apertures, respectively, of the GHRS.
1.3 Where to find additional information, changes, errata, etc.
As we mentioned, the Call for Proposals provides an overview ofHST capabilities and
describes how a Phase I proposal is to be prepared. It goes hand­in­hand with the
Phase I Proposal Instructions and documents for the Remote Proposal Submission Sys­
tem (RPSS). If your proposal is successful, you will need to submit a Phase II proposal
that provides all the specific details we need to ensure that your observations are
obtained in the form you intend. ThisHandbook provides much of the information you
will need in Phase II, with the proposal procedures themselves in aPhase II Proposal
Instructions book. There is also a separate document that describes how to process and
reduce GHRS data. Contact the User Support Branch of STScI for further details.
(Please note, incidentally, that the Target Acquisition Handbook is no longer produced
because it is redundant.)
There is a separate document titledHST Data Handbook that describes how HST data
are reduced by the ``pipeline'' system and how you can reproduce those steps to tailor
the reduction to your needs. A copy may be obtained from the User Support Branch.
This Instrument Handbook is written to apply to the Goddard High Resolution Spec­
trograph as it will be configured and will operate in Cycle 5 of theHST science pro­
gram. The installation of COSTAR has changed the dimensions of the apertures and
other elements of GHRS as measured in arcsec projected on the sky. The new dimen­
sions have been used throughout this document. ThisHandbook supersedes all previous
versions, but if another document conflicts with thisHandbook, you should use the one
with the most recent date­of­issue.
You should also be aware of STEIS, theSpace Telescope Electronic Information Ser­
vice. STEIS provides an easy way to check for updates to existing documents and to get
HST­related information and news. To use STEIS, ftp to stsci.edu, enter anonymous as
the user and your last name as the password. Transfer the README file in the highest­
level directory with a get command to get basic information. Various subdirectories pro­
vide details on specific subjects. For more information on STEIS, contact the User Sup­
port Branch. The xterm interface toftp called Mosaic is an especially effective way to
access STEIS.
You are always welcome to call us, the STScI GHRS team, to get information when you
find yourself confused or at a loss. We prefer e­mail (to the addresses on the back of the
title page), but you may contact us by telephone if you wish.
Finally, you will find some additional sources of information in Chapter 9.
Do not use out­of­
date documents
as a source of
information! If this
Handbook does
not contain the
information you
need, please
consult us.

GHRS Instrument Handbook 5.0 11
Chapter 2 Instrument Summary ---
Why Use the GHRS?
2.1 Fundamental properties of the instrument 12
2.1.1 Entrance Apertures 12
2.1.2 Useful Wavelength Range 12
2.1.3 Available Resolving Powers 12
2.1.4 Photometric Precision and Accuracy 13
2.1.5 Time Resolution 13
2.1.6 Operational Complexity 13
2.2 A Brief Description of the Instrument and Its Operation 13
2.3 A Little About How HST and GHRS Work 15
2.4 GHRS Modes of Operation 17
2.4.1 Target Acquisition Mode 17
2.4.2 Science Data Acquisition Modes 19

Instrument Summary --- Why Use the GHRS?
12 GHRS Instrument Handbook 5.0
The Goddard High Resolution Spectrograph was built to obtain high­quality spectra of
astronomical sources efficiently. The GHRS can also record images of the objects it
observes, but that is mostly as an adjunct to its spectroscopic properties to confirm
pointing.
2.1 Fundamental properties of the instrument
Here we provide a brief overview of the basic properties of the GHRS. Each of these
aspects is described in more detail in the next two chapters. Chapters 6, 7, and 8 provide
illustrations of the GHRS and tables of instrument parameters.
2.1.1 Entrance Apertures
The source to be observed may be centered in aLarge Science Aperture (LSA) or a
Small Science Aperture (SSA) 1 . Because of the installation of COSTAR, the LSA is
1.74 arcsec square and the SSA is 0.22 arcsec square, although they retain their pre­
COSTAR names (2.0 and 0.25, respectively). The high­quality images that COSTAR
produces mean that spectra with good spectroscopic resolution result when the LSA is
used. The SSA has about 50 to 70% of the throughput of the LSA; using the LSA will
degrade resolution by 10 to 20% compared to the SSA because of the wings to the
instrumental profile.
The LSA has a shutter which automatically closes when an observation with the SSA is
being performed, in order to reduce stray light.
2.1.2 Useful Wavelength Range
The GHRS can obtain spectra from about 1150 to 3200 å. These limits are set by the
magnesium fluoride coatings on HST's optics and by the nature of the detectors. The
additional two reflections introduced by COSTAR's mirrors significantly reduce
throughput at the very shortest wavelengths (i.e., below Lyman­a) so that even very
bright stars (e.g., µ Col) have failed to produce detectable flux below 1150 å. It is possi­
ble to observe bright stars out to 3400 å.
2.1.3 Available Resolving Powers
With Side 1, observations may be madefrom 1150 to 1800 å at R = 2,000, 25,000, and
80,000 (gratings G140L, G140M, and Ech­A, respectively). With Side 2, the options are
R = 25,000 from 1150 to 3200 å (G160M, G200M, and G270M) and R = 80,000 from
1680 to 3400 å (Ech­B). For certain applications it can be advantageous to use grating
G270M to wavelengths as low as 2100 å because of its high efficiency.
1. We will use ``LSA'' and ``SSA'' to denote the two science apertures of the GHRS in this docu­
ment in order to lessen ambiguity about their apparent size. Note that the official nomenclature of
``2.0'' and ``0.25'' does not change despite the fact that these two apertures are now 1.74 and
0.22 arcsec square respectively.

GHRS Instrument Handbook 5.0 13
Instrument Summary --- Why Use the GHRS?
2.1.4 Photometric Precision and Accuracy
Routine calibrations on standard stars provide flux­calibrated spectra that are accurate to
10% 1 . Relative fluxes obtained at different wavelengths should be good to better than
5%. The repeatability of fluxes is even better, being better than 1%; i.e., it is possible to
compare measures of the same wavelength in the same star at different times to within
1% for observations with the LSA.
Within a single bandpass (i.e., one grating setting), relative photometric precision is
limited by photon statistics for S/N < 30 and by detector non­uniformities above that,
provided that the detectors are being used within the linear portion of their response.
With suitable observing strategies, it is possible to achieve relative S/N as high as 900
(Lambert et al. 1994) 2 .
We have found that the photometric sensitivity of the GHRS has not changed with time
to within 1% or less.
2.1.5 Time Resolution
Most observers use the GHRS to accumulate photons for the time needed to reach the
signal­to­noise they desire. In ACCUM mode the exposures may be as short as 0.2 sec­
onds, although use of standard procedures for improving S/N usually limits exposures
to no shorter than about 15 seconds. The GHRS has a rapid readout mode (
RAPID) that
can obtain spectra as often as every 50 milliseconds, but that can only be done by sacri­
ficing many features that are important for producing high­quality spectra.
2.1.6 Operational Complexity
The limited availability of memory on theHST spacecraft means that there exists a max­
imum number of operating commands that can be in place for a single set of observa­
tions. That can be a limit for use of the GHRS in certain cases, described later
(Section 4.6 on page 47).
2.2 A Brief Description of the Instrument and Its Operation
The GHRS has the usual components of an astronomical spectrograph: entrance aper­
tures, a collimator, dispersers, camera mirrors, and detectors. There are also a wave­
length calibration lamp, flat field lamps, and mirrors to acquire and center objects in the
observing apertures. The apertures were described above in basic terms, and are illus­
trated in Section 6.1 on page 56. The collimator and camera mirrors are unexceptional
and need no further description here (see Section6.2 on page 60 for details). The impor­
tant elements are the dispersers and the detectors.
1. Starting in Cycle 4, the routine fluxes delivered by the pipeline data reduction system are no
longer on the IUE system but instead have been adjusted to conform to models of the white dwarf
G191B2B. This can produce systematic differences when comparing observations.
2. References are listed in Chapter 9.

Instrument Summary --- Why Use the GHRS?
14 GHRS Instrument Handbook 5.0
The dispersers are mounted on a rotating carrousel, together with several plane mirrors
used for acquisition. The first­order gratings are designated as G140L, G140M, G160M,
G200M, and G270M, where ``G'' indicates a grating, the number indicates the blaze
wavelength (in nm), and the ``L'' or ``M'' suffix denotes a ``low'' or ``medium'' resolution
grating, respectively. The GHRS medium resolution first­order gratings are holographic
in order to achieve very high efficiency within a limited wavelength region. G140L is a
ruled grating. The first two first­order gratings, G140L and G140M, have their spectra
imaged by mirror Cam­A onto detector D1, which is optimized for the shortest wave­
lengths. The other three gratings have their spectra imaged by Cam­B onto detector D2,
which works best at wavelengths from about 1700 to 3200 å, but which is also useful
down to 1200 å.
The carrousel also has an echelle grating. The higher orders are designated as mode
Ech­A, and they are imaged onto D1 by the cross­disperser CD1. The lower orders are
designated as mode Ech­B, and they are directed to D2 by CD2. Finally, mirrors N1 and
A1 image the apertures onto detector D1, and mirrors N2 and A2 image onto D2. The
``N'' mirrors are ``normal,'' i.e., unattenuated, while the ``A'' mirrors (``attenuated'')
reflect a smaller fraction of the light to the detectors, so as to enable the acquisition of
bright stars. (To be precise, the mode designated as N1 actually uses the zero­order
image produced by grating G140L.)
Use of the various gratings or mirrors in concert with the camera mirrors produces one
of three kinds of image at the camera focus: 1) an image of the entrance aperture, which
may be mapped to find and center the object of interest; 2) a single­order spectrum; or 3)
a cross­dispersed, two­dimensional echelle spectrum.
The flux in these images is measured by Digicon detectors, and the portion of the image
plane that is mapped onto the Digicon is determined by magnetic deflection coils. The
detectors are the heart of the GHRS and they involve subtleties that must be understood
if the instrument is to be used competently.
First, there are two Digicons: D1 and D2. D1 has a cesium iodide photocathode on a
lithium fluoride window; that makes D1 effectively ``solar­blind,'' i.e., the enormous
flux of visible­light photons that dominate the spectrum of most stars will produce no
signal with this detector, and only far­ultraviolet photons (1100 to 1800 å) produce
electrons that are accelerated by the 23 kV field onto the diodes. D2 has a cesium tellu­
ride photocathode on a magnesium fluoride window. Each Digicon has 512 diodes that
accumulate counts from accelerated electrons. 500 of those are ``science diodes,'' plus
there are ``corner diodes'' and ``focus diodes'' (see Chapter 6).
Second, both photocathodes have granularity -- irregularities in response -- of about
0.5% (rms) that can limit the S/N achieved, and there are localized blemishes that pro­
duce irregularities of several percent. The Side 1 photocathode also exhibits ``sleeking,''
which is slanted, scratch­like features that have an amplitude of 1 to 2% over regions as
large as half the faceplate. The effects of these irregularities could in principle be
removed by obtaining a flat field measurement at every position on the photocathode,
but that is impractical. Instead, the observing strategy is to rotate the carrousel slightly
between separate exposures and so use different portions of the photocathode. This pro­
cedure is called an FP­SPLIT, and with it each exposure is divided into two or four
separate­but­equal parts, with the carrousel moving the spectrum about 5.2 diode widths

GHRS Instrument Handbook 5.0 15
Instrument Summary --- Why Use the GHRS?
each time in the direction of dispersion. These individual spectra can be combined
together during the reduction phase.
Third, the diodes in the Digicons also have response irregularities, but these are very
slight. The biggest effect is a systematic offset of about 1% in response of the odd­num­
bered diodes relative to the even­numbered ones. This effect can be almost entirely
defeated by use of the default COMB addition procedure. COMB addition deflects the
spectrum by an integral number of diodes between subexposures and has the additional
benefit of working around dead diodes in the instrument that would otherwise leave
image defects.
Fourth, the Digicons' diodes are only slightly smaller than the image of the SSA pro­
duced by the optics, and are larger than the point spread function (PSF) forHST. Thus
the true resolution of the spectrum cannot be realized unless it is adequately sampled.
That is done by making the magnetic field move the spectrum by fractions of the width
of a diode, by either half­ or quarter­diode widths, and then storing those as separate
spectra in the onboard memory. These are merged into a single spectrum in the data
reduction phase. The manner in which this is done is specified by theSTEP­PATT
parameter, described in more detail later. The choice of STEP­PATT also determines
how the background around the spectrum is measured.
Defaults exist for these parameters and they have been set to yield the best quality of
spectrum for the configuration to which they apply (except forFP­SPLIT, which
must be invoked explicitly). Details on the defaults are provided later (Section4.3 on
page 43), but we strongly encourage you to use the defaults unless there are compelling
reasons not to.
2.3 A Little About HowHST and GHRS Work
Because of the difficulties of working with and communicating with a satellite in low­
earth orbit, and in order to makeHST more efficient, virtually all actions taken by the
spacecraft are planned weeks in advance. Only a small fraction ofHST's time can be
used for real­time actions that are at the discretion of the observer, and even then the
realm of possible actions is very limited, being restricted to deciding which object in the
field should be centered in the aperture before a subsequent observation is begun.
This need for detailed planning ofHST observations lies at the heart of the apparent
complexity of the use of the spacecraft and its instruments. At the same time, by care­
fully laying out every aspect of what you want done you will find yourself with a better
understanding of what actually happens and more confidence that the desired results
will be achieved.
All HST observations begin with an acquisition. An acquisition can be as simple as
blindly pointing to particular celestial coordinates, although such a procedure is
unlikely to succeed with the GHRS because its entrance apertures are small. For the
GHRS, an acquisition usually means a pointing to precisely specified coordinates, small
motions of the telescope in a spiral pattern to sample the region of sky in the vicinity of
the coordinates, and then a peakup motion to center a star in the aperture after on­board
software has determined its location. Variations include offsetting from the acquired star
to another nearby object or moving the star to the small aperture. In rare cases it may be

Instrument Summary --- Why Use the GHRS?
16 GHRS Instrument Handbook 5.0
necessary to perform an interactive acquisition, in which the observer specifies the
object in real time. An intermediate possibility is to take an image with one ofHST's
cameras (or with the GHRS itself) in advance of the spectroscopic observation (by one
to two months) and to then derive precise coordinates from that image (an early acquisi­
tion). For very faint objects, especially those to be observed with Side 1 of the GHRS, it
is possible to acquire the object with the Faint Object Spectrograph before moving it to
the LSA.
Once the star has been properly positioned in the appropriate aperture of the GHRS, sci­
ence observations may begin. In some cases you may wish to useIMAGE mode, which
can map the LSA at ultraviolet wavelengths, but in general this part means dispersing
the light with one of the gratings and adding up the counts to form a spectrum. A
RAPID mode also exists to record spectra that change on very short time scales. The
GHRS has no independent microprocessor and so depends on the spacecraft's computer
and memory for control of its operations. One implication of that dependence is that
there is a maximum number of commands that can be stored at any one time. Since
those commands are generally loaded into the spacecraft only a few times per day, that
limitation restricts the total number of GHRS exposures that may be made in a 24 hour
period. At the same time, image motion within the instrument that is induced by the
earth's magnetic field (see Section8.6.5 on page 98) is best dealt with by making indi­
vidual exposures no longer than about 5 minutes, thereby increasing the total number of
exposures you need to make to get a science observation. In some cases these require­
ments come into conflict and compromises must be made to accommodate science
goals.
Some other relevant aspects of schedulingHST observations are:
. Objects in most regions of the sky ``rise'' and ``set'' and will be available for science
observations for about half of an orbit (about 45 to 50 minutes). Longer exposures
get spread over several orbits, with a reacquisition at the beginning of each orbit, but
this occurs at no practical cost in science terms since the GHRS' detectors are pho­
ton counters. Some objects sometimes fall withinHST's Continuous Viewing Zones
(CVZs), which enables them to be observed for long times at high efficiency; see the
Call for Proposals for details on taking advantage of the CVZs.
. The orbit of HST passes through the South Atlantic Anomaly (SAA), which is a
region in which the background count rate is very high. At present the scheduling
software simply stops the counting of photons during times when the spacecraft is
within the SAA.

GHRS Instrument Handbook 5.0 17
Instrument Summary --- Why Use the GHRS?
2.4 GHRS Modes of Operation
GHRS has several operational modes for target acquisition and obtaining science data.
See also Section 4.1 on page 35 for more discussion of acquisitions.
2.4.1 Target Acquisition Mode
2.4.1.1 Onboard Acquisitions
Most targets observed with the GHRS can be automatically acquired with an onboard
acquisition (ONBOARD ACQ). An onboard target acquisition observation consists of dis­
tinct phases, which are executed in ascending numerical order. Phases 1 and 2 perform
initialization and internal calibration functions, and need not concern the observer. The
third phase is called Target Search. A series of small angle maneuvers, called a ``spiral
search,'' scans an area of the sky centered on the initial position. The flux coming
through the Large Science Aperture (LSA) is measured at each dwell point in the
search. If the BRIGHT=RETURN option has been chosen (and it is recommended), the
telescope returns to that dwell point which had the greatest number of counts. If
BRIGHT and FAINT limits have instead been specified, the flux is compared to these
upper and lower limits at each step in the spiral, and if the measured value falls between
these limits the target is assumed to be within the aperture and the search immediately
stops. You may request that a field map be generated at the final dwell point by means of
the MAP optional parameter. You should be aware, though, that approximately two min­
utes is required for each map, and that many pointings may be made during the search.
(If you intend to analyze the maps in real time, the search phase must be done as an
interactive acquisition.) If you wish to confirm the spacecraft's pointing, we recommend
obtaining an IMAGE after the acquisition instead of using the MAP option -- see
Chapter 4.
The fourth phase is target locate (ACQ/PEAKUP). This process measures the precise
location of the target within the aperture, and requests a small angle maneuver to move
it to the center. The field map of the LSA may be madebefore the centering maneuver is
performed by specifying MAP=END­POINT. If done after the centering (in IMAGE
mode), the map can be helpful for confirming that the object was placed precisely in the
center of the aperture 1 . The final phase of an acquisition is a flux measurement in which
the flux entering the GHRS through the final target aperture is measured and inserted
into the data. After centering, a second maneuver will automatically translate the object
to the SSA if that is the aperture specified for the observation. AnACQ/PEAKUP with
``0.25'' as the specified aperture will also center the object in the SSA.
1. Please don't get the wrong impression. Getting an image of the aperture to confirm pointing is
rarely necessary or useful and we mention it here mostly for completeness. If you are working in
a crowded field, it might help to know exactly what was in the aperture after the fact.

Instrument Summary --- Why Use the GHRS?
18 GHRS Instrument Handbook 5.0
For some kinds of difficult targets an onboard acquisition may not work. Possible causes
might be:
. The error in the coordinates is greater than a few arcsec in either declination or right
ascension, so that the target lies outside the largest area that the GHRS can search in
its onboard procedure.
. The object is a moving target whose coordinates could not be predicted with±5 arc­
sec accuracy when the proposal was written. Features in the atmosphere of a planet,
and comets are possible examples.
. The object has a poorly known or unpredictably variable ultraviolet flux.
. The target has nearby neighbors of similar brightness -- the onboard search process
could center up on the wrong object.
. The object has a spatial extent greater than two arcsec. The automatic centering
algorithms may not produce acceptable results on objects comparable in size or
larger than the Large Science Aperture.
. The object is too faint to get adequate counts with the maximum permissible integra­
tion time of 12.75 seconds.
In many cases these problems can be worked around by using an onboard acquisition on
a nearby star and then offsetting to the object of interest, or, perhaps, by using the FOS
to acquire before slewing the target to the GHRS.
2.4.1.2 Early and Interactive Acquisitions
You may choose to obtain an early acquisition ( EARLY ACQ) image with WFPC2, FOC,
or GHRS itself. In some cases an acquisition image would be helpful, but the field of
view of the WFPC2 is not needed. Stationary point sources in crowded but recognizable
fields would be examples. The GHRS has its own ``field map'' capability which will pro­
duce an image of the sky as seen through the LSA. Each map is a square array of 16â
16 pixels, covering arcsec with 0.11 arcsec spatial resolution. A single field
map requires a minimum of two minutes to take the data and send it to the ground, and
much longer if each point in a spiral search is mapped or if aSTEP­TIME longer than
the default (0.2 sec) is used. One WFPC2 image requires from three to five minutes, but
covers a much larger area of the sky. As a practical matter, if more than one field map
would be needed, a WFPC2 image may be a more efficient choice. The FOC could also
be an appropriate choice for ultraviolet­bright objects.
If an interactive acquisition (INT ACQ) is required, the observer must be present at
STScI, prepared to inspect the image and identify the target in a timely fashion. Real­
time observations are subject to many constraints and are difficult to schedule (they are
occasionally impossible). Early acquisition should therefore be chosen in preference to
interactive acquisition whenever possible. The HST (Phase I) Proposal Forms require
justification of requests for real­time observation.
1.74 1.74
â

GHRS Instrument Handbook 5.0 19
Instrument Summary --- Why Use the GHRS?
2.4.2 Science Data Acquisition Modes
There are several modes of science data acquisition, including Accumulation Mode,
Rapid Readout Mode, and Image Mode. Each of these modes may be used in conjunc­
tion with any of the optical configurations described earlier.
Accumulation Mode
Accumulation Mode (ACCUM) is the normal way of obtaining a spectrum with the
GHRS. The name refers to the fact that data can be accumulated in the onboard com­
puter during a long exposure. All of the features of the flight software are available in
this mode, making it the most powerful, flexible, and automatic way to use the GHRS.
There are two types of benefits which one can expect by using the flight software in
Accumulation Mode.
The first is the ability to make long duration observations with effective and automatic
control of the process. The time varying Doppler shift caused by the orbital velocity of
the spacecraft is compensated for automatically. The software constantly monitors a set
of data quality criteria and can flag, reject, or reobserve individual integrations that fail
the tests. Finally, the software can suspend the observation during scheduled or unex­
pected interruptions, such as occultation of the target by the Earth or passage through
the South Atlantic Anomaly, and then resume when the interruption ends. The very low
background count rate and absence of readout noise in the Digicons make exposures of
hours duration feasible, though it is strongly suggested that these be broken into shorter
segments to aid in scheduling and protect against catastrophic data loss in the event of
an unexpected problem.
The second category of benefits results from the ability of the software to perform pat­
terns of integrations at closely spaced positions on the photocathode, a process which is
referred to as substepping. There are four purposes for this. At the beginning of an
observation, the software executes a procedure called Spectrum Y Balance (
SPYBAL)
to find the optimum centering of the image on the diode array. This compensates for
minor changes in the image location due to thermal or electrical drifts. The second use
is to make multiple (2 or 4) samples per resolution element (1 diode width) to ensure
that the digital data satisfy the Nyquist sampling criterion. This isvery important when
the ultimate spectral resolution of narrow features is required. Third, the background
adjacent to the spectrum or in the echelle interorder region can be measured. Finally,
comb addition allows the effect of small diode­to­diode sensitivity variations to be min­
imized and eliminates the holes in the data due to a few inoperative channels. When
substepping is used to define the detailed sampling of the spectrum and background, the
data obtained at each step are accumulated into one of up to seven distinct ``bins'' in the
memory of the onboard computer.
This overview of the flight software features is not exhaustive, but summarizes those
capabilities which are immediately relevant to the acquisition of spectra in accumula­
tion mode. Several items, namely substepping and exposure control, require the
observer to specify certain parameters. These will be described in more detail later in
this Handbook.

Instrument Summary --- Why Use the GHRS?
20 GHRS Instrument Handbook 5.0
Rapid Readout Mode (sometimes called Direct Downlink)
Rapid Readout Mode (RAPID) is intended to provide very fast time resolution without
the overhead times associated with Accumulation Mode. The sample time can be
between 50 ms and 12.75 seconds, in increments of 50 milliseconds (i.e., 1 to 255 times
50 ms). At the end of each integration the data are read out, either directly to the TDRSS
satellite or to the spacecraft science data tape recorder. The flight software cannot exe­
cute all of its functions and still allow readouts every 50 ms. When the Rapid Readout
Mode is entered, substepping, data quality checks and exposure control features are
deactivated. The primary factor governing the choice betweenACCUM and RAPID is
time resolution. In accumulation mode, the time between exposures can be as short as
about one minute. If higher time resolution is required, if the source is bright enough to
give useful counts in a shorter integration, and if one is willing to sacrifice the flight
software control, then direct downlink is a useful alternative. InRAPID mode, a SAM­
PLE­TIME of less than 0.33 sec requires the use of the 1 Mb data channel (see
Section 4.4 on page 45). Such a high data rate stressesHST's data­handling capabilities
and means that only about 20 minutes of observations can be stored.
Image Mode
Images may be obtained in this mode by deflecting the image of the photocathode over
the 0.11 â 0.11 arcsec focus diodes. The result is a map similar to that obtained during
target acquisition, but without an acquisition being performed. Also, aMAP as part of an
acquisition can cover more of the sky than the LSA subtends at one time by small move­
ments of the telescope, whereas anIMAGE is limited to the 1.74 by 1.74 arcsec area of
the LSA; see Section4.2 on page 42.
WSCAN and OSCAN Modes
These are really modifications of theACCUMulation mode designed for higher efficiency
in multiple observations, and they may be requested during Phase II of the proposal pro­
cess. WSCAN obtains a series of spectra within a given order, incrementing by a speci­
fied wavelength increment between each. The result is a spectrum spanning a broader
wavelength range than is possible with a single exposure.OSCAN works with the
echelle, and uses the magnetic deflection of the Digicon to obtain spectra over a range
of echelle orders. The grating carrousel is not rotated, and spectra are obtained at equal
values of ml, where m is the echelle order. OSCAN is not ordinarily used for science
observations.

GHRS Instrument Handbook 5.0 21
Chapter 3 Phase I:
What the T
AC Sees
3.1 Essential questions 22
3.2 Creating an Observation Scenario 23
3.2.1 The Simplest Case: One Spectrum for One Star 23
3.2.2 Several Spectra for One Star 23
3.2.3 Spectra of Several Stars 24
3.2.4 RAPID Mode Observations 24
3.2.5 Adding Calibrations 24
3.3 Specifying Target Acquisition 24
3.3.1 Very Bright Stars 25
3.3.2 Moving Targets 25
3.3.3 Crowded Fields 25
3.3.4 Extended and Very Faint Objects 26
3.3.5 Variable Objects 26
3.4 Calculating the Exposure Time 26
3.4.1 Sensitivity 26
3.4.2 Reddening 27
3.4.3 Background 27
3.4.4 Scattered light 28
3.4.5 Signal­to­noise 29
3.4.6 A Simple Example 31

Phase I: What the TAC Sees
22 GHRS Instrument Handbook 5.0
A proposal for theHubble Space Telescope is written in two phases. In Phase I, you are
asked to provide the minimum information needed for the Telescope Allocation Com­
mittee and STScI to judge the scientific merit and technical feasibility of what you wish
to do. If your proposal is successful, you will be asked in Phase II to provide the specific
details and parameters that are needed to turn your proposal into a series of commands
that the spacecraft can execute. At the time thisHandbook is being written the proce­
dures for submission ofHST proposals are under review; consult theCall for Proposals
for the procedures in effect. In particular, Cycle 5 proposals will ask for time in units of
orbits instead of total spacecraft time and an orbit calculator has been constructed to
allow for planning.
These instructions for completing a Phase I proposal are meant to work with the Cycle 5
Call for Proposals and Phase I Proposal Instructions.
3.1 Essential questions
The essential questions you must answer in filling out anHST proposal form are:
. Do I need real­time acquisition for my targets?
. How long will my exposures take?
. Are standard calibrations adequate for my needs?
. Does my science call for any special requirements?
All of these lead up to:
. What is the total number of orbits my program asks for?
The information you provide on the execution of observations is divided into Phase I
and Phase II. Phase I proposal processing takes place before the T AC meets and at that
time you are asked to provide only the information that they need to arrive at a decision
on the scientific merit and feasibility of your proposal. The TAC needs to know, of
course, how much telescope time your program is likely to need. Phase II takes place
only after your proposal is successful and at that time you are required to specify all the
details needed to transform your observational needs into spacecraft commands, and
you must do so within the total spacecraft time that you have been allotted.
The distinction between Phase I and Phase II is somewhat arbitrary and in some cases
the Phase I forms actually prevent you from supplying information (such as the entrance
aperture to be used) that could be helpful in assessing the proposal. The intent is to
make writing the Phase I proposal easier, but you may find that your proposal will be
better if you understand fully the operation and use of the GHRS right from the start. We
encourage you to think in terms of the Phase II requirements even when writing the
Phase I proposal because in most cases that will not require much more work and you
may be able to save some time later on. Thus there is information in this section that is
only entered on the Phase II forms but which has been included here to provide a picture
of how the instrument works.

GHRS Instrument Handbook 5.0 23
Phase I: What the TAC Sees
3.2 Creating an Observation Scenario
3.2.1 The Simplest Case: One Spectrum for One Star
Let us assume that in answering the above questions you have decided that an onboard
acquisition will suffice (more on that in a moment), that standard calibrations are ade­
quate, and that you wish to obtain a single spectrum of a single star. By specifying the
resolving power you desire for a particular wavelength, you have, in effect, chosen to
observe with either Side 1 or Side 2 and with a particular grating (see Chapter 7). This
simplest of observation scenarios then involves one acquisition sequence and one
ACCUM observation. (An IMAGE mode observation may be specified in place of the
ACCUM with no loss of generality. A WSCAN or OSCAN is just a minor variation on an
ACCUM. See below forRAPIDs.)
The choice of Side 1 or Side 2 for obtaining the spectrum need not necessarily force you
to that Side for the acquisition; for example, you might wish to observe a cool star with
grating G140L on Side 1, but the far ultraviolet flux would be too low to acquire the
object with mirror N1. In that case you can use mirror N2; it is permissible to mix Sides
in a scenario, but there may be a cost in observing time for doing so (see Section4.1.5.1
on page 38). Some difficult targets may be best acquired with the FOS before they are
observed with the GHRS.
For this simple case this sequence of events can be compared to thePhase I Proposal
Instructions:
. A guide star acquisition.
. The target acquisition with the GHRS.
. The ACCUM (or IMAGE).
. Some overhead time to read the observation.
3.2.2 Several Spectra for One Star
If more than one spectrum is desired for the star, the ACCUMs + overheads are repeated
as necessary, bearing in mind the need to reacquire the star at the start of each orbit. The
multiple spectra could be either an assortment of wavelengths or repeats at a single
wavelength to follow an object in time or to improve signal­to­noise. In all cases over­
head time must be added for each separate exposure:
. A guide star acquisition.
. The target acquisition with the GHRS.
. The first ACCUM or IMAGE.
. Some overhead time to read the observation.
. The second ACCUM or IMAGE, followed by an overhead allowance.
. The third ACCUM...

Phase I: What the TAC Sees
24 GHRS Instrument Handbook 5.0
3.2.3 Spectra of Several Stars
This instance is just multiple versions of the previous case since changing stars requires
a new visit, meaning a full guide star acquisition, acquisition of the object into the
GHRS LSA, etc.
3.2.4 RAPID Mode Observations
RAPID mode observations can be planned just as forACCUMs. The overhead time is
added only once at the very end of the wholeRAPID sequence.
3.2.5 Adding Calibrations
The only kind of calibration exposure that an observer will ordinarily need is one of the
wavelength calibration lamp. An exposure of 30 to 60 seconds is adequate for almost all
settings with the first­order gratings; otherwise the exposure is just anotherACCUM.
Note that for most applications the information contained in theSPYBAL that accompa­
nies each first use of a grating is likely to suffice and a separate wavelength calibration
is superfluous; see Section4.5 on page 46.
3.3 Specifying Target Acquisition
For most situations, a standard onboard acquisition that automatically centers the
brightest object in the field into the desired aperture is all that is needed. Such a proce­
dure is especially appropriate for isolated point sources that are beyond our solar system
and which have fairly predictable ultraviolet fluxes, i.e., most stars. In other cases, it is
possible to use a variation on the automatic procedure to acquire other objects. For
example, an extended object or some moving objects may be acquired by first automati­
cally centering on a nearby pointlike source and then offsetting to the object of interest.
Some potential problem cases are:
. very bright stars, which can saturate the detectors;
. moving targets, such as planets and their satellites;
. crowded fields, in which the automatic centering procedure mightget confused;
. extended objects that do not have a sharply peaked source to center on;
. very faint objects for which few counts would be accumulated in the maximum per­
missible integration time (12.75 sec).
. objects that are so variable that their brightness relative to nearby objects may be
unpredictable.
These situations may require an interactive (or real­time) acquisition, although an on­
board acquisition may still work in many cases. In an interactive acquisition, the space­
craft obtains a picture of the target's field with one of the cameras (WFPC2 or FOC) or
with the field mapping capability of the GHRS itself. This picture is relayed immedi­
ately to STScI where the observer is available to study the image and decide where the
telescope should be pointed. Interactive acquisitions are obviously helpful in difficult
situations, but the requirement for real­time contact between the ground andHST,
together with the need to set aside a block of telescope time for the pointing decision to

GHRS Instrument Handbook 5.0 25
Phase I: What the TAC Sees
be made, makes interactive acquisitions consume much more spacecraft time than is
needed for a standard onboard acquisition. Interactive acquisitions require special
scheduling and so require greater­than­average work on the part of the planners of
HST's time. Real­time contact withHST is a limited resource (it cannot exceed 20% of
the total time) which must be reserved for genuine need.
A variation on this procedure is to get the field image two months or so in advance of
the time the spectrum will be obtained. This is called an early acquisition (
EARLY ACQ)
and it takes more time than an on­board acquisition but much less than an interactive
acquisition and imposes no burden of real­time contact. The observer must be prepared
to analyze the early acquisition image quickly (within a week or two) if the positions
from it are to be incorporated into the telescope schedule.
If you wish to use either WFPC2 or FOC for early­ or interactive acquisitions you must
refer to documents specific to those instruments. Details on the use of the GHRS' imag­
ing capability are provided in the next chapter.
3.3.1 Very Bright Stars
When is a star too bright for an onboard acquisition? In practice we are unaware of any
real need to use interactive acquisition just because a star is very bright. GHRS acquisi­
tions are done with ultraviolet light, so it is the UV flux of the star that matters. There
are no stars too bright to acquire with the attenuated mirror A1, for example. Even with
Side 2 it is not necessary to specify an interactive acquisition for a very bright star if the
BRIGHT=RETURN option is used. Since the few very bright stars which could cause
problems are always the brightest point sources in their immediate area, there is no
apparent reason not to useBRIGHT=RETURN with an onboard acquisition.
3.3.2 Moving Targets
Sophisticated pointing at moving objects (i.e., objects within the solar system) often
requires interactive acquisition to be sure the desired portion of the object's surface is
centered in the observing aperture. There are cases where an on­board acquisition will
suffice, especially if the object is small (essentially point­like) and has a well­deter­
mined orbit. An on­board acquisition can often work well even for a large moving
object like Jupiter by first centering on a small object nearby whose relative position is
well known (one of Jupiter's moons, for example), and then offsetting to the position of
interest on the planetary disk. Solar system astronomers may wish to consult with a
moving­target specialist at STScI before specifying the acquisition mode.
3.3.3 Crowded Fields
Work in crowded fields can usually be done by obtaining an early acquisition, so that
you have an image to work from to specify the object to be observed, an image that has
been obtained with HST's full spatial resolution. The camera observation is usually best
done at about the same wavelength that the spectroscopic observations will be made.
The Point Spread Function (PSF) of the GHRS has been restored by the COSTAR mir­
rors, making it possible to separately observe stars that are very close together. For
example, in an Early Release Observation in April, 1994, two stars in R136a separated

Phase I: What the TAC Sees
26 GHRS Instrument Handbook 5.0
by only 0.25 arcsec were observed independently. One of these stars was only 0.1 arcsec
from a brighter neighbor. This was done by centering on a bright object in the field and
then offsetting to the targets of interest. We suggest that you consult with us if you wish
to work in crowded fields. Also, see Section8.6.1 on page 94.
3.3.4 Extended and Very Faint Objects
For most extended objects, it may be possible to offset from a nearby point source or at
the least the pointing can be specified from an early acquisition image. Interactive
acquisitions should be necessary only rarely. Another method is to acquire the object
with the FOS and then offset to the GHRS' LSA.
In Cycle 5 observers can use a centering option designed expressly for extended objects,
especially uniform ones like the Gallilean satellites of Jupiter. Chapter 4 contains more
information on this option, known asLOCATE = EXTENDED.
3.3.5 Variable Objects
Objects whose ultraviolet brightness varied often caused acquisitions to fail when it was
necessary to specify bothBRIGHT and FAINT count limits. The advent of software that
automatically finds the brightest object in the field(BRIGHT=RETURN) obviates that
problem in most cases. A variable object in a crowded field might benefit from an early
acquisition to determine precise coordinates, but an interactive acquisition should gen­
erally be unnecessary.
3.4 Calculating the Exposure Time
3.4.1 Sensitivity
The sensitivity of the GHRS andHST Optical Telescope Assembly (OTA), using the
LSA, has been determined from observations of stars with known ultraviolet fluxes. The
sensitivity is designated as , and has units of (counts diode ­1 sec ­1 ) for each incident
(erg cm ­2 sec ­1 å). It varies as a function of wavelength for each grating. You must first
estimate the intrinsic flux of your target and then multiply that by the appropriate value
of to yield an estimate of the count rate to be expected for a particular grating config­
uration at the chosen wavelength. Sensitivity curves for the first­order gratings are pro­
vided in Section 8.2.3 on page 85 and for the echelles in Section8.3.1 on page 87.
In the echelle configurations, the sensitivity varies with wavelength across each order.
This behavior is characteristic of all echelle spectrographs, and is called the Blaze Func­
tion. The basic nature of the variation with wavelength is similar for all orders, and can
be parameterized in terms of the product , where m is the order number andl is the
wavelength (å). The shape of the blaze function, normalized to a peak value of unity,
and plotted as a function of is shown in Section8.3.3 on page 91. The sensitivity at
any wavelength in any order can be estimated by multiplying the peak response of that
order by the relative response shown. The blaze function, relative to the center of an
order, falls as low as 0.25 at the end of the free spectral range and its ef fect should not be
omitted in exposure calculations.
S l
S l
ml
ml

GHRS Instrument Handbook 5.0 27
Phase I: What the TAC Sees
3.4.2 Reddening
Corrections for ultraviolet extinction in the interstellar medium are included in
Section 8.5 on page 93. These are standard values, and their applicability in specific sit­
uations is left to the judgment of the observer.
3.4.3 Background
There are several potential sources of background counts, including detector dark count,
electrical interference or cross talk with devices either within the GHRS or the space­
craft, and effects caused by the charged particle radiation environment of theHST orbit.
The intrinsic sources of dark count are very small. During ``thermal vacuum'' testing
prior to launch the detector dark count rates were observed to be approximately 0.0004
counts per diode per second. On­orbit, the background is caused primarily by Cerenkov
radiation bursts induced in the faceplate of the Digicon by cosmic rays. This causes the
actual background to range from to about four times that, depending on the
orbital position of HST. For planning purposes these mean values suffice: 0.011 counts
s ­1 for D2 and 0.008 counts s ­1 for D1. The counts appear to be randomly distributed in
time, so that the ``noise'' in the dark count is the square root of the total counts accumu­
lated during the observation. If one is observing very faint objects with low count rates
the background can influence the signal to noise ratio of the data. Formulae for making
quantitative estimates of S/N are given in Section3.4.5.1 on page 29. At the present
time there are no known sources of interference or cross talk which affect the detector
count rates.
The GHRS is equipped with both hardware (automatic) and software capabilities to rec­
ognize and respond to cosmic ray and trapped particle events. You may invoke the soft­
ware capability by specifying CENSOR = YES on an Exposure Logsheet line in
Phase II. This causes rejection of individual STEP­TIME segments of data if they
included a specified number of photons arriving within a short (
8 µs) interval, as hap­
pens with cosmic rays. Any rejected integration is repeated, so there is no loss of total
exposure time. You should only use this anticoincidence rejection on faint targets, since
on bright targets the interval between actual photon events will be small and real counts
would be rejected. We recommend using CENSOR = YES only for count rates less than
about 0.1 counts per diode per second. The expected dark count reduction is a few tens
of per cent (For more details onCENSOR, see Section 8.6.2 on page 95).
For extremely faint sources for which the expected count rate is well below the expected
dark level, it is possible to use a special commanding option calledFLYLIM. This
option, if pertinent to your needs, should be explored with a GHRS Instrument Scientist.
See also Section 8.6.3 on page 97.
An external source of background which can potentially be a problem during the acqui­
sition (and sometimes the observation) of faint targets is geocoronal Lyman­a. This
problem and what to do about it are discussed in Section7.4.2 on page 80.
The final cause of background counts is passage through the dip in the Earth's Van Allen
radiation belt called the South Atlantic Anomaly (SAA). SAA passage occurs on 7 of 16
daily orbits of HST. During the most central of these passages, dark count rates increase
about two orders of magnitude, to about 1 count per diode per second. A contour around
the SSA which corresponds to 0.02 cts/s/diode is known and no GHRS observations are
4 10 3
--
â

Phase I: What the TAC Sees
28 GHRS Instrument Handbook 5.0
scheduled when the HST is within this zone. At the time of this writing the SAA con­
tours for the GHRS are being reviewed to allow for more efficient usage.
3.4.4 Scattered light
The presence of stray and scattered light in a spectrograph is an effect which can influ­
ence the planning and execution of an observation, as well as the reduction and interpre­
tation of the data. None of the optical configurations which include first order gratings
has any serious problem with scattered light. The high quality of the imaging optics and
holographic diffraction gratings and the effectiveness of the baffles have successfully
minimized the stray light. On­orbit measurements indicate that it amounts to less than
10 ­3 when using the SSA, and at most a few times 10 ­3 when using the LSA (these are in
units of the peak intensity).
In the echelle configuration, both the echelle and the cross­dispersers are ruled gratings.
This fact, plus the presence of light from sixteen orders simultaneously on the photo­
cathode, results in a detectable level of background radiation. The irradiance on the pho­
tocathode due to scattered light (measured as count rate per unit area) amounts to a few
percent of the signal in the order. Two factors complicate this effect. The first is a geo­
metrical effect caused by the fact that the science diodes are 400µm tall, while the
image of the spectrum is only about 55µm high. Thus about 1/8 of the diode is illumi­
nated by the spectrum+background, while the rest is measuring background, meaning
that a weak background irradiance is multiplied to the point that a significant fraction
(anywhere from 2 to 50%) of the gross count rate on a diode may be due to background.
The measured scattered light background can be calculated from information in
Section 8.3.1 on page 87. It varies significantly with order number.
The second complication arises at the short wavelength ends of the echelle format.
Below a wavelength of about 1800 å with Echelle B (or 1250 å with Echelle A), the
spacing between orders is comparable to the length of the diodes, and it is difficult to
make a clean measurement of a single order. The diode array has four large ``corner
diodes'' which are long (1 mm) in the direction of the echelle's dispersion, but narrow
(100 µm) in the cross­dispersion direction. These diodes may be used to sample the
interorder background without the problem of contamination by in order light, but they
do not provide any spatial resolution. TheHST scheduling system will default to use of
the corner diodes when that is appropriate. At a minimum, the time spent measuring the
background should be about 10% of the time spent on the spectrum. If the goal is to
achieve a very high signal to noise ratio in the net spectrum, it may be necessary to
devote a greater fraction of time to the background measurement. Suggestions for esti­
mating signal to noise ratios are made in the next section.
In order to reduce stray light, there is a shutter over the LSA which automatically closes
whenever the SSA is being used for an observation. There is no shutter on the SSA.
Thus a wavelength calibration exposure obtained with a bright star in the SSA will
result in a combined spectrum of the two because the aperture for the wavelength cali­
bration lamp (SC2) is displaced from the SSA in the same sense as the direction of dis­
persion. Usually you can subtract the stellar spectrum to recover the wavelength
calibration.
More detailed quantitative information on background and scattered light in the GHRS
is provided in Section8.6.1 on page 94.

GHRS Instrument Handbook 5.0 29
Phase I: What the TAC Sees
3.4.5 Signal­to­noise
There are several factors which influence the signal to noise, including statistical (Pois­
son) noise in the detected spectrum, dark count noise in the detector, scattered light in
the spectrograph, diode to diode gain variations, and granularity in the photocathode
sensitivity. For signal to noise ratios up to approximately 50, statistical fluctuations in
the signal and background will dominate. Diode to diode variations are extremely small,
and are accounted for in the routine calibration procedures. Cathode granularity will
become important if signal to noise greater than 50 is required, and must be treated sep­
arately. For sources observed through the small aperture the sky background should not
contribute significantly to the noise, except, perhaps, when observing at Lyman­a.
3.4.5.1 Photon Noise
The following equations may be used to estimate signal to noise ratio, depending on the
relative importance of scattered light and dark count.
Case 1. Neither scattered light nor dark count are important.
Let:
s = signal strength (counts per diode per second) estimated by multiplying the stellar
flux by the sensitivity at the desired wavelength.
t = duration of the observation in seconds. This total time will be divided among the
separate substep bins.
= the number of adjacent diodes that will be binned together to produce an effec­
tive resolution element. Usually n s = 1. This is not the merging of substep bins,
but the deliberate averaging to increase signal to noise at the expense of resolu­
tion.
Then
This formula would be appropriate for relatively bright objects observed with any first
order grating, when substep pattern 1, 2, or 3 is used (see Section8.4 on page 92).
Case 2. Dark count is important, scattered light is not.
Let:
d = dark count rate in counts per diode per second.
Then
If the signal is less than about ten times the dark count rate, the factor in parentheses
should be included in the estimate. This formula would be useful ifSTEP­PATT 5, for
example, were used with a first order grating to measure a faint source (see Section8.4
on page 92).
n s
S N
/
( ) 2 sn s t
=
S N
( ) 2 s d
/
1 s d
+
­­­­­­­­­­­­­­­­­­
Õ Ü
Ô Æ sn s t
=

Phase I: What the TAC Sees
30 GHRS Instrument Handbook 5.0
Case 3. Scattered light is important, dark count is not.
Let:
f = fraction of time spent measuring the spectrum. (See Section8.4 on page 92)
b = scattered light as a fraction of the signal in the adjacent orders.
Then
This formula gives a good estimate of the performance for observations with the ech­
elles when stepping patterns 6, 7, 8 or 9 are used. This formula assumes that the back­
ground bins are heavily smoothed. Most of the high frequency statistical noise in the
background bins is thus suppressed.
Case 4. Both scattered light and dark count are important.
Let:
= number of adjacent diodes to smooth the background bins over before subtract­
ing. Experiments with ground­based data indicate that gives the best
results.
Then
There are two ways to use these formulae. If you need a certain S/N to do the scientific
analysis, use the appropriate equation to solve for the required exposure timet. Alter­
nately, you can decide to devote a fixed length of time to the observation, and use the
equations to estimate what S/N will be achieved.
3.4.5.2 Fixed Pattern Noise
The formulae just presented suggest that the signal to noise ratio increases in proportion
to the square root of the exposure time. These relations only hold true until S/N» 50 or
so is reached. At higher signal levels the photocathode granularity described in
Section 4.3 on page 43 will become the limiting factor. Observing standard stars to pro­
vide a precise ``flat field'' observation is too inefficient, and there is no onboard contin­
uum lamp that illuminates the optics and detectors in exactly the same way as the stellar
spectrum. The best practice is to use theFP­SPLIT option (Section 4.3.1 on page 43).
Rather than merely averaging the fourFP­SPLIT sub­exposures, the data analysis pro­
cedure solves for the two vectors representing photocathode granularity and the spec­
trum. S/N well in excess of 100 has been obtained this way on bright targets.
Achieving extremely high signal­to­noise (200 or more) is possible by obtaining a num­
ber of spectra, each with FP­SPLIT but at slightly different grating positions. See
Lambert et al. (1994) for a discussion.
S N
( ) 2 f
1 b
+
­­­­­­­­­­­­ sn s t
»
n b
n b 10
»
S N
/
( ) 2 s 2 t
s 1 b
+
n s f
­­­­­­­­­­­­ b
n b 1 f
--
( )
­­­­­­­­­­­­­­­­­­­­­­­
+ d 1
n s f
­­­­­­ 1
n b 1 f
--
( )
­­­­­­­­­­­­­­­­­­­­­­­
+
+
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
=

GHRS Instrument Handbook 5.0 31
Phase I: What the TAC Sees
3.4.6 A Simple Example
Here is a very simple example to illustrate how an integration time may be computed.
Suppose that the goal is to obtain a spectrum of a 13th magnitude B0 star at 1900 å,
with the G160M grating and with a signal­to­noise of 25 per diode in the continuum. In
this case we will assume that this star has not been previously observed in the ultraviolet
so that there is noa priori knowledge of the UV flux.
To be specific, take the star to have a spectral type of B0I, , and
. The unreddened color for this spectral type is , so
that . The total visual extinction is then , leading
to a dereddened magnitude of . The dereddened flux at 5500 å is then
.
What flux can we anticipate at 1900 å? The model atmospheres of Kurucz (1979, ApJS,
40, 1) predict for a star with K. This leads to a flux of
at 1900 å for the unreddened star. Reddening will
diminish this by a factor of , where the absorption at 1900 å can be deter­
mined from the data in Section8.5 on page 93; the result in this case is .
We therefore predict this star to have a flux at 1900 å of .
The next step is to determine the detected count rate. For G160M at 1900 å, the sensi­
tivity is , in units of counts per second per diode per incident erg s ­1
cm ­2 å ­1 . This leads to an expected count rate of . An inte­
gration time of about 2.25 hours would lead to approximately 625 detected counts per
diode, or the required signal­to­noise of 25. This neglects the effects of dark, which
should be an order­of­magnitude below this count rate.
V 12.89
=
B V
--
( ) 0.03
= B V
--
( ) 0 0.24
--
=
E B V
--
( ) 0.27
= 3.1 0.27
â 0.84
=
V 0 12.05
=
F 5500 5.4 10 14
-- erg s 1
-- cm 2
-- å 1
--
â
=
F 1900 F 5500
/ 23
= T eff 25000
=
F 1900 1.2 10 12
-- erg s 1
-- cm 2
-- å 1
--
â
=
10 0.4 A 1900
â
--
A 1900 2.26
=
1.5 10 13
-- erg s 1
-- cm 2
-- å 1
--
â
S l 5.2 10 11
â
=
7.8 10 2
-- counts s 1
-- diode 1
--
â

Phase I: What the TAC Sees
32 GHRS Instrument Handbook 5.0

GHRS Instrument Handbook 5.0 33
Chapter 4 Your Phase II Pr
oposal
4.1 Acquisitions 35
4.1.1 Initial Pointing 35
4.1.2 Interactive Acquisitions 35
4.1.3 Early Acquisitions 36
4.1.4 Onboard Acquisitions 36
4.1.5 Special Onboard Strategies for Special Situations 38
4.1.6 MAPs 40
4.1.7 Acquisition Parameters --- A Summary 40
4.2 Image Mode 42
4.2.1 Image Mode Parameters 42
4.2.2 The GHRS as a Slitless Spectrograph 43
4.3 Accumulation Mode 43
4.3.1 Optimizing Data Quality 43
4.3.2 Summary of Accumulation Mode Parameters 44
4.3.3 WSCAN mode 45
4.3.4 OSCAN mode 45
4.4 Rapid Readout Mode 45
4.5 The Precision and Accuracy of Standard Calibrations 46
4.6 Other Considerations 47

Your Phase II Proposal
34 GHRS Instrument Handbook 5.0
This chapter supplements the Phase II Proposal Instructions that will be issued for
Cycle 5. You may also wish to consultPhase II Instructions for the Solar System Target
List if your program is to observe solar system objects.
Most users of the GHRS will find that a simple sequence of commands will work most
of the time to obtain the data they desire:
. ACQ in LSA withBRIGHT=RETURN.
. ACQ/PEAKUP to center star in LSA or SSA, whichever is appropriate to the science
observations that follow.
. IMAGE, if you wish to verify target centering or to obtain an image of the object.
. Wavelength calibration exposure, if desired.
. ACCUMs at wavelengths of interest.NO GAP is recommended as a Special Require­
ment in order to ensure thePEAKUP operation is properly applied to the spectro­
scopic exposures.
. ACQ/PEAKUP in the SSA if the previous observation was in the LSA.
. ACCUMs for that different aperture, again withNO GAP.
. Repeat the above as needed for more stars.
Target acquisitions always take place with the LSA because the SSA is too small to
enable a field to be mapped effectively. Additional ACCUMs may be specified after the
first so as to obtain spectra at several wavelengths.
One task of the Instrument Scientists at STScI is to check the feasibility of successful
proposals after the Phase II proposals have been submitted.It is in your interest to
help us with that by using standard and consistent formats for the Phase II propos­
als because that can greatly reduce ambiguity about the intentions of the General
Observer. We suggest the following:
. Follow the examples in the next chapter. The examples usually specify all the
defaults for clarity, which is not strictly necessary, but, like comments in a computer
program, can help to confirm that a procedure conforms to the intents of the
observer.
. Although the submission software does not strictly require it, we ask that you give
the commands in the order just listed: First the acquisition, then embellishments to
the acquisition (PEAKUP, OFFSET, etc.), then anIMAGE and/or wavelength calibra­
tion exposure (if desired), and then the actual science observation (usually an
ACCUM).
CAUTION
The procedures for cr
eating a Phase II pr
oposal are being r
eviewed and
revised as this is written. W
e strongly r
ecommend that users check the
Phase II documentation carefully. W
e also r
ecommend checking on STEIS
at that time for a r
evised version of this Instrument Handbook.

GHRS Instrument Handbook 5.0 35
Your Phase II Proposal
. Add plenty of comment lines (RPSS imposes a limit) to explain what you want to
have happen so that we can see whether or not your proposal actually accomplishes
that. In particular, please explain briefly how the exposure time was determined.
4.1 Acquisitions
Most objects observed with the GHRS are point sources (stars), and the majority of the
remainder can be observed by first centering on a nearby point source and then offset­
ting to the object of interest. Point sources with accurate coordinates are very, very easy
to acquire with the GHRS: just specifyACQ with BRIGHT=RETURN to have the instru­
ment automatically center on the brightest object found within the LSA.
4.1.1 Initial Pointing
A blind pointing withHST is likely to place the object of interest within 2 arcsec of the
center of the aperture. That accuracy is limited in part by the quality of coordinates pro­
vided by users and partly by errors in the positions of the FGSs relative to the GHRS
apertures (see the FOS Instrument Handbook for a discussion of pointing errors). Using
J2000 coordinates tied to the GSC reference frame can help to reduce the possibility of
a failed acquisition. And don't forget to include proper motions if appropriate and to
check the equinox and epoch of astrometric quantities.
4.1.2 Interactive Acquisitions
We discuss interactive acquisitions (INT ACQ) first in the hope of dissuading you from
using that capability. An INT ACQ requires real­time contact between the ground and
HST. Real­time contact is a limited and expensive resource that should only be used as a
last resort. In almost all cases where an onboard acquisition will not work (because the
object is in a crowded field, or is variable, or is a moving target), it is sufficient to use
EARLY ACQ to get a WFPC2 or FOC image a few weeks in advance of the GHRS
observation. The image can then be analyzed to pinpoint the source to be observed with­
out requiring real­time contact. INT ACQ may be needed in a few instances where the
object changes in its ultraviolet brightness unpredictably. We suggest that you consult
with us before requestingINT ACQ.
If an interactive acquisition with the GHRS has been specified, a spiral search will be
run after HST makes its initial pointing. A map of the LSA is made at each dwell point
and each map is then downlinked to STScI in real time and may be viewed almost
immediately in OSS. After the spiral is complete, the telescope remains at its final dwell
point, awaiting instructions. The final 9 (or 25) maps are assembled into a mosaic and
displayed for the observer to identify the target, either from a cursor position or from
calculation of a centroid. The motion needed to center the specified position in the LSA
is computed and uplinked toHST. The recentering of the target usually takes place about
an orbit after the spiral search, and must be scheduled for a specific time. If you have so
requested in your proposal, an image of the LSA will be made after the recentering so
that you may confirm the position of the target (but additional interaction at that time is
not normally possible).

Your Phase II Proposal
36 GHRS Instrument Handbook 5.0
4.1.3 Early Acquisitions
In an early acquisition (EARLY ACQ), an image of the field of interest is obtained sev­
eral weeks (8 or more) in advance of the spectroscopic observation that the GHRS is to
make. The image may be obtained with WFPC2, with the FOC (especially if an ultravi­
olet image is desired), or with the imaging capability of the GHRS itself. The GHRS is
relatively slow at getting images, so if you wish to map an area much larger than about
2 â 2 arcsec we recommend that you consider WFPC2 or FOC. However, the GHRS has
the capability of obtaining a monochromatic map (see Section4.2.2 on page 43) in
IMAGE mode, which can be useful in some situations. For anyEARLY ACQ, be sure to
note the relationship of the image to the spectroscopic observations as a Special
Requirement (see the first example in the next chapter). Also, you should plan ahead so
that the early acquisition image can be analyzed quickly and the positions measured sent
back to STScI for incorporation into the telescope observing schedule.
4.1.4 Onboard Acquisitions
After the initial pointing, a GHRS onboard target acquisition begins with a spiral search
centered on the field of view. The motions are made by the telescope, and at each point
of the search either a single flux measurement (with 8 science diodes) or a map of the
LSA is made. The default is a pattern (SEARCH­SIZE=3) of maps in a square
4.6 arcsec on a side. Other options available are a pattern (SEARCH­SIZE=5),
7.7 arcsec on a side, or a single integration (
SEARCH­SIZE=1) that is 1.74 arcsec
square (this latter option can be useful for obtaining aMAP after an object is centered).
The telescope motions are made in thex and y coordinate system of the GHRS with a
step­size of 1.53 arcsec, not the U2, U3 system of the telescope.
For stars with good coordinates, the default ( ) acquisition strategy should suffice,
but the pattern usually costs little more in total time and guards against minor
coordinate uncertainties (the time needed increases in proportion to [
SEARCH­SIZE] 2 ,
but the STEP­TIME for an acquisition is usually so low that the total time involved is
small).
You should use ONBOARD ACQ whenever:
. The object is a point source, or
. The object can be reached by offsetting from a nearby object that meets the above
description, or
. An extended object is small enough that LOCATE=EXTENDED will work (see
Section 4.1.5.4 on page 39).
Also, the object to be centered should be the brightest object within the area searched
(about arcsec for a 3â 3 search) but with allowance for about 3 arcsec uncer­
tainty in positioning as well. In other words, you should ensure that your object is the
brightest one that HST will find within a box whose total size is about 8â 8 arcsec. Note
that the flux is measured in the ultraviolet (see Section7.4.1 on page 78).
The items you must specify for theONBOARD ACQ are:
. The mirror to use. N2 and A2 should suffice for virtually all targets you might wish
to observe. Mirrors N1 or A1 may also be used, especially for spectroscopic obser­
vations that utilize Side 1 (i.e., detector D1). Mirror N2 provides a flat reflectivity
3 3
â
5 5
â
3 3
â
5 5
â
4.8 4.8
â

GHRS Instrument Handbook 5.0 37
Your Phase II Proposal
over a broad range of ultraviolet wavelengths (see Section7.4.1 on page 78). Mirror
A2 has a similar spectrum response but reflects much less light than N2, in order to
acquire bright objects. Both detectors may be active at the same time, so it is permis­
sible to specify mirror N2 for an acquisition to observe with Side 1; this may be
desirable, for example, when observing cool stars. However, doing this may cost
you observing time; see below.
. BRIGHT and FAINT flux limits so the instrument knows when the object has been
found. However, in almost all cases it is better to useBRIGHT=RETURN, which is a
feature that automatically centers the brightest object found. IfBRIGHT=RETURN
is specified, any FAINT limit given is ignored.
. The size of the spiral search pattern to execute (
SEARCH­SIZE). The default is a
3 â 3 grid (which covers about 4.6 arcsec square), but you may also request a 5â 5
search over a square 7.7 arcsec on a side.
. Whether or not to record a map of the field at the search points so that you can con­
firm the telescope's pointing after the fact. AMAP is usually unnecessary and so
wastes spacecraft time. At most, aMAP=END­POINT should suffice. Note that such
a MAP occurs after the return to the brightest point in the field but before the object is
centered in the LSA by anACQ/PEAKUP. To determine the position of an object in
the LSA before spectroscopic observations are begun, we recommend obtaining an
IMAGE on a separate Exposure Logsheet line. TheMAP=ALL­POINTS option may
not be used with anONBOARD ACQ.
. The offset to apply once the object is centered (if appropriate).
4.1.4.1 Explicitly Specifying BRIGHT, FAINT, and STEP­TIME
Explicit BRIGHT and FAINT limits may be specified if you desire, although there is an
increased risk of a failed acquisition unless you are fairly confident of those fluxes.
Also, a few very bright stars cannot be acquired with Side 2 if an explicitBRIGHT
value is given but they can be acquired automatically withBRIGHT=RETURN.
Details on computing BRIGHT and FAINT limits are given in Section7.1 on page 68.
Please note that although we discourage the use of explicitBRIGHT and FAINT values
unless they are unavoidable, you still need to estimate the target acquisition count rate in
order to ensure that you choose the acquisition mirror correctly and that theSTEP­
TIME is determined properly.
. Note that using BRIGHT=RETURN and explicitly specifying BRIGHT and FAINT
limits result in fundamentally different acquisition procedures. If BRIGHT and
FAINT are specified, the acquisition stops as soon as those conditions are met and
the point at which that happened is moved to the center of the LSA. With
BRIGHT=RETURN, the entire spiral search region is sampled and the brightest
object in it determined before any movement is made to center on the target. Both
procedures require the same amount of telescope time because the schedule must
allow for the entire region to be sampled.
4.1.4.2 Peakups
After the initial acquisition, a peakup helps to precisely center the object in the aperture.
Specifying a ACQ/PEAKUP before starting LSA observations will help to ensure the

Your Phase II Proposal
38 GHRS Instrument Handbook 5.0
reliability of measured fluxes. A ACQ/PEAKUP before starting SSA observations is
vital for achieving the best throughput with the small aperture.
In the past we recommended using aSTEP­TIME value of 1.6 seconds for aACQ/
PEAKUP, but that is unnecessary for the post­Servicing Mission observatory. We recom­
mend aiming to achieve 1,000 to 10,000 counts in the peak step, as for the acquisition,
but levels as low as 100 will suffice for faint targets.
4.1.5 Special Onboard Strategies for Special Situations
4.1.5.1 Side 2 Acquisitions for Side 1 Science Observations
There are situations in which an object can be observed satisfactorily with Side 1 but for
which the count rates for acquisition mirrors N1 or A1 are extremely low. One possibil­
ity is to increase the exposure time for the acquisition, but the maximum permitted
STEP­TIME is 12.75 seconds. A better option may be to acquire with mirror N2. Both
detectors, D1 and D2, may be active in the GHRS at the same time, but there is an over­
head involved in making one primary and the other secondary; to go from Side 2 to Side
1, that time is approximately 40 minutes. Whether or not that is a ``cost'' or not to your
program depends on specific details. It is often the case that an acquisition takes place
over the first orbit, followed by science observations in later orbits. In that case, most or
all of the 40 minutes can take place during the part of the orbit when the target is inac­
cessible. But for CVZ viewing the cost can be real.
4.1.5.2 Complex Targets
Given the centering algorithm for the GHRS, which we will now describe, you can usu­
ally predict the results of an onboard target acquisition. Stepping, in both thex and y
directions, is done in 0.027 arcsec steps, and on a point source the centering is expected
to be good to within two steps. If the target is extended enough that the fluxes in the
areas which are compared do not change significantly when a step is made, the center­
ing accuracy will be degraded. An example is the case in which there is more than one
source of light within the LSA.
Consider, for instance, two stars which are separated by 1.0 arcsec and for which the
second star is 1 magnitude fainter than the primary star. Exact results will depend on the
position angle between the two stars. Thex balancing algorithm begins by placing the
brightest source on the fourth of the eight diodes that are used during an acquisition (the
LSA is imaged onto eight diodes), and moving until the flux on diodes 4 and 5 is bal­
anced. The second star would not affect this balance at all unless its light fell on one of
the same diodes as the primary star. In that case it would affect centering by a fraction of
a diode.
In the y direction the results are different. If the second star is ``above'' or ``below'' the
primary, it will ``pull'' the centering in that direction. In the case described, an extra
source of light 40% as bright as the primary would be present in the upper or lower half
of the LSA. The flux­balancing algorithm would divide the primary image 70­30, rather
than 50­50, with the image displaced towards the half of the LSA which did not contain
the second star. In this case the centering error should be less than 0.1 arcsec. If the LSA
acquisition were to be followed by a slew to the SSA,PEAKUP, and an observation, the
primary object should be successfully centered and observed. More complicated

GHRS Instrument Handbook 5.0 39
Your Phase II Proposal
images, or sources more similar in brightness may not be suitable for onboard acquisi­
tion. (Note that balancing in they direction is done before thex direction is balanced.)
4.1.5.3 Acquiring Faint Targets with the GHRS or FOS
Sometimes a star may be just plain faint to the point where geocoronal L
yman­a inter­
feres. Some guidance for when this may be a problem is provided in Section7.4.2 on
page 80. If it is, we recommend that you specifyDARK TIME as a special requirement
on the acquisition line on your Phase II form. Doing so constrains the scheduling of
your proposal and is likely to result in greater resource charges to you, soDARK TIME
should only be requested when it is necessary.
Another way to acquire very faint targets reliably is to use the Faint Object Spec­
trograph. This can be especially useful for acquiring extragalactic objects to observe
with grating G140L because the acquisition mirrors for Side 1 of the GHRS reflect only
far­ultraviolet light and because the maximum permissible integration time per dwell
point is only 12.75 seconds. FOS­assisted acquisitions for the GHRS will be tested in
Cycle 4, so we suggest that you consult us if you wish to explore this option.
4.1.5.4 Acquiring Extended Sources with the GHRS
There are three classes of extended sources we can consider:
. Objects larger than the LSA that have roughly uniform surface brightness.
. Objects smaller than the LSA with roughly uniform surface brightness.
. Objects with significant structure, some of which is on scales smaller than the LSA.
The first class might be typified by Jupiter, and such objects are impossible to acquire
directly with the GHRS because there is no clear photometric ``center'' to align on. In
such cases it is necessary to offset from a smaller object which can be centered.
The second class of objects includes the Gallilean satellites of Jupiter, and it is these for
which the LOCATE=EXTENDED acquisition option was written. In a normalLOCATE,
the object to be observed is moved in thex direction until the signal seen by the center
two diodes (of the eight onto which the LSA is imaged) is balanced.
LOCATE=EXTENDED in ACQUISITION mode balances the four left diodes against
the four right­hand ones to roughly center an object. InACQ/PEAKUP mode, the
EXTENDED option allows you to specify that the balancing be done over the central
four, six, or eight diodes (specified asEXTENDED=2, 3, or 4).
The third class of objects can be the most problematic, especially if the target is an
extragalactic one at high latitude. In such cases there may be no nearby star from which
you could offset, but the source itself often contains point­like sources that can be cen­
tered on; in these cases an early acquisition or a pre­existing image is invaluable. The
problem is then one of predicting acquisition count rates; that is treated in Section7.1
on page 68. You may also wish to consider an acquisition with the FOS, as described in
the previous section.
4.1.5.5 Offsetting
Even if an ONBOARD ACQuisition will not work for your target, it may still be possible
to acquire a nearby reference star and to then offset to your target. Such an offset will

Your Phase II Proposal
40 GHRS Instrument Handbook 5.0
happen automatically if the coordinates given for an acquisition exposure are different
from those given for the science exposure. You would normally use two or three lines
on the Phase II Exposure Logsheet to achieve this: acquisition of a reference star, offset,
peakup on the target (if desired), and a science observation. The first line would request
an onboard acquisition of the reference star. It should specify ONBOARD ACQ FOR
. Line 2 should then be anACQ/PEAKUP, and it should specify ONBOARD
ACQ FOR . The next line would specify an offset to move from the reference
star to the target, and the final line should be your intended science observation.
You must, of course, include the reference star as one of the objects on your T
arget
Logsheet. It should be designatedxxx­OFFSET, where xxx is the name of the target
object. If desired, you may give the position of your target by using RA­OFF, DEC­
OFF, or XI­OFF, ETA­OFF and FROM relative to the offset star. See the Proposal
Instructions for details and notes on proper units. On the Exposure Logsheet, the T
arget
Name for lines 1 and 2 arexxx­OFFSET, and in the example above, the Target Name
for line 3 is xxx.
To make a successful offset, the relative positions of the offset star and target must be
very well known -- about as well as 1/4 the size of the aperture. (e.g., rms errors of 0.05
arcsec for the SSA.) One way of obtaining such positions is by requesting anEARLY
ACQuisition WFPC2 image, and measuring relative positions from it (at least 2 months
prior to the science observation). The offset positioning accuracy of theHST is expected
to be very good (of the order of 0.03 to 0.05 arcsec for a 30 arcsec offset), and the accu­
racy of the placement will be primarily determined by the accuracy of your positions.
An offset of more than 30 arcsec may require the telescope to acquire new guide stars,
which would worsen the accuracy of the positioning.
4.1.6 MAPs
The GHRS has the ability to make aMAP of the LSA by raster scanning one or both of
its small focus diodes over the aperture. You may, for example, want a map to confirm
the pointing at the time your spectrum was taken. The default forONBOARD ACQuisi­
tions is to make no map. If you ask forMAP=END­POINT, you will get a map after the
spiral search has found your target, but before it has been centered (withLOCATE) in
the LSA. If you want a map after the final centering, you can add a single Exposure
Logsheet line in IMAGE mode. AnIMAGE may also be obtained of the SSA, which can
be a useful a posteriori means of determining what was observed in a crowded field.
The MAP=ALL­POINTS option may not be used with an onboard acquisition.
4.1.7 Acquisition Parameters --- A Summary
Step 1: Mode=ACQ
. Aperture is always LSA (``2.0'').
. MIRROR is usually N2 or N1 unless object is too bright (then useA2 or A1; see
Section 7.1 on page 68). Mirrors A1 and N1 may also be used and it is permissible
to acquire with one side (mirrorN2, say) and observe with the other (grating G140L,
perhaps), but with a possible cost in time.
. SEARCH­SIZE=3 is the default and adequate almost all the time. Values of 1 or 5
may also be used.

GHRS Instrument Handbook 5.0 41
Your Phase II Proposal
. BRIGHT=RETURN is the default for finding the target and should be used unless
you are forced not to. Do not specifyFAINT unless you must specify an explicit
BRIGHT limit. (FAINT is ignored if BRIGHT=RETURN is used.)
. LOCATE: Default is YES for an ONBOARD ACQ and NO for EARLY ACQ or INT
ACQ. We recommend these defaults. Note that LOCATE=EXTENDED is now avail­
able. With an ONBOARD ACQ, LOCATE=NO may be used only ifMAP=END­POINT
is specified.
. MAP: The defaults provide for an image to be transmitted to the ground ifINT ACQ
or EARLY ACQ is specified. No image is generated by default for anONBOARD ACQ;
MAP=END­POINT will provide one with the target in the LSA, but it will not be
centered. As we have pointed out, if you wish to determine the actual position of the
object in the LSA before spectroscopic observations are begun, you should obtain an
IMAGE as a separate Exposure Logsheet line and you should not specify aMAP at
all. MAP=ALL­POINTS may not be used with anONBOARD ACQ.
. The time per exposure can be calculated from
where (i.e., 1, 9, or 25), and is the number
of dwell points mapped (=1 if MAP=END­POINT is chosen and = if
MAP=ALL­POINTS. MAP=ALL­POINTS can only be used withINT ACQ and
EARLY ACQ.). Please note the value ofSTEP­TIME you want as aCOMMENT on
the Exposure Logsheet.
. Special Requirements are INT ACQ, EARLY ACQ, or ONBOARD ACQ.
Step 2: Mode=ACQ/PEAKUP
. The aperture can be either the LSA (``2.0'') or SSA (``0.25''); specify the one to be
used for the science observations that immediately follow.
. Specify the MIRROR as for Mode=ACQ; i.e., N1, A1, N2, or A2 depending on target
brightness.
. The time per exposure can be calculated from
where f Aperture = 102 if the LSA is used and = (
SEARCH­SIZE) 2 if the SSA is
used. Note that the throughput of the SSA is half to 2/3 that of the LSA so that in
general you should specify aSTEP­TIME that is larger than the one you used for a
PEAKUP in the LSA.
. We urge you to be precise and explicit about the way in which you specify anACQ/
PEAKUP and the order in which observations are to be made. The defaults that apply
to ACQ and ACQ/PEAKUP modes will usually accomplish what you wish, but the
way to be sure is to specify the details. Confusion can arise particularly when a pro­
gram mixes LSA and SSA observations. We would recommend that you do anACQ
in the first line of the Exposure Logsheet, then on line 2 specifyACQ/PEAKUP and
indicate the lines to which it applies (all of which should use the same aperture).
t exp 128 N MAP
â N SEARCH
+
( ) STEP­TIME
â
=
N SEARCH SEARCH­SIZE
( ) 2
= N MAP
N SEARCH
t exp f Aperture STEP­TIME
â
=

Your Phase II Proposal
42 GHRS Instrument Handbook 5.0
Also indicate a NO GAP Special Requirement for that group of lines. Then specify
another ACQ/PEAKUP before starting observations in the other aperture, and again
specify NO GAP to ensure that they are treated as a group.
4.2 Image Mode
The GHRS is, of course, primarily a spectrograph, but it includes useful imaging capa­
bilities, especially because the detectors of the GHRS are blind to much of the visible
light that dominates the flux of most stars. You may wish to request anIMAGE or MAP,
for example, to confirm that the telescope had your object properly centered in the data­
taking aperture before the exposure was taken.
Note the following in using the imaging capability:
. GHRS IMAGEs and MAPs are obtained with the focus diodes (see Section6.3 on
page 62) at the ends of the array of main science diodes. The focus diodes are
smaller and square, making them more useful for focusing, but at the price of a
lower count rate. The total count rate over the LSA is, of course, unchanged, and it is
that which is predicted with the information in Section7.1 on page 68. Multiply the
count rate estimated for the regular diodes by approximately 0.3 to get the value
appropriate to the focus diodes when they are centered on the star.
. A MAP is obtained as an integral part of an acquisition whereas anIMAGE is a sepa­
rate observation that may or may not have anything to do with an acquisition. AMAP
with SEARCH­SIZE=3 or 5 is made as the acquisition procedure causes the tele­
scope to make small motions in a square spiral pattern, thereby enabling it to record
a larger portion of the sky than the LSA itself subtends. AnIMAGE can only record
the light in the arcsec region of the LSA. A singleMAP (SEARCH­
SIZE=1) is equivalent to an IMAGE. Note that MAP=ALL­POINTS may not be
used with an ONBOARD ACQ.
. A standard IMAGE will have a pixel spacing of 0.109 arcsec and will cover the
entire LSA aperture of 1.74 â 1.74 arcsec. You may also select pixel spacings of
0.055 or 0.027 arcsec, with proportionately smaller regions of the sky covered in
a (the default) IMAGE. You may also useIMAGE with the SSA.
4.2.1 Image Mode Parameters
. Either the LSA (``2.0'') or SSA (``0.25'') may be selected as the aperture. The SSA
is so small that it is generally pointless to image it, although there may be special
cases where IMAGE mode is of use, particularly for confirming pointing in a
crowded field.
. A mirror is the usual choice as optical element. A grating may also be specified -- see
below.
. The number of pixels in the x and y directions can be chosen separately and can
range from 1 to 512 pixels. However, a large number of pixels only oversamples the
region of sky subtended by the LSA and does not make theIMAGE include a larger
area. The parameters to specify areNX, NY, DELTA­X, and DELTA­Y, for which the
defaults are 16, 16, 4, and 4, respectively. The product ofNX and NY may not exceed
512. An image that is critically sampled in thex direction may be obtained by speci­
fying NX=32, NY=16, DELTA­X=2, and DELTA­Y=4.
1.74 1.74
â
16 16
â

GHRS Instrument Handbook 5.0 43
Your Phase II Proposal
Only the N1 mirror intercepts the full beam diameter, meaning that images of the
LSA with the other acquisition mirrors will not yield an accurate Point Spread Func­
tion (PSF).
. The PRECISION parameter may be specified asNORMAL (the default) or HIGH.
PRECISION may only be specified if DELTA­Y=4. Using PRECISION=HIGH
causes the image to be obtained with only one focus diode instead of two (thereby
eliminating uncertainty over the relative response of the two), but the time per expo­
sure you must list is the same in either case (but usingPRECISION=NORMAL takes
less actual time to execute and the differences in response of the diodes are known to
be small).
. There are two focus diodes available to raster over the LSA. Thus the total time
needed is the dwell time per pixel (0.2 seconds is the default) times the number of
pixels in the x direction (default is NX=16) times the number ofy pixels (default is
NY=16), all divided by 2. The maximum permissible dwell time per pixel is 12.75
seconds. (Note that the Phase II Proposal Instructions for Cycle 4 required you to
calculate the total time without dividing by the factor of two. The situation for
Cycle 5 should be confirmed before a value is entered.)
4.2.2 The GHRS as a Slitless Spectrograph
In IMAGE mode you may specify a grating instead of a mirror as the spectrum element
(note that this may not be done in Acquisition Mode). Doing so for a target that emits
primarily in lines can yield the equivalent of using a slitless spectrograph over a very
small portion of the sky (the 1.74 arcsec square region of the LSA). Thus the focus
diodes would be swept over the image of the line to produce a picture that is resolved
spatially in the y direction and spectroscopically in thex direction. This mode of use
would be very slow if all you wanted was the spatial structure of a small object (the
FOC would probably be better), but there might be interesting uses for obtaining spec­
trophotometrically pure, spatially resolved images in the ultraviolet. Please consult us if
you wish to explore this option.
4.3 Accumulation Mode
4.3.1 Optimizing Data Quality
The previous chapter provided the information needed to estimate an exposure time to
achieve a given level of signal­to­noise. We reiterate several factors having to do with
the detectors that must be taken into account to achieve the best data quality. Note that it
is not necessary to explicitly specify these parameters (except forFP­SPLIT) because
the defaults that apply to each mode of operation will automatically invoke them. More­
over, you should not deviate from the defaults without good reason.
The Digicon detectors have faceplates with some granularity (uneven response). The
diodes onto which the faceplate is imaged also have response irregularities and some of
them have been turned off because of misbehavior. Both of these effects are relatively
small but enough to prevent you from obtaining a spectrum with S/N much in excess of
50. They can also produce ``glitches'' that can mimic spectrum features. TheFP­
SPLIT parameter causes the carrousel to move slightly between each of the two or four

Your Phase II Proposal
44 GHRS Instrument Handbook 5.0
separate subexposures. The COMB parameter suppresses diode­to­diode gain variations
and allows one to work around the dead diodes. Both features should be used, especially
since they cost little or nothing in exposure time and improve data quality.
The Digicon diodes also undersample the spectrum by about a factor of two. The
parameter STEP­PATT causes electronic motions of the spectrum so as to sample the
spectrum fully. It is possible to STEP­PATT at two samples per diode width, but we
recommend using four samples per diode to yield optimum results, and again at no net
cost. You can always rebin a quarter­stepped spectrum into a half­stepped one during
your data analysis, but the process cannot make a quarter­stepped spectrum out of a
half­stepped one. Deconvolution has worked best with quarter­stepped spectra (the
default); see Gilliland et al. (1992).STEP­PATT also determines the way in which the
background is measured (see Section8.4 on page 92).
We also remind you to break up long exposures into subexposures that are no longer
than about 5 minutes each, so as to defeat the effects of geomagnetically­induced image
motion. Bear in mind that a 20 minute exposure, for example, specified withFP­
SPLIT=4 will result in four 5­minute exposures.
4.3.2 Summary of Accumulation Mode Parameters
. Specify the aperture as ``2.0'' (LSA) or ``0.25'' (SSA). The object will automati­
cally be moved to the correct aperture even if the acquisition was into the other. If a
SSA spectroscopic observation follows an LSA spectroscopic observation, we rec­
ommend an ACQ/PEAKUP in the SSA withSEARCH­SIZE=5 before beginning an
ACCUM.
. If wavelength accuracy is needed that exceeds the default (see Section4.5 on
page 46), then specify WAVE as the target with an aperture of SC2. Get a WAVE
before the ACCUM to which it is to apply.
. Specify the grating to be used, either first­order or echelle. If you wish to force an
echelle observation to be done in an order other than the default, you may do so by
specifying the grating as, for example,ECH­B24, where 24 was the order chosen.
. STEP­PATT may be chosen as a number from 1 to 15, and specific pattern numbers
go with specific spectrograph configurations. We recommend using the default that
pertains to the setup you have chosen. The details of how the substepping is per­
formed and the background measured are given in Section8.4 on page 92.
. FP­SPLIT=STD is recommended. The default forFP­SPLIT is NO, which will
not yield a spectrum with the best signal­to­noise.
. COMB=FOUR is the default value and is recommended for the best results.
. DOPPLER=DEF is recommended. This activates compensation for the velocity
shifts of astronomical spectra over the course of an orbit but turns it off for internal
exposures.
. STEP­TIME may be specified as a number from 0.2 to 12.75 seconds, in incre­
ments of 0.05 seconds. STEP­TIME specifies the length of the individual subspec­
tra that are accumulated to form the final spectrum, and there is no good reason to
not use the default of 0.2 sec.
. The CENSOR parameter may also be specified. The default isNO, which is appropri­
ate in almost all cases. IfCENSOR=YES is used, individual subspectra (of duration

GHRS Instrument Handbook 5.0 45
Your Phase II Proposal
STEP­TIME, which should be used at the default value of 0.2s) are examined
onboard the spacecraft and are discarded if multiple counts have occurred within a
8 µs interval. This allows for the lowering of background noise in cases where the
object being observed is very faint, i.e., less than about 0.1 counts per second per
diode. Rejected exposures are repeated by the GHRS, leading to a longer total
elapsed time for the observation, but only by about 2%. Since the observation must
end at a specific time that is predetermined in the spacecraft schedule, usingCEN­
SOR involves a risk of losing all the data if too many subexposures are rejected. This
is guarded against by adding some padding time in the observation planning process
(that is done at STScI and does not affect the exposure time you estimate). See
Section 8.6.2 on page 95 for more information onCENSOR.
. A special commanding option called FLYLIM can also be used to reject noise in
cases where the object is substantially fainter than the background. Please consult
with us if you wish to explore this possibility.
. If you have any doubts about the manner in which your program will be executed
(which spectra first, whether a peakup is done, etc.), remove the ambiguity by
explicitly indicating the nature and order of the exposures on the Exposure
Logsheet.
4.3.3 WSCAN mode
Use of WSCAN can result in a spectrum covering a broader total bandpass than is possi­
ble with a single exposure. All the parameters listed above for anACCUM exposure are
available in WSCAN mode. The most important parameter to specify isWAVE­STEP,
which is the spacing (in ångstroms) between each subexposure. If WAVE­STEP=DEF
is specified, the central wavelengths of the separate exposures will be equally spaced so
as to cover the range of wavelengths that you specify, with at least 20% overlap from
one subspectrum to the next.
You may also explicitly give aWAVE­STEP value. If is the central wavelength of
the shortest­wavelength exposure, and is the central wavelength of the longest­
wavelength exposure, then choose these values in concert withWAVE­STEP so as to
yield an integral number ofWAVE­STEPs between and .
4.3.4 OSCAN mode
This mode makes it possible to scan across echelle orders at a fixed value ofml, where
m is the order number andl is the wavelength. It is rare that adjacent orders both have
features of astrophysical interest and so this mode is primarily used for calibrations and
not for science observations. If you do use this mode, all the parameters of anACCUM
observation are available.
4.4 Rapid Readout Mode
This mode is sometimes referred to as Direct Downlink. A normalACCUM exposure is
the best way to get a good spectrum because all the features of the spectrograph are
available to you: automatic compensation for the motion of the spacecraft along the
line­of­sight, rejection of high­noise subspectra with CENSOR, use of FP­SPLIT,
l min
l max
l min l max

Your Phase II Proposal
46 GHRS Instrument Handbook 5.0
COMB, and STEP­PATT to optimize data quality, and so on. However, there are times
when ACCUM cannot obtain successive spectra as quickly as is needed to probe a partic­
ular phenomenon.
In those cases you can useRAPID mode. The data are read from the detector at the end
of each short integration, either to the science tape recorder onHST or through TDRSS
to the ground. Data obtained inRAPID mode require special handling by the observer
to correct for some of the effects (especially doppler shifts) that are automatically com­
pensated for in ACCUM mode.
As for anACCUM, you should specify the science aperture and the spectral element. You
may also choose to observe WAVE as target to get a wavelength calibration. The only
other parameter you may specify isSAMPLE­TIME, which is the length of each sepa­
rate exposure that is read to the ground. The defaultSAMPLE­TIME is also the mini­
mum, 0.05 seconds. SAMPLE­TIME may be incremented in 0.05 second values up to a
maximum of 12.75 seconds. Use of a very shortSAMPLE­TIME and/or use of RAPID
mode for extended periods can cause scheduling problems because of the very high data
volumes that are generated. In particular, a SAMPLE­TIME of less than 0.33 sec
records data at the 1 Mb rate and so can proceed for no more than about 20 minutes
before filling the onboard tape recorder. A SAMPLE­TIME of 0.33 seconds or more, but
less than 2.57 seconds, results in a 32k data rate, while aSAMPLE­TIME in excess of
2.57 seconds results in a 4k data rate. This latter low rate can be sustained almost indef­
initely.
4.5 The Precision and Accuracy of Standard Calibrations
Our job of calibrating the GHRS on a routine basis ensures that you can rely on the
wavelength scale and flux calibration that you are provided. The quality of the flux cali­
bration is limited primarily by innate factors that are present in tying together different
photometric systems. The GHRS has proved to be a reliable instrument whose response
has not been seen to vary with time. Routine calibrations will deliver absolute fluxes
accurate to 10% and photometrically precise to better than 1% with the LSA 1 . In other
words, we monitor the sensitivity of the GHRS on a regular basis (approximately every
four months) and have not seen changes in the count rates for the standard stars that
exceed 1%, once the effects of telescope focus are taken into account. However, there
are undoubtedly systematic effects present that preclude knowing absolute fluxes to bet­
ter than 5 to 10%. The solid performance of the GHRS means that it is impossible to
improve substantially on the flux­calibration by obtaining observations on your own.
The GHRS dark count is so low for most objects as to be irrelevant (but see the discus­
sion on CENSOR in Section 8.6.2 on page 95) and high signal­to­noise can be obtained
without the need for flat­field exposures.
The one area in which you might wish to consider a special calibration is for the wave­
length scale. However, even here the default wavelength scale provided by the pipeline
1. Starting in Cycle 4, the fluxes from the data reduction pipeline in the ultraviolet can differ sys­
tematically from earlier values by up to 1% because of the use of models of the white dwarf
G191B2B as the fundamental standard.

GHRS Instrument Handbook 5.0 47
Your Phase II Proposal
data reduction system has been improved to take account of several systematic effects.
As a result those wavelengths are good to better than 1 km s
­1 for the first­order grat­
ings, except at the shortest wavelengths (Ly­a). For Echelle­B, the rms scatter in fitted
wavelengths is about 0.6 km s ­1 . These values are uncertainties for the wavelength zero
point of a spectrum; the dispersion of spectra differ negligibly from the routine values.
Our specifications for routine wavelength calibration are to have them good to only
about one diode rms, estimated as follows:
. First, errors in carrousel positioning can amount to 0.2 diode.
. Second, if a wavelength calibration is not available for the precise carrousel position
you select then it is necessary to interpolate in the dispersion constants, and that can
lead to an error of 0.5 diode.
. Third, changing temperatures within the spectrograph can lead to wavelength shifts
of about 0.5 diode over the course of an orbit.
. Finally, geomagnetically­induced image motion can lead to oscillations of up to 1
diode peak­to­peak amplitude over half an orbit.
. The net result is that about 1 1/4 diode accuracy is what is routinely expected.
Precision of about 0.2 diode can be achieved by requesting that anACCUM with a TAR­
GET of WAVE be made immediatelybefore your science exposure. Nearly the same pre­
cision can be realized by using information in the SPYBAL exposures that are
automatically taken at the start of a sequence of observations with a new grating. Also,
you should specify NO GAP for the exposure lines to which theWAVE pertains. See
Chapter 5 for an example. Bear in mind that the several separate exposures you list may
not necessarily be obtained one right after the other (unless you so specify in Phase II),
so that separate WAVE calibrations may be needed.
We also recommend breaking long exposures into subexposures that are no longer than
about 5 minutes each. This is done to ensure that geomagnetically­induced image
motion will not degrade the quality of your data. Any shifts in the different spectra of
the same object can generally be determined by cross­correlating them during your data
analysis.
4.6 Other Considerations
There are many factors that may influence how you specify the manner in which your
observations should be obtained. Here we mention two that have arisen in particular
instances. The first has to do with Targets of Opportunity (TOOs) and/or coordinated
observations. The Call for Proposals should be consulted for information about propos­
ing to observe Targets of Opportunity with HST and GHRS. Observations of TOOs
often need to be coordinated with other satellites or ground­based observatories. The
long lead times for planning HST observations, even for TOOs, are an impediment to
that coordination. We encourage you to explain in detail exactly what is or is not
required for the successful completion of your program.
Another problem that can occur arises when a science program specifies a large number
of separate GHRS exposures. The problem is caused by the relatively small amount of
memory available on HST in which to store GHRS commands. It is usually possible to

Your Phase II Proposal
48 GHRS Instrument Handbook 5.0
break up such a program so that the separate exposures are not all together, but occa­
sionally the science goals cannot allow that and some other compromise must be made.
Roughly speaking, about 40 total spectra can be scheduled in a single block (aWSCAN
with n set­points counts asn exposures and anFP­SPLIT counts as 2 or 4). Once that
number is exceeded the remaining observations must be scheduled in a new block of
time, and that means a new target acquisition will be needed, with the concomitant over­
head time.

GHRS Instrument Handbook 5.0 49
Chapter 5 Phase II Pr
oposal
Examples

Phase II Proposal Examples
50 GHRS Instrument Handbook 5.0
We present in this chapter several examples of how to actually execute proposals to
achieve what you want, since abstract instructions are, at best, difficult to follow. The
examples are shown separately, partly for formatting reasons, since they require a side­
ways orientation of the page, and partly to assemble them in one place for easy refer­
ence.
We begin by showing how the first entry on the next page would look as input to RPSS.
The remainder of this chapter shows how Phase II proposals look to us. In most of these
examples, the various parameters have been explicitly listed for clarity, even though
they often correspond to the defaults that would apply anyway.
We remind you that any if differences exist between this document and, say, the Phase II
Proposal Instructions, the one with the most recent date of issue should be followed or
you should consult us to resolve the discrepancy.
exposure_logsheet:
linenum: 1.0
targname: MU­COL
config: HRS
opmode: ACQ
aperture 2.0
sp_element: MIRROR­A2
num_exp: 1
time_per_exp: 10.75M
fluxnum_1: 1
priority: 1
param_1: SEARCH­SIZE=5,
param_2: MAP=ALL­POINTS
req_1: CYCLE 5/ 1­2;
req_2: EARLY ACQ FOR 2.0
comment_1: STEP­TIME=0.2S;
comment_2: EXPECT 21000 COUNTS IN STEP­TIME
!
linenum: 2.0
targname: MU­COL
config: HRS
opmode: ACCUM
aperture: 2.0
sp_element: ECH­B
num_exp: 1
time_per_exp: 4.8S
fluxnum_1: 1
priority: 1
param_1: STEP­PATT=13

Phase II Proposal Examples
GHRS Instrument Handbook 5.0 51
1.
Example
of
an
observation
that
specifies
an
early
acquisition.
Note
that
the
TIME
in
column
(1 1)
is
0.2
sec
(the
STEP­TIME)
times
(128
x
25)
+
25,
in
accord
with
the
formula
in
Section
4.1.13
.
Note
also
that
the
expected
number
of
counts
for
the
acquisition
is
mentioned.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|10
|
11
|
12
|13
|14|
15
LN
|
SEQ
|
TARGET
|INSTR
|
OPER.
|
APER
|SPECTRAL|CENTRAL|
OPTIONAL
|NUM|
TIME
|
S/N
|FLX|PR|
SPECIAL
NM
|
NAME
|
NAME
|CONFIG|
MODE
|OR
FOV
|ELEMENT
|WAVELN
|
PARAMETERS
|EXP|
|
|REF|
|
REQUIREMENTS
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1.0
MU­COL
HRS
ACQ
2.0
MIRROR­
SEARCH­SIZE=5,
1
10.75M
1
1
CYCLE
5/
1­2;
A2
MAP=ALL­POINTS
EARLY
ACQ
FOR
2.0
COMMENTS:
STEP­TIME=0.2S;
EXPECT
21000
COUNTS
IN
STEP­TIME
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 2.0
MU­COL
HRS
ACCUM
2.0
ECH­B
1600
STEP­PATT=13
1
4.8S
1
1
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 2.
The
same
observation
as
above
specified
as
an
interactive
acquisition. ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|10
|
11
|
12
|13
|14|
15
LN
|
SEQ
|
TARGET
|INSTR
|
OPER.
|
APER
|SPECTRAL|CENTRAL|
OPTIONAL
|NUM|
TIME
|
S/N
|FLX|PR|
SPECIAL
NM
|
NAME
|
NAME
|CONFIG|
MODE
|OR
FOV
|ELEMENT
|WAVELN
|
PARAMETERS
|EXP|
|
|REF|
|
REQUIREMENTS
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 10.0
MU­COL
HRS
ACQ
2.0
MIRROR­
SEARCH­SIZE=5,
1
10.75M
1
1
CYCLE
5/
10­20;
00
A2
MAP=ALL­POINT
INT
ACQ
FOR
20;
S
COMMENTS:
STEP­TIME=0.2S;
EXPECT
21000
COUNTS
IN
STEP­TIME
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 20.0
MU­COL
HRS
ACCUM
2.0
ECH­B
1600
STEP­PATT=13
1
4.8S
1
1
00
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­

Phase II Proposal Examples
52 GHRS Instrument Handbook 5.0
3.
A
GHRS
acquisition
specified
with
explicit
BRIGHT
and
F
AINT
limits.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|10
|
11
|
12
|13
|14|
15
LN
|
SEQ
|
TARGET
|INSTR
|
OPER.
|
APER
|SPECTRAL|CENTRAL|
OPTIONAL
|NUM|
TIME
|
S/N
|FLX|PR|
SPECIAL
NM
|
NAME
|
NAME
|CONFIG|
MODE
|OR
FOV
|ELEMENT
|WAVELN
|
PARAMETERS
|EXP|
|
|REF|
|
REQUIREMENTS
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 100.
BD28D4211
HRS
ACQ
2.0
MIRROR­
BRIGHT=65535,
1
9S
1
1
ONBOARD
ACQ
FOR
2
000
A2
FAINT=700
00;
COMMENTS:
STEP_TIME=1.0S;
EXPECT
1800
COUNTS
IN
STEP­TIME;
FAINT
SET
TO
APPROX.
40%
OF
EXPECTED
COUNTS.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 4.
Example
of
an
acquisition
with
an
offset.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|10
|
11
|
12
|13
|14|
15
LN
|
SEQ
|
TARGET
|INSTR
|
OPER.
|
APER
|SPECTRAL|CENTRAL|
OPTIONAL
|NUM|
TIME
|
S/N
|FLX|PR|
SPECIAL
NM
|
NAME
|
NAME
|CONFIG|
MODE
|OR
FOV
|ELEMENT
|WAVELN
|
PARAMETERS
|EXP|
|
|REF|
|
REQUIREMENTS
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 900.
HII­POS1
HRS
ACQ
2.0
MIRROR­
BRIGHT=RETURN
1
1.8S
1
1
ONBOARD
ACQ
FOR
9
000
N2
10
COMMENTS:
STEP­TIME=0.2S
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 910.
HII­POS2
HRS
ACCUM
2.0
G160M
1500
1
272S
50
1
1
000
COMMENTS:
RA­OFF
=
+0.227S
+/­
0.001S,
DEC­OFF
=
+6.0''
+/­
0.01'',
FROM
HII­POS1
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­

Phase II Proposal Examples
GHRS Instrument Handbook 5.0 53
5.
A
fairly
typical
GHRS
observation,
starting
with
an
acquisition
using
BRIGHT=RETURN
and
proceeding
to
several
spectroscopic
exposures
that
use
alternative
variants
of
ACCUM
specifications.
Not
all
of
these
would
be
used
in
one
program.
Note
the
IMAGE
(line
300)
that
is
obtained
to
later
confirm
centering
of
the
target
in
the
LSA.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|10
|
11
|
12
|13
|14|
15
LN
|
SEQ
|
TARGET
|INSTR
|
OPER.
|
APER
|SPECTRAL|CENTRAL|
OPTIONAL
|NUM|
TIME
|
S/N
|FLX|PR|
SPECIAL
NM
|
NAME
|
NAME
|CONFIG|
MODE
|OR
FOV
|ELEMENT
|WAVELN
|
PARAMETERS
|EXP|
|
|REF|
|
REQUIREMENTS
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 100.
BD28D4211
HRS
ACQ
2.0
MIRROR­
BRIGHT=RETURN,
1
9S
1
1
ONBOARD
ACQ
FOR
2
000
A2
00;
COMMENTS:
STEP_TIME=1.0S;
EXPECT
1800
COUNTS
IN
STEP­TIME.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 200.
BD28D4211
HRS
ACQ/PE
2.0
MIRROR­
1
102S
1
1
ONBOARD
ACQ
FOR
3
000
AKUP
A2
00­800;
COMMENTS:
STEP_TIME=1.0S
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 300.
BD28D4211
HRS
IMAGE
2.0
MIRROR­
NX=16,
NY=16
1
256.0S
1
1
000
A2
COMMENTS:
STEP­TIME=1.0S
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 400.
BD28D4211
HRS
ACCUM
2.0
G160M
1500
STEP­PATT=5,
S
1
217.6S
50
1
1
000
TEP­TIME=0.2, FP­SPLIT=NO,
C
OOMB=FOUR
COMMENTS:
EXPOSURE
TIME
IS
A
INTEGRAL
NUMBER
OF
MINIMUM
EXPOSURE
TIME
(27.2S
IN
THIS
CASE);
S/N
IS
PER
DIODE;
STEP­PATT
WILL
RESULT
IN
4
PIXELS/DIODE.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 450.
BD28D4211
HRS
ACCUM
2.0
G160M
1500
STEP­PATT=5,
S
1
217.6S
50
1
1
000
TEP­TIME=0.2, FP­SPLIT=FOUR, COMB=FOUR
COMMENTS:
FP­SPLIT
WILL
PRODUCE
4
SPECTRA
EACH
WITH
54.4S
EXPOSURE
TIME.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 475.
BD28D4211
HRS
ACCUM
2.0
G160M
1500
10
217.6S
50
1
1
000
COMMENTS:
TOTAL
EXPOSURE
=
2176S­­BREAK
UP
LONG
EXPOSURE
TO
AVOID
DRIFT
DUE
TO
GIMP.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­

Phase II Proposal Examples
54 GHRS Instrument Handbook 5.0
6.
A
sequence
of
exposures
that
could
be
added
to
Example
5
if
additional
SSA
observations
were
desired.
The
ACCUM,
RAPID,
and
WSCAN
lines
illustrate
a
range
of
options.
Note
the
ACQ/PEAKUP
with
the
SSA
(line
550)
to
move
the
star
to
that
aperture.
If
line
550
were
absent
the
star
would
still
be
moved
to
the
SSA,
but
it
would
not
be
as
well
centered.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|10
|
11
|
12
|13
|14|
15
LN
|
SEQ
|
TARGET
|INSTR
|
OPER.
|
APER
|SPECTRAL|CENTRAL|
OPTIONAL
|NUM|
TIME
|
S/N
|FLX|PR|
SPECIAL
NM
|
NAME
|
NAME
|CONFIG|
MODE
|OR
FOV
|ELEMENT
|WAVELN
|
PARAMETERS
|EXP|
|
|REF|
|
REQUIREMENTS
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 550.
BD28D4211
HRS
ACQ/PE
0.25
MIRROR­
SEARCH­SIZE=5
1
25S
1
1
ONBOARD
ACQ
FOR
AKUP
A2
600.00;
COMMENTS:
STEP_TIME=1.0S
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 575.
WAVE
HRS
ACCUM
SC2
G160M
1500
1
60S
1
1
CALIB
FOR
525;
000
SEQ
500­600
NO
GAP
COMMENTS:
STANDARD
SPECTRAL
CAL
LAMP
SPECTRUM.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 600.
BD28D4211
HRS
ACCUM
0.25
G160M
1500
STEP­PATT=5,
S
1
326.4S
50
1
1
000
TEP­TIME=0.2, FP­SPLIT=NO,
C
OMB=FOUR
COMMENTS:
THROUGHPUT
OF
SSA
APPROX.
0.67
OF
LSA
AT
1500A.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 700.
BD28D4211
HRS
RAPID
0.25
G160M
1500
SAMPLE­TIME=1.
1
20M
25
1
1
000
0
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 800.
BD28D4211
HRS
WSCAN
0.25
G160M
1400­1
WAVE­STEP=30.0
1
217.6S
50
1
1
000
610
COMMENTS:
WSCAN
TO
PRODUCE
7
SPECTRA
W/
CENTRAL
WAVELENGTHS:
W1=1415.0,W2=1445,W3=1475,W4=1505,
W5=1535,W6=1565,W7=1595.
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­

GHRS Instrument Handbook 5.0 55
Chapter 6 Design and Construction
of the GHRS
6.1 The HST Focal Plane and the GHRS Apertures 56
6.2 Gratings and Optical Elements 60
6.3 The Digicon Detectors 62

Design and Construction of the GHRS
56 GHRS Instrument Handbook 5.0
6.1 The HST Focal Plane and the GHRS Apertures
We provide here a description of the instrument in largely pictorial terms. More illustra­
tions and full technical descriptions of the GHRS may be found in the references (see
Section 9.2 on page 102).
Figure 6­1. The Hubble Space Telescope and its components, with the
locations of important operational elements shown.

GHRS Instrument Handbook 5.0 57
Design and Construction of the GHRS
Figure 6­2. The focal plane of HST and the definitions of the V2, V3 and U2,
U3 coordinate systems of the spacecraft.

Design and Construction of the GHRS
58 GHRS Instrument Handbook 5.0
Figure 6­3. Locations of GHRS apertures relative to spacecraft axes. Note
that the sense of the x and y motions are shown by the arrows, but that the
zero point for each aperture (SSA and LSA) is located at its center. COSTAR
does not, of course, change the layout of the entrance apertures, but it does
alter the way that the sky is imaged onto the focal plane. The sense is easy
to remember: the COSTAR mirrors invert the sense of the original image,
which means that the signs of motions in both coordinates, V2 and V3, (or U2
and U3) are reversed.

GHRS Instrument Handbook 5.0 59
Design and Construction of the GHRS
Figure 6­4. Optical schematic of the GHRS.

Design and Construction of the GHRS
60 GHRS Instrument Handbook 5.0
6.2 Gratings and Optical Elements
Note that the ``CD'' gratings are cross­dispersers for the echelles. CD­A has a focal
length of 1460 mm and CD­B has a focal length of 1340 mm. Note also that the ``M''
gratings are holographic and that the blaze angle quoted formally is that which correctly
predicts the center of the wavelength region the grating is optimized for. G140L is a
ruled grating. ``Ech­A'' and ``Ech­B'' refer to two modes of operation that use the same
echelle grating but different cross­dispersers and detectors.
a. Mirror N2 is actually ``D'' shaped, being a circle with a small slice off one
side. It is about 60â 80 mm.
TABLE 6­1 Properties of GHRS Gratings
Name
Grooves
per mm
Blaze
Angle
Order of
use
Angle of
Incidence
Diffraction
Angle
Deviation
Angle Detector
G140L 600 2.6 1 9.0 -- 10.3 ­5.3 -- ­4.0 14.25 D1
G140M 6000 23 1 26 -- 38 11 -- 24 14.25 D1
G160M 4960 19 1 21 -- 33 14 -- 27 6.25 D2
G200M 4320 26 1 23 -- 34 17 -- 28 6.25 D2
G270M 3600 28 1 27 -- 38 20 -- 32 6.25 D2
Ech­A 316 63.4 33 -- 53 68 -- 74 54 -- 61 13.25 D1
Ech­B 316 63.4 17 -- 33 63 -- 72 58 -- 66 5.75 D2
CD­A 194.6 0.75 1 D1
CD­B 85.7 0.54 1 D2
TABLE 6­2 Properties of Other GHRS Optical Components
Name
Clear Aperture
(mm)
Focal Length
(mm) Detector
LSA = ``2.0'' 0.559 D1, D2
SSA = ``0.25'' 0.067 D1, D2
Collimator 80 1850 D1, D2
Mirror N2 a 80 D2
Mirror A2 20 D2
Mirror N1 80 D1
Mirror A1 20 D1
Cam­A 84 1425 D1
Cam­B 86 1350 D2
D1 22 â 28
D2 22 â 28

GHRS Instrument Handbook 5.0 61
Design and Construction of the GHRS
Figure 6­5. Schematic diagram of GHRS' acquisition optics. The ``main
acquisition mirror'' is N2.

Design and Construction of the GHRS
62 GHRS Instrument Handbook 5.0
6.3 The Digicon Detectors
Figure 6­6. Cutaway view of a Digicon.

GHRS Instrument Handbook 5.0 63
Design and Construction of the GHRS
Figure 6­7. Cross­sectional view of a Digicon and views of its faceplate and
diode arrays.

Design and Construction of the GHRS
64 GHRS Instrument Handbook 5.0
Figure 6­8. A view from the cross­dispersers toward the Digicon detectors to
illustrate the senses of x and y motions and of increasing wavelength.

GHRS Instrument Handbook 5.0 65
Design and Construction of the GHRS
Figure 6­9. A detailed layout of the diodes in the Digicon detectors. Note
the 6 large ``corner diodes'' and the 6 ``focus diodes'' (numbers 4, 5, and 6, for
example).

Design and Construction of the GHRS
66 GHRS Instrument Handbook 5.0

GHRS Instrument Handbook 5.0 67
Chapter 7 Target Acquisition
Reference Information
7.1 Predicting Target Acquisition Count Rates for Stars 68
7.1.1 Alternate Method for Predicting Target Acquisition Count Rates 69
7.1.2 Two Examples 69
7.2 Constraints on the Value of the STEP­TIME Parameter 71
7.3 Acquisition Count Rates for Extended Objects 76
7.4 Other Acquisition Information 78
7.4.1 Effective Areas of the Acquisition Mirrors 78
7.4.2 Geocoronal Lyman­a Background 80

Target Acquisition Reference Information
68 GHRS Instrument Handbook 5.0
7.1 Predicting Target Acquisition Count Rates for Stars
We have calculated GHRS target acquisition count rates for the spectra of a subset of
the 175 stars contained in the Bruzual­Persson­Gunn­Stryker (BPGS) Library of Stellar
Spectra by using calcphot, a task in the synphot package in stsdas. GHRS target
acquisition count rates for objects similar to those in the BPGS catalog can be predicted
by following the procedures described here. Constraints on the value ofSTEP­TIME
are discussed. Please note that the values tabulated are the total count rate for a star, and
that the count rate for a particular diode will depend on that portion of the Point Spread
Function that strikes it. That can influence the degree, for example, to which the paired­
pulse correction applies. However, the acquisition procedure sums the counts over the
eight science diodes upon which the LSA is imaged, so for most objects these values
may be used straightforwardly.
Do not forget to reduce these values by a factor of 0.3 if the focus diodes are being used
for an IMAGE; this is because of the reduced area of the focus diodes compared to using
eight normal diodes for an acquisition. This factor applies to when a focus diode is cen­
tered on a point source.
The flux distributions in the BPGS catalog include ultraviolet wavelengths and can be
used for planning GHRS target acquisitions. Each spectrum in the catalog was dered­
dened and scaled toV 0 = 0.0. Thecalcphot task in thesynphot package of stsdas was
used to convolve the catalog flux distributions with the effective areas of the acquisition
mirrors. Table 7­1 on page 74 contains columns giving the BPGS catalog object name,
spectral type, (B--V) 0 , count rate for the acquisition mirror with no reddening, and scale
factors (per unit magnitude) indicating the relative count rate observed at given amounts
of reddening compared to the count rate with no reddening.
To use Table 7­1 to predict target acquisition count rates:
. Determine the intrinsic color, (B -- V) 0 , and magnitude, V 0 , of your object as well as
its color excess, E(B -- V).
. Find an entry in Table 7­1 that has similar spectral characteristics to your object (by
spectral type or (B -- V) 0 and note that luminosity class is important for the coolest
stars). The table is sorted by increasing(B -- V) 0 . Make sure you pick from the col­
umn corresponding to the acquisition mirror that you plan to use.
. Scale the predicted count rate found in the previous step by the ratio of apparent
brightness of your object to an object withV 0 = 0.0, i.e., multiply by .
. To obtain the scale factor by which the unreddened count rate will be reduced for an
amount of reddening appropriate to your object, multiply the count rate from the
previous step by this factor:
. The GHRS detectors are nonlinear at high count rates: this phenomenon is referred
to as the ``dead­time'' or ``paired­pulse'' effect. Consequently, the predicted count
rate from the previous step must be reduced to yield the actual count rate that GHRS
will measure. Multiply the count rate you just determined by the ``fraction detected''
value determined from Figure2 on page 71 to obtain the final predicted count rate.
. This final value is probably reliable to within a factor of two, which is adequate for
acquisition purposes in almost all instances.
10 0.4V 0
--
10 scale factor E B V
--
( )
â

GHRS Instrument Handbook 5.0 69
Target Acquisition Reference Information
7.1.1 Alternate Method for Predicting Target Acquisition Count Rates
Figure 7­1 on page 70 shows mean predicted count rates as a function of
color for the four acquisition mirrors. Also shown are the fits to the predicted count rates
for various amounts of reddening. The label for each curve represents the color excess,
E(B -- V), applied to the spectra.
You can estimate target acquisition count rates using the plots instead of Table 7­1:
. Determine the intrinsic color and magnitude of your object as well as its color
excess.
. Read from the Figures (depending on the mirror used) the predicted count rate.
. Scale the predicted count rate by the ratio of apparent brightness of your object to an
object with zero magnitude.
. Use Figure 2 on page 71 to correct for the ``paired­pulse'' effect.
7.1.2 Two Examples
First, suppose you want to observeµ Col, which has the following properties:
Using the table, you would see that HR 8023, an O6 star with is the
closest match, giving a predicted count rate for the A2 mirror of 1.3x 10 7 counts s --1 for
a V 0 = 0 star. Multiplying this count rate by 10 ­0.4â5.13 gives 1.2 x 10 5 counts s --1 . Red­
dening will decrease the counts slightly; calculation of the scale factor indicates that you
should multiply by 0.93, giving a new count rate of 1.1x 10 5 counts s ­1 . The dead­time
correction factor estimates that only 83% of those counts will be detected, so one would
expect approximately 94000 counts s ­1 with this star. In fact, whenµ Col was observed
early in Cycle 4, 19,600 counts were obtained in 0.2seconds with the A2 mirror, which
works out to 98,000counts s ­1 , which is within 5% of the calculated value.
Second, consider a very red star such as Aldebaran:
Using the table, you would see that BD--1 o 3113 has a similar spectral type (K5III) and
color (1.61). The calculated count rate for mirror A2 is then counts per sec­
ond. The adjustment for apparent magnitude is , which yields a count rate of
530 per second, or 210 in 0.2 seconds. Early in Cycle 4 a T
au was acquired with mirror
A2 and the count rate seen was 246 in 0.2 seconds, within 15% of the predicted value.
Name Sp. Type. V B -- V E(B -- V)
µ Columbae O9V 5.16 --0.29 0.01
Name Sp. Type. V B -- V E(B -- V)
a Tauri K5III 0.84 1.54 0.00
B V
--
( ) 0
B V
--
( ) 0.313
--
=
1.6 10 3
â
10 0.4 0.84
â
--

Target Acquisition Reference Information
70 GHRS Instrument Handbook 5.0
Figure
7­1.
Mean
target
acquisition
count
rates
for
stars
with
the
four
mirrors
of
the
GHRS.
The
numbers
at
the
right
indicate
the
appropriate
E(B
--
V)
for
each
curve.

GHRS Instrument Handbook 5.0 71
Target Acquisition Reference Information
Figure 7­2. Fraction of counts detected as a function of the true count rate,
i.e., before the paired pulse correction.
7.2 Constraints on the Value of the STEP­TIME Parameter
Once you have the predicted countrate, you then need to extract the piece of information
required to plan your target acquisition: STEP­TIME. This is necessary to calculate the
time per exposure that is entered on the proposal logsheet. The goal is to have GHRS
see 10 3 to 10 4 counts at the peak dwell point of the spiral search for either an acquisition
or a peakup. A minimum of 100 counts are needed to have a chance at a successful tar­
get acquisition. The STEP­TIME then is just the number of counts desired (10 3 to 10 5 ,
but at least 100) divided by the predicted count rate. Remember, however, that the mini­
mum STEP­TIME permitted is 0.2 seconds. Also bear in mind that theSTEP­TIME
may not exceed 12.75 seconds.
To avoid the possibility of a failed target acquisition, it is important that the combination
of your target flux and the acquisition mirror used should not result in the eight science
diodes used for the target acquisition seeing more than a total of about 65,000 counts
(the number accommodated in a 16 bit register) in a givenSTEP­TIME. Counting more
than 65,000 counts effectively causes the register to wrap back to zero and thus con­
founds the target acquisition algorithm. This problem is avoided if BRIGHT=RETURN
is specified because a 32­bit on­board register is used. If aBRIGHT limit is explicitly
given, the register that holds it is limited to 16 bits.
Figure 7­3 and Figure 7­4 are provided to allow for a visual check of potential problems
arising from the choice ofSTEP­TIME. These Figures show the constraints placed on
using a particular mirror andSTEP­TIME. Figure 7­4 is analogous for the Side 1 mir­
rors.

Target Acquisition Reference Information
72 GHRS Instrument Handbook 5.0
Figure 7­3. Target acquisition constraints for the Side 2 mirrors N2 and A2.
N2R: Constraints and Restrictions Document (CARD) upper limit for use of Mirror­N2. Observing
objects that are bluer and brighter than indicated by this line would result in degraded perfor­
mance and possible damage to the instrument. Brighter and bluer objects should be acquired with
mirrors A2 or A1.
N2­0.2: GHRS will count 65,000 counts in 0.2 seconds with the N2 mirror for objects on this contour. To
the left of this curve, more than 65,000 counts will be detected leading to a probable failure to
acquire the object if aBRIGHT value is specified. However, use of BRIGHT=RETURN will
result in a satisfactory acquisition.
A2­0.2: GHRS will count 65,000 counts in 0.2 seconds with the A2 mirror for objects on this contour. To
the left of this curve, more than 65,000 counts will be detected leading to a probable failure to
acquire the object if aBRIGHT value is specified. However, use of BRIGHT=RETURN will
result in a satisfactory acquisition.

GHRS Instrument Handbook 5.0 73
Target Acquisition Reference Information
Figure 7­4. Target acquisition constraints for the Side 1 mirrors N1 and A1.
N1R: Constraints and Restrictions Document (CARD) upper limit for use of Mirror­N1. Observing
objects that are bluer and brighter than indicated by this line would result in degraded perfor­
mance and possible damage to the instrument. Brighter and bluer objects should be acquired with
mirrors A2 or A1.
N1­0.2: GHRS will count 65,000 counts in 0.2 seconds with the N1 mirror for objects on this contour. To
the left of this curve, more than 65,000 counts will be detected leading to a probable failure to
acquire the object if aBRIGHT value is specified. However, use of BRIGHT=RETURN will
result in a satisfactory acquisition.!
Note that no constraints apply to the use of the A1 mirror, i.e., no objects are too bright.

Target Acquisition Reference Information
74 GHRS Instrument Handbook 5.0
TABLE
7­1
Predicted
target
acquisition
count
rates
for
stars,
reduced
toV
0
=
0.
Star
Name
Spectral Type
(B--V)
0
count
rate
for
mirror
reddening
reduction
factor
a
N2
A2
N1
A1
N2
A2
N1
A1
9
Sgr
O5
­0.337
8.595E9
2.098E7
4.598E8
2.657E5
­3.034
­3.108
­3.428
­3.130
HR
8023
O6
­0.313
5.684E9
1.348E7
2.545E8
1.690E5
­2.955
­3.021
­3.382
­3.114
60
Cyg
B1V
­0.269
5.367E9
1.293E7
2.794E8
1.767E5
­3.002
­3.070
­3.393
­3.123
69
Cyg
B0IB
­0.234
4.192E9
9.901E6
1.869E8
1.209E5
­2.935
­3.003
­3.384
­3.110
i
Her
B3V
­0.203
2.764E9
6.529E6
1.232E8
7.970E4
­2.935
­3.003
­3.384
­3.110
HR
7467
B3III
­0.182
2.436E9
5.755E6
1.087E8
7.028E4
­2.936
­3.004
­3.384
­3.110
20
Aql
B3IV
­0.156
2.268E9
5.357E6
1.012E8
6.542E4
­2.936
­3.004
­3.384
­3.110
38
Oph
A1V
­0.139
1.776E9
4.108E6
6.421E7
4.582E4
­2.891
­2.953
­3.347
­3.101
HR
7346
B7III
­0.108
1.368E9
3.165E6
4.948E7
3.531E4
­2.892
­2.953
­3.347
­3.101
HD
189689
B9V
­0.081
1.008E9
2.303E6
2.763E7
2.306E4
­2.864
­2.921
­3.317
­3.097
59
Her
A3III
­0.045
5.948E8
1.363E6
1.700E7
1.407E4
­2.873
­2.932
­3.320
­3.099
11
Sge
B9IV
­0.027
5.745E8
1.293E6
1.168E7
1.182E4
­2.819
­2.873
­3.281
­3.088
69
Her
A2V
0.000
4.918E8
1.107E6
9.997E6
1.011E4
­2.819
­2.873
­3.281
­3.088
HR
8020
B8IA
0.027
1.063E9
2.386E6
1.940E7
2.154E4
­2.820
­2.872
­3.255
­3.089
78
Her
B9V
0.036
4.532E8
1.018E6
8.272E6
9.187E3
­2.820
­2.872
­3.255
­3.089
58
Aql
A0V
0.057
4.291E8
9.634E5
7.830E6
8.696E3
­2.820
­2.872
­3.255
­3.089
60
Her
A3IV
0.085
3.197E8
7.086E5
3.280E6
5.497E3
­2.758
­2.810
­3.217
­3.077
HR
6570
A7V
0.107
2.999E8
6.647E5
3.079E6
5.161E3
­2.759
­2.811
­3.217
­3.077
HD192285
A4IV
0.124
2.628E8
5.771E5
1.444E6
3.569E3
­2.719
­2.771
­3.184
­3.071
q
1
Ser
A5V
0.143
2.669E8
5.862E5
1.466E6
3.623E3
­2.718
­2.770
­3.184
­3.071
KW
114
0.175
2.395E8
5.240E5
9.954E5
2.843E3
­2.703
­2.755
­3.168
­3.069
KW
154
0.219
2.050E8
4.439E5
4.497E5
1.636E3
­2.666
­2.715
­3.166
­3.063
c
Ser
F0IV
0.240
1.829E8
3.958E5
4.004E5
1.457E3
­2.665
­2.714
­3.166
­3.062
HD5132
F0IV
0.287
2.011E8
4.354E5
4.412E5
1.605E3
­2.666
­2.715
­3.166
­3.063
HD508
A9IV
0.305
1.718E8
3.720E5
3.771E5
1.372E3
­2.666
­2.716
­3.166
­3.063
r
Cap
F2IV
0.339
1.125E8
2.322E5
2.619E4
1.014E2
­2.484
­2.517
­3.181
­3.023
KW
332
0.389
1.177E8
2.431E5
2.751E4
1.065E2
­2.486
­2.519
­3.181
­3.023
HD7331
F7IV
0.427
9.807E7
2.012E5
2.256E4
8.772E1
­2.451
­2.482
­3.179
­3.015
BD+63
o
13
F5IV
0.444
1.139E8
2.362E5
6.070E4
2.395E2
­2.488
­2.523
­3.171
­3.024
HD35296
F8V
0.489
1.089E8
2.259E5
5.804E4
2.290E2
­2.488
­2.523
­3.171
­3.024
vB
1
0.530
6.757E7
1.374E5
2.147E4
8.448E1
­2.407
­2.437
­3.175
­3.006
HD154760
G2V
0.586
6.393E7
1.285E5
3.115E3
1.151E1
­2.365
­2.386
­3.206
­2.998

GHRS Instrument Handbook 5.0 75
Target Acquisition Reference Information
a.
use
factor
f
to
reduce
count
rate
by
.
HD139777A
K0V
0.631
6.040E7
1.215E5
2.954E3
1.091E1
­2.367
­2.387
­3.206
­2.998
HR
6516
G6IV
0.651
5.167E7
1.039E5
2.518E3
9.301E0
­2.365
­2.386
­3.206
­2.998
HD136274
G8V
0.672
4.901E7
9.855E4
2.392E3
8.837E0
­2.366
­2.387
­3.206
­2.998
HD150205
G5V
0.700
4.650E7
9.352E4
2.275E3
8.404E0
­2.367
­2.388
­3.206
­2.998
31
Aql
G8IV
0.732
4.042E7
8.123E4
1.960E3
7.242E0
­2.364
­2.384
­3.206
­2.997
vB
21
0.766
3.890E7
7.818E4
1.887E3
6.972E0
­2.364
­2.385
­3.206
­2.997
BD--2
o
4018
G5IV
0.791
3.516E7
7.068E4
1.713E3
6.328E0
­2.365
­2.386
­3.206
­2.998
HD190571
G8V
0.816
1.587E7
3.086E4
1.165E1
4.76E­4
­2.233
­2.244
­3.719
­3.247
HD11004
G5IV
0.825
1.587E7
3.086E4
1.169E1
4.78E­4
­2.234
­2.244
­3.719
­3.247
HD56176
G7IV
0.857
1.510E7
2.937E4
1.109E1
4.54E­4
­2.233
­2.244
­3.719
­3.247
HD190470
K3V
0.895
1.237E7
2.403E4
8.968E0
3.67E­4
­2.230
­2.241
­3.719
­3.246
q
1
Tau
G8III
0.904
1.950E7
3.862E4
2.639E1
2.67E­4
­2.306
­2.320
­3.750
­3.262
HD170527
G5IV
0.918
2.157E7
4.277E4
2.944E1
2.98E­4
­2.309
­2.323
­3.750
­3.263
HD191615
G8IV
0.947
1.596E7
3.159E4
2.136E1
2.16E­4
­2.303
­2.317
­3.750
­3.261
HD4744
G8IV
0.989
1.087E7
2.119E4
8.241E0
3.37E­4
­2.240
­2.250
­3.719
­3.248
91
Aqr
K0III
1.010
8.535E6
1.662E4
6.381E0
2.61E­4
­2.237
­2.247
­3.719
­3.247
HD95272
K0III
1.041
8.184E6
1.593E4
6.086E0
2.49E­4
­2.236
­2.246
­3.719
­3.247
y
UMa
K1III
1.081
6.894E6
1.342E4
5.143E0
2.10E­4
­2.237
­2.247
­3.719
­3.247
BD+1
o
3131
K0III
1.143
4.403E6
8.511E3
7.928E0
2.54E­4
­2.210
­2.221
­3.718
­3.242
vB
173
1.202
6.753E6
1.337E4
2.396E1
7.80E­4
­2.296
­2.311
­3.721
­3.260
HD166780
K5III
1.323
2.059E6
3.986E3
3.791E0
1.21E­4
­2.214
­2.225
­3.718
­3.243
RZ
Her
M6III
1.380
5.352E6
1.042E4
1.057E1
3.39E­4
­2.228
­2.239
­3.718
­3.246
HD116870
M0III
1.413
1.414E6
2.734E3
2.228E0
7.87E­5
­2.207
­2.218
­3.717
­3.241
M67
IV­202
1.463
1.170E6
2.263E3
1.857E0
6.56E­5
­2.208
­2.219
­3.717
­3.242
HD104216
M2III
1.523
1.423E6
2.795E3
4.603E0
1.50E­4
­2.271
­2.287
­3.721
­3.255
BD--1
o
3113
K5III
1.609
8.070E5
1.588E3
2.681E0
8.72E­5
­2.277
­2.293
­3.721
­3.256
HD142804
M1III
1.694
7.419E5
1.463E3
2.517E0
8.19E­5
­2.283
­2.299
­3.721
­3.258
Gl
15B
M6V
1.707
2.391E6
4.742E3
8.704E0
2.83E­4
­2.303
­2.318
­3.721
­3.262
Gl
65
M5V
1.768
1.075E7
2.122E4
3.642E1
1.18E­3
­2.284
­2.300
­3.721
­3.258
HD
151658
M2III
1.770
7.242E5
1.423E3
2.359E0
7.67E­5
­2.272
­2.288
­3.721
­3.255
R
Leo
1.918
4.550E6
8.879E3
1.308E1
4.26E­4
­2.245
­2.260
­3.721
­3.250
WZ
Cas
N
2.636
1.987E5
3.867E2
5.89E­1
1.92E­5
­2.246
­2.263
­3.721
­3.250
AW
Cyg
N
3.790
1.951E5
3.796E2
5.70E­1
1.85E­5
­2.243
­2.260
­3.721
­3.249
TABLE
7­1
Predicted
target
acquisition
count
rates
for
stars,
reduced
toV
0
=
0.
(Continued)
Star
Name
Spectral Type
(B--V)
0
count
rate
for
mirror
reddening
reduction
factor
a
N2
A2
N1
A1
N2
A2
N1
A1
10
f
E
B
V
--
(
)
â

Target Acquisition Reference Information
76 GHRS Instrument Handbook 5.0
7.3 Acquisition Count Rates for Extended Objects
Chapter 4 mentions acquisition methodologies for extended objects, and, in particular,
the use of theEXTENDED optional parameter. In this section we provide some guidance
on predicting the count rates to be expected during an acquisition of an extended object,
especially one beyond our own Galaxy. The renewed availability of Side 1 of the GHRS
and its G140L grating make it possible to get high­quality spectra of faint objects effi­
ciently.
Whenever possible, we recommend that faint objects be acquired by offsetting from a
nearby and brighter point source. If accurate coordinates are used this method should be
reliable. However, such objects are not always present next to targets of astrophysical
interest. Also, obtaining a good astrometric position of an extended source can be pre­
vented by its large saturated area on the photographic plates upon which the Guide Star
Catalog is based. In these cases a direct target acquisition will need to be attempted, and
it should succeed if the object provides enough ultraviolet photons. The procedure
closely follows that for point sources just described:
. Find a star in Table 7­1 with a spectral energy distribution like that of your object, or
consult the IUE Atlas of Star­Forming Galaxies, by Kinney et al. (1993, ApJS, 86,
5). This publication provides representative spectra of many classes of galaxies and
compares their shapes to ones of stellar spectra (whence the ``spectral types'' listed).
. Estimate the flux that will fall within the LSA from either Table 7­1 or Table 7­2 on
page 77, as appropriate.
. Calculate STEP­TIME in the same manner as for stars. If necessary, you may use
mirror N2 on Side 2 for your acquisition, even if Side 1 is being used to observe, but
doing so will add about 40 minutes of overhead time.
. If your object does not fall within these categories, please consult us.

GHRS Instrument Handbook 5.0 77
Target Acquisition Reference Information
a. use factor f to reduce count rate by .
TABLE 7­2 Predicted count rates for non­stellar objects
``Spectral
Type''
count rate for mirror reddening reduction factor a
N2 A2 N1 A1 N2 A2 N1 A1
o3_6v 8.843E9 2.116E7 3.990E8 2.726E5 ­3.059 ­3.112 ­3.362 ­3.134
o4_9i 8.047E9 1.934E7 3.731E8 2.427E5 ­3.054 ­3.109 ­3.380 ­3.133
o5_6iii 8.935E9 2.140E7 3.961E8 2.687E5 ­3.068 ­3.121 ­3.364 ­3.136
o7_b0v 7.407E9 1.771E7 3.282E8 2.065E5 ­3.028 ­3.086 ­3.388 ­3.129
o9_b0iv 6.413E9 1.533E7 2.931E8 1.784E5 ­3.012 ­3.072 ­3.396 ­3.125
b0_2i 4.314E9 1.005E7 1.372E8 9.530E4 ­2.949 ­3.005 ­3.378 ­3.113
b0_2iii 5.144E9 1.214E7 1.908E8 1.231E5 ­2.994 ­3.051 ­3.388 ­3.122
b2_4v 2.889E9 6.765E6 1.037E8 7.131E4 ­2.964 ­3.021 ­3.364 ­3.116
b2_5iv 2.754E9 6.444E6 9.673E7 6.712E4 ­2.960 ­3.017 ­3.367 ­3.115
b3_5i 1.694E9 3.836E6 3.581E7 3.042E4 ­2.859 ­2.910 ­3.328 ­3.095
b3_6iii 2.344E9 5.448E6 7.404E7 5.400E4 ­2.943 ­2.998 ­3.353 ­3.112
b5_8v 1.713E9 3.952E6 4.849E7 3.716E4 ­2.922 ­2.976 ­3.337 ­3.108
b6_9i 9.689E8 2.187E6 2.021E7 1.765E4 ­2.831 ­2.882 ­3.313 ­3.089
b7_9iii 8.700E8 1.975E6 1.845E7 1.663E4 ­2.867 ­2.918 ­3.305 ­3.098
b8_9iv 9.701E8 2.216E6 2.337E7 1.984E4 ­2.889 ­2.941 ­3.316 ­3.102
f2_7iv 1.007E8 2.108E5 2.968E4 1.206E2 ­2.547 ­2.580 ­3.201 ­3.043
f2_8i 4.536E7 9.480E4 1.403E4 5.634E1 ­2.535 ­2.569 ­3.201 ­3.041
f5_7v 1.079E8 2.254E5 1.782E4 ­2.538 ­2.571 ­3.238
f6iii 9.009E7 1.862E5 8.586E2 ­2.499 ­2.526 ­3.830
f8_9v 6.664E7 1.367E5 3.996E3 ­2.460 ­2.484 ­3.245
g0_2iv 4.451E7 9.041E4 3.614E2 ­2.415 ­2.434 ­3.604
g0_3i 1.971E7 4.015E4 ­2.429 ­2.444
g0_5iii 3.349E7 6.822E4 ­2.431 ­2.450
g0_5v 5.372E7 1.095E5 ­2.428 ­2.449
g5_8i 4.802E6 9.721E3 ­2.399 ­2.414
g5_8iv 2.123E7 4.242E4 ­2.344 ­2.354
g5_k0iii 6.969E6 1.384E4 ­2.320 ­2.330
g6_9v 2.588E7 5.189E4 ­2.360 ­2.372
g8_k1iv 8.008E6 1.583E4 ­2.297 ­2.304
k0_1v 1.307E7 2.606E4 ­2.337 ­2.348
k0_2iii 5.918E6 1.165E4 ­2.285 ­2.290
k1_3i 2.497E6 5.057E3 ­2.397 ­2.411
k2_3v 9.035E6 1.800E4 ­2.327 ­2.336
k2iii 2.092E6 4.107E3 ­2.268 ­2.273
k3iii 1.157E6 2.266E3 ­2.259 ­2.264
k4_5iii 5.490E5 1.085E3 ­2.290 ­2.296
k5_m0v 2.607E6 5.167E3 ­2.304 ­2.311
k5_m5i 9.714E5 1.976E3 ­2.413 ­2.426
k7_m3iii 5.420E5 1.081E3 ­2.323 ­2.330
10 f E B V
--
( )
â

Target Acquisition Reference Information
78 GHRS Instrument Handbook 5.0
7.4 Other Acquisition Information
7.4.1 Effective Areas of the Acquisition Mirrors
Table 7­3 on page 79 lists the effective areas of the four acquisition mirrors (in ) as
a function of wavelength, to use to predict acquisition count rates. These values are
those from the Science Verification Report for the GHRS, adjusted by the observed ratio
of post­ to pre­COSTAR sensitivities (see Section8.1 on page 82). Note that A1 and N1
may only be used with detector D1 and A2 and N2 with detector D2.
Also note that the proper use of this table requires compensation for the different ener­
gies of photons of different wavelengths, hence the last column, which is in picoergs per
photon.
Figure 7­5. Relative sensitivities of the GHRS acquisition mirrors. The effec­
tive areas shown are from Table 7­3 on page 79.
cm 2

GHRS Instrument Handbook 5.0 79
Target Acquisition Reference Information
TABLE 7­3. Effective areas of the four GHRS acquisition mirrors
Wavelength (å)
Effective Area (cm 2 )
perg
photon ­1
A1 N1 A2 N2
1100 0.0025 0.00085 0.175 18.1
1150 4.89 0.12 23.36 17.3
1200 16.79 0.29 54.18 16.6
1250 28.54 0.34 76.50 15.9
1300 34.10 0.37 96.22 15.3
1350 36.64 0.34 97.03 14.7
1400 36.05 0.28 96.88 14.2
1450 35.35 0.28 105.46 13.7
1500 35.49 0.27 113.43 13.2
1550 0.000172 34.43 0.29 133.84 12.8
1600 0.00298 30.18 0.31 155.76 12.4
1650 0.0317 29.37 0.37 165.92 12.0
1700 0.0843 25.91 0.44 176.10 11.7
1750 0.0923 18.11 0.56 216.91 11.4
1800 0.0680 11.82 0.68 259.65 11.0
1850 0.0261 7.38 0.74 281.80 10.7
1900 0.0094 3.04 0.79 302.40 10.5
1950 0.0021 1.04 1.04 362.78 10.2
2000 0.55 1.04 363.32 9.93
2050 0.30 1.08 406.51 9.69
2100 0.16 1.13 449.71 9.46
2200 1.43 594.61 9.03
2300 1.55 663.74 8.64
2400 1.75 776.52 8.28
2500 1.87 847.76 7.95
2600 1.85 854.42 7.64
2700 1.83 866.65 7.36
2800 1.68 813.29 7.09
2900 1.50 754.81 6.85
3000 0.76 398.89 6.62
3100 0.58 309.31 6.41
3200 0.38 194.15 6.21
3300 0.21 116.67 6.02
3400 0.14 80. 5.84

Target Acquisition Reference Information
80 GHRS Instrument Handbook 5.0
7.4.2 Geocoronal Lyman­a Background
The acquisition mirror sensitivity curves illustrated above show that the mirrors still
reflect well at the Lyman­a line at 1216 å. Note that Ly­a is suppressed by mirror A1.
Tests have shown that the count rate from geocoronal Lyman­a can be as high as 12
counts per second per diode. If you wish to acquire a faint target with the least Ly­a
contamination, we suggest that you specifyDARK TIME as a Special Requirement in
Phase II.

GHRS Instrument Handbook 5.0 81
Chapter 8 Reference T
ables for
Instrument Performance
8.1 The Effect of COSTAR on the GHRS 82
8.2 Properties of the First­Order Gratings 83
8.2.1 Useful Wavelength Ranges 83
8.2.2 Resolving Power 83
8.2.3 Sensitivity Functions for the First­Order Gratings 85
8.3 Properties of the Echelle Gratings 87
8.3.1 Wavelength Coverage, Bandpass, and Sensitivity 87
8.3.2 Echelle Wavelength Formats 89
8.3.3 Echelle Blaze Function 91
8.4 Standard Patterns for Substepping and Background Measurement 92
8.5 The Effects of Reddening in the Ultraviolet 93
8.6 Instrumental Properties 94
8.6.1 The Point Spread Function 94
8.6.2 Detector Dark Count and the CENSOR Option 95
8.6.3 Noise Rejection with FLYLIM 97
8.6.4 Count Rate Linearity 98
8.6.5 Image Stability 98
8.6.6 Wavelength Calibrations 98

Reference Tables for Instrument Performance
82 GHRS Instrument Handbook 5.0
8.1 The Effect of COSTAR on the GHRS
Prior to the Servicing Mission, it was believed that the COSTAR mirrors would have lit­
tle effect on the throughput of the GHRS. It was known that the magnesium fluoride
coatings would severely attenuate light below 1150 å, but at longer wavelengths we
anticipated that the light lost from the extra two reflections would be compensated for
by the improved throughput of the restored Point Spread Function.
The actual situation is more complex, as shown below. What is plotted is a mean rela­
tion that was fitted to observations of the same star that were made both before and after
the Servicing Mission. The most prominent feature is the dip at 2000 å. The high ratio
at longer wavelengths is just due to the improved PSF, but the peak near 1300 å is not
understood.
Figure 8­1. The observed ratio of counts for µ Col, made before the Servic­
ing Mission (in Cycle 2) and after the deployment of the COSTAR mirrors.

GHRS Instrument Handbook 5.0 83
Reference Tables for Instrument Performance
8.2 Properties of the First­Order Gratings
8.2.1 Useful Wavelength Ranges
The following table summarizes the useful wavelength range for each of the first­order
gratings of GHRS. More precise sensitivity values are enumerated below. Note that lit­
tle or no flux below 1150 å is reflected by the COSTAR mirrors because of their magne­
sium fluoride coatings.
The last three gratings are used with detector D2, which admits some second­order
light, hence the comments. For example, Lyman­a light (1216 å) can appear at 2432 å
in second order. Except for this possibility of geocoronal contamination, many cool
stars have very little short­wavelength flux, so that the best resolution can be achieved
without undue extraneous light by observing in first order near the high­wavelength
limit.
Note that the G270M grating has an order­sorting filter which eliminates light below
about 1650 å so that no cross­order contamination occurs below 3300 å.
8.2.2 Resolving Power
The following figures illustrate the resolving power as measured for each of GHRS'
gratings. In this case the resolving power was computed asR = l/Dl, where is the
measured full­width­at­half­maximum (FWHM) of lines from an exposure of a spec­
trum calibration lamp. Tests have shown that the measured FWHM does not change sig­
nificantly with wavelength (for the first­order gratings) or withml, the product of the
wavelength and order number (for the echelle gratings). The nominal design specifica­
tion for the GHRS wasR = 20,000 for the first­order gratings, but in fact one can exceed
a resolving power of 25,000 at virtually all wavelengths. Similarly, the low­dispersion
grating G140L has R in excess of 2,000 over most of its useful wavelength range. The
true resolving powers for the echelle gratings are closer to 80,000 than the nominal
100,000.
By providing a sharper image of a point source, COSTAR restores the resolving power
achieved with the LSA to within about 20% of that possible with the SSA. There is no
effective change for the SSA, however.
TABLE 8­1
Useful wavelength ranges for first­order gratings.
Grating Useful Range (å) å per diode Bandpass (å) Comment
G140L 1100 -- 1900 0.572 -- 0.573 286 -- 287
G140M 1100 -- 1900 0.056 -- 0.052 28 -- 26
G160M 1150 -- 2300 0.072 -- 0.066 36 -- 33 2nd order overlap above 2300 å
G200M 1600 -- 2300 0.081 -- 0.075 41 -- 38 2nd order overlap above 2300 å
G270M 2000 -- 3300 0.096 -- 0.087 48 -- 44 2nd order overlap above 3300 å
Dl

Reference Tables for Instrument Performance
84 GHRS Instrument Handbook 5.0
Figure 8­2. Spectrum resolving power as a function of wavelength for the
GHRS medium­resolution (holographic) gratings. From left to right the curves
are for G140M, G160M, G200M, and G270M, respectively.
Figure 8­3. Resolving power for grating G140L.

GHRS Instrument Handbook 5.0 85
Reference Tables for Instrument Performance
8.2.3 Sensitivity Functions for the First­Order Gratings
Below are given sensitivities for the first­order gratings, using the Large Science Aper­
ture, in units of . These
values will be updated with precise numbers determined in Cycle 4 but what is listed
here has been adjusted for the known effects of the COSTAR mirrors. SSA sensitivity is
about 50 to 70% of these values, with the larger value applying at longer wavelengths.
TABLE 8­2
Sensitivities for first­order gratings.
Grating
Wavelength (å) G140L G140M G160M G200M G270M
1100 0.20 0.006
1150 20.9 0.75 0.64
1200 82.3 2.27 2.87
1250 152. 4.95 5.77
1300 168. 7.13 7.96
1350 166. 5.89 7.33
1400 146. 5.23 6.41
1450 114. 4.74 6.08
1500 89.3 3.41 4.34
1550 67.0 2.34 4.87
1600 43.9 1.36 5.17 4.48
1650 40.0 1.14 4.89 4.63
1700 32.0 0.88 4.74 5.56
1750 20.7 4.80 6.53
1800 10.8 4.78 7.42
1850 4.03 5.02 8.21
1900 1.09 5.20 8.61
1950 5.20 8.40 3.64
2000 5.00 8.23 4.72
2050 4.75 8.57 6.72
2100 4.68 8.92 9.25
2150 9.18 12.31
2200 9.35 16.19
2250 9.02 20.87
2300 8.57 23.82
2350 8.14 23.98
2400 8.70 26.29
2450 9.24 32.34
2500 9.76 41.63
10 11 (counts sec 1
-- diode 1
-- ) per incident (erg cm 2
-- sec 1
-- å 1
-- )

Reference Tables for Instrument Performance
86 GHRS Instrument Handbook 5.0
2550 49.06
2600 53.21
2650 54.37
2700 53.01
2750 50.28
2800 46.73
2850 42.44
2900 37.71
2950 32.19
3000 25.76
3050 19.46
3100 14.14
3150 10.07
3200 6.53
3250 3.83
3300 2.64
TABLE 8­2 (Continued)
Sensitivities for first­order gratings.
Grating
Wavelength (å) G140L G140M G160M G200M G270M

GHRS Instrument Handbook 5.0 87
Reference Tables for Instrument Performance
8.3 Properties of the Echelle Gratings
8.3.1 Wavelength Coverage, Bandpass, and Sensitivity
The following tables summarize basic properties of the two echelle gratings. The dis­
persion in each order has not been listed but does not change if it is computed in veloc­
ity units. At the center of each order, the dispersion is 3.0 km s ­1 per diode, at the long­
wavelength end of each order it is 2.9 km s
­1 per diode, and at the short­wavelength end
of each order it is 3.1 km s ­1 per diode.
a. ``d'' coefficient of Cardelli, Ebbets, and Savage (1993; see Section9.2 on page 102). These apply only to
the SSA.
b. Sensitivity function at blaze peak, in units of 10 11 (counts diode ­1 sec ­1 ) per incident (erg cm ­2 sec ­1 å ­1 ).
TABLE 8­3
Properties of Grating Echelle­A
Order
Central
Wavelength (å)
Order
Coverage (å)
Bandpass per
exposure (å)
Scattered
Light a S l b
51 1102 1091 -- 1113 5.90 -- 5.55
50 1124 1113 -- 1135 6.05 -- 5.65
49 1147 1135 -- 1159 6.15 -- 5.75 0.061 0.12
48 1171 1159 -- 1183 6.25 -- 5.90 0.058 0.27
47 1196 1183 -- 1209 6.40 -- 6.00 0.055 0.46
46 1222 1209 -- 1235 6.55 -- 6.10 0.052 0.8
45 1249 1235 -- 1263 6.70 -- 6.25 0.049 0.92
44 1277 1263 -- 1292 6.85 -- 6.40 0.046 1.15
43 1307 1292 -- 1322 7.05 -- 6.55 0.044 1.16
42 1338 1322 -- 1354 7.20 -- 6.70 0.041 1.34
41 1371 1354 -- 1387 7.40 -- 6.85 0.039 1.23
40 1405 1387 -- 1423 7.55 -- 7.00 0.037 1.47
39 1441 1423 -- 1460 7.80 -- 7.20 0.035 1.04
38 1479 1460 -- 1498 8.00 -- 7.40 0.034 0.91
37 1519 1498 -- 1539 8.25 -- 7.55 0.032 0.77
36 1561 1539 -- 1583 8.45 -- 7.75 0.031 0.64
35 1606 1583 -- 1629 8.70 -- 7.95 0.030 0.56
34 1653 1629 -- 1677 8.95 -- 8.20 0.030 0.44
33 1703 1677 -- 1729 9.25 -- 8.40 0.029 0.40

Reference Tables for Instrument Performance
88 GHRS Instrument Handbook 5.0
a. ``d'' coefficient of Cardelli, Ebbets, and Savage (1993; see Section9.2 on page 102). These apply only to
the SSA.
b. Sensitivity function at blaze peak, in units of 10 11 (counts diode ­1 sec ­1 ) per incident (erg cm ­2 sec ­1 å ­1 ).
TABLE 8­4
Properties of Grating Echelle­B
Order
Central
Wavelength (å)
Order
Coverage (å)
Bandpass per
exposure (å)
Scattered
Light a
S l b
33 1703 1677 -- 1729 9.3 -- 8.4 0.045 0.35
32 1756 1729 -- 1784 9.6 -- 8.6 0.043 0.74
31 1813 1784 -- 1842 9.9 -- 8.9 0.041 1.06
30 1873 1842 -- 1905 10.3 -- 9.2 0.039 1.62
29 1938 1905 -- 1971 10.7 -- 9.5 0.037 2.26
28 2007 1971 -- 2043 11.1 -- 9.8 0.035 3.04
27 2082 2043 -- 2120 11.5 -- 10.1 0.033 4.29
26 2162 2120 -- 2203 11.9 -- 10.5 0.031 6.18
25 2248 2203 -- 2293 12.4 -- 10.9 0.030 8.59
24 2342 2293 -- 2391 13.0 -- 11.3 0.028 9.96
23 2444 2390 -- 2497 13.6 -- 11.7 0.026 11.2
22 2555 2497 -- 2613 14.2 -- 12.2 0.024 13.0
21 2676 2613 -- 2740 14.9 -- 12.7 0.022 13.4
20 2810 2740 -- 2880 15.8 -- 13.3 0.020 12.8
19 2958 2880 -- 3036 16.7 -- 13.9 0.018 9.57
18 3122 3036 -- 3209 17.6 -- 14.6 0.016 4.20

GHRS Instrument Handbook 5.0 89
Reference Tables for Instrument Performance
8.3.2 Echelle Wavelength Formats
The illustrations below provide a means of estimating where particular wavelengths fall
within the echelle formats and where they lie relative to the blaze peak. Note that these
layouts are purely schematic; the actual length of the free spectral range changes from
order to order. The center of each order is atml = 56200.
Figure 8­4. Schematic format of wavelengths for Echelle­B.

Reference Tables for Instrument Performance
90 GHRS Instrument Handbook 5.0
Figure 8­5. Schematic format of wavelengths for Echelle­A.

GHRS Instrument Handbook 5.0 91
Reference Tables for Instrument Performance
8.3.3 Echelle Blaze Function
The figure below shows the echelle blaze function (also known as the ``Ripple Func­
tion'') in the form of sensitivity relative to the peak of the blaze as the product of wave­
length and order number.
Figure 8­6. Normalized blaze function for the GHRS echelle gratings.

Reference Tables for Instrument Performance
92 GHRS Instrument Handbook 5.0
8.4 Standard Patterns for Substepping and Background
Measurement
This table shows how eachSTEP­PATT pattern measures the background for a science
exposure. Listed are the STEP­PATT number, the number of spectrum bins for which
substepping occurs, the number of background bins measured, the diodes used to mea­
sure the background, the fraction of the total time spent measuring flux on the science
diodes, the gratings for which theSTEP­PATT is appropriate, and the shortest exposure
that can be used for that pattern. The shortest exposure has been computed assuming
COMB=4, which is the recommended value. These minimum exposures can be reduced
by half if COMB=2 is used and by a factor of four forCOMB=NO.
TABLE 8­5
STEP­PATT specifications
STEP­
PATT
number
Bins Measured Spectrum/
Backgr
Ratio
Diodes used for
Background
On­target
Efficiency
Appropriate
Gratings
Minimum
Exposure
Time (sec)
Spectrum Background
1 1 0 1 1.00 all 0.8
2 2 0 1 1.00 all 1.6
3 4 0 1 1.00 all 3.2
4 2 2 8 science 0.89 first­order 14.4
5 4 2 8 science 0.94 first­order 27.2
6 2 2 8 science 0.89 echelle 14.4
7 4 2 8 science 0.94 echelle 27.2
8 2 2 8 corner 0.89 echelle 14.4
9 4 2 8 corner 0.94 echelle 27.2
10 2 2 1 science 0.50 first­order 3.2
11 4 2 1 science 0.67 first­order 4.8
12 2 2 1 science 0.50 echelle 3.2
13 4 2 1 science 0.67 echelle 4.8
14 2 2 1 corner 0.50 echelle 3.2
15 4 2 1 corner 0.67 echelle 4.8
TABLE 8­6 Default STEP­PATT for science modes
Grating Order STEP­PATT
First­order 1 5
Ech­A ¨ 51 9
< 51 7
Ech­B ¨ 31 9
< 31 7

GHRS Instrument Handbook 5.0 93
Reference Tables for Instrument Performance
8.5 The Effects of Reddening in the Ultraviolet
The table above lists extinction from interstellar reddening at ultraviolet wavelengths.
More information on the effects of reddening may be found in the bibliography in Chap­
ter 9. The above values are from Code et al. (1976). Seaton (1979) has provided conve­
nient fits to ultraviolet extinction forx = 1/l, with l in microns:
TABLE 8­7
Average normalized ultraviolet extinction as a function of wavelength.
Wavelength (å) A l /E(B­V) Wavelength (å) A l /E(B­V)
1100 11.70 2160 10.10
1200 10.20 2200 9.85
1300 9.19 2300 8.75
1400 8.54 2400 7.92
1500 8.29 2500 7.30
1600 8.03 2600 6.82
1700 7.85 2700 6.41
1800 7.90 2800 6.10
1900 8.38 2900 5.85
2000 9.05 3000 5.65
2100 9.90 3300 5.16
x range X(x) = A l /E(B--V)
2.70 ¸ x ¸ 3.65
3.65 ¸ x ¸ 7.14
7.14 ¸ x ¸ 10
1.56 1.048x 1.01 x 4.60
--
( ) 2 0.280
+
[ ]
+ +
2.29 0.848x 1.01 x 4.60
--
( ) 2 0.280
+
[ ]
+ +
16.17 3.20x
-- 0.2975x 2
+

Reference Tables for Instrument Performance
94 GHRS Instrument Handbook 5.0
8.6 Instrumental Properties
8.6.1 The Point Spread Function
The illustration below shows the PSF for the GHRS after the installation of the COS­
TAR mirrors, to allow for estimation of the degree of scattered light in the vicinity of a
bright object. Note that this PSF is only one­dimensional, and was only measured on
one side of the center of the aperture at that; in fact the true PSF has two­dimensional
structure. The relative throughput of the SSA at the wavelength at which this measure­
ment was done (1450 å) is 0.55.
Figure 8­7. Normalized Point Spread Function for the GHRS. The upper
curve is for the LSA and the lower for the SSA, both normalized to 1.0 at the
origin.

GHRS Instrument Handbook 5.0 95
Reference Tables for Instrument Performance
8.6.2 Detector Dark Count and the CENSOR Option
Each Digicon diode has its own discriminator, and if they are properly set the dark count
rate measured is very low. On­orbit measurement has shown that both GHRS detectors
have a dark rate less than the design goal of 0.01 counts diode ­1 sec ­1 at low geomag­
netic latitudes and that the dominant noise source is cosmic rays. During SAA passage,
the noise increases to a maximum of about 1 count diode ­1 sec ­1 . The scheduling soft­
ware for HST uses known contours of the SAA and does not accumulate counts when
the spacecraft is within those contours.
Even outside the SAA, observations show that ``dark'' counts tend to come in bursts.
Approximately 15% of dark counts are produced by events that occur within a time of8
µs or less. The CENSOR feature in Accumulation Mode allows you to ignore integra­
tions with such high dark rates. CENSOR works by summing all 512 channels of the
Digicon every 8 µs, and if that sum exceeds a threshold a coincident event is recorded.
The flight software maintains a record of how many coincidence events occurred within
a given STEP­TIME and can reject and repeat a subexposure for which the coincidence
sum exceeds a specified level. The coincidence circuit reacts only to strong events that
trigger about 8 diodes simultaneously, so that only the high­amplitude tail of back­
ground events can be rejected, even if the specification is to reject anySTEP­TIME
with a non­zero coincidence sum. It is estimated that use ofCENSOR can reduce the
dark count rate by about 20%. UsingCENSOR=YES is unlikely to degrade an observa­
tion (unless the object is bright enough to cause a consistently high count rate), but the
benefit is also low: only about a 10% gain in S/N in favorable cases.
The dark count rate is highly uniform over the diodes, so that a mean dark count rate is
an excellent representation of what happens at the detector.
Summary:
. The pre­flight noise specification was 0.01 counts diode ­1 sec ­1 . The average mea­
sured value is 0.005 for D1 and 0.008 for D2.
. The background is sensibly constant between --20 and +20 o geomagnetic latitude.
. At ±40 o geomagnetic latitude, the extrema ofHST's orbit, the rate is twice that at the
geomagnetic equator. Typical average rates are 0.007 counts per diode per second
for D1 and 0.011 for D2.
. The background rate is correlated with the cosmic ray and trapped particle flux. Cal­
culations show that the dark noise can be accounted for by cosmic­ray­induced Cer­
enkov radiation in the faceplates of the Digicons.
. The background due to the direct penetration of cosmic rays into diodes is very low
(0.0004 counts diode ­1 sec ­1 ).
When using CENSOR:
. Use the default STEP­TIME of 0.2 sec.
. Do not use CENSOR if the expected count rate exceeds about 100 counts per second
per diode because real photon events will be rejected. In a severe case, all the data
could be lost.
. Using CENSOR can drop the noise level by about 20%. The nominal Side 2 dark
count rate is 0.011 counts per second per diode, soCENSOR=YES can lower it to
0.0088.

Reference Tables for Instrument Performance
96 GHRS Instrument Handbook 5.0
. The following table shows the expected effects of using CENSOR. Rate is the target
count rate per diode per second; the second column shows the standard S/N and the
third column lists the S/N achieved withCENSOR=YES if the dark is 80% of its
value without CENSOR. Signal­to­noise was calculated on a per diode basis for a
nominal exposure time of 10,000 seconds (which is reduced to 9,800 ifCENSOR is
used because of the loss of 2% of the exposures).
Note that for rates exceeding one per second the effective loss of exposure time (due
to 2% of the exposures being rejected) more than offsets any reduction of the noise.
At a rate of 0.1CENSOR has essentially no effect on S/N but for lower ratesCEN­
SOR can help.
TABLE 8­8 Effects of CENSOR
Rate Standard S/N S/N with CENSOR=YES
100 999.94 989.91
10 316.05 312.91
1 99.45 98.56
0.1 30.02 30.01
0.01 6.90 7.22
0.005 3.95 4.21
0.001 0.91 1.00

GHRS Instrument Handbook 5.0 97
Reference Tables for Instrument Performance
8.6.3 Noise Rejection with FLYLIM
FLYLIM is a special commanding option for rejecting noise in cases where the source
signal is much weaker than the noise level. The idea is that asHST circles the Earth it
finds itself in different noise environments due to the changing magnetic field, and that
influences the detected noise background. Also, the background noise occurs as discrete
events from radiation in the space environment. If the source count level is well below
the background noise, then spectra integrated over a sufficiently short interval that con­
tain multiple counts are probably just noise, and should be rejected, whereas spectra
with single counts are more likely to contain real information. The rejected spectra are
discarded, which wastes observing time, but there is a net gain in signal­to­noise. A test­
in­principle run in 1993 showed that the net background rate could be reduced to as low
as 0.002 counts sec ­1 diode ­1 for the Side 2 detector, which was a factor of four
improvement over the mean dark rate. This gain was achieved at the cost of the loss of
about 25% of the individual 0.2 secondSTEP­TIME integrations.
By ``special commanding'' we mean that FLYLIM is requested as a comment on the
Phase II proposal, rather than as an optional parameter. The normal scheduling system
cannot automatically invoke FLYLIM; manual intervention is necessary. A detailed
description of FLYLIM cannot be provided here, but if users believe thatFLYLIM may
be of help in their science program they should note the following:
. The use of the FLYLIM parameter may not be assumed by the proposer but instead
must be arranged in advance, i.e., before the Phase I proposal deadline, through con­
sultation with a GHRS Instrument Scientist. The TAC will be made aware that
FLYLIM may require additional resources to implement.
. If your program that usesFLYLIM is approved, we will attempt to execute it on a
best efforts basis, but limited resources may prevent that.
. At the time this is writtenFLYLIM has not been fully tested and its use is at the
observer's risk.

Reference Tables for Instrument Performance
98 GHRS Instrument Handbook 5.0
8.6.4 Count Rate Linearity
Deviations from linearity in the way in which the Digicons count photons at high rates
were illustrated above in Figure7­2 on page 71. The effective deadtime for the GHRS
detectors has been measured to be 10.2µs for detector D1. The same value has been
assumed to hold for D2. Deviations from linearity are imperceptible below 10 3 and can
be corrected to an accuracy of 1% up to a measured count rate of 20,000 (in units of
counts diode ­1 s ­1 ).
8.6.5 Image Stability
The images formed by the Digicons are vulnerable to the effects of the Earth's magnetic
field. Over the course of a full orbit, the amplitude of the motion is about 50 microns per
Gauss for D2 (about 15 microns peak­to­peak) and 10 microns per Gauss for D1. The
50 micron motion seen in D2 corresponds to the size of a diode. This geomagnetically­
induced image motion (``GIMP''), together with thermal effects, is the underlying reason
for breaking up long exposures into segments of no more than about 5 minutes each.
8.6.6 Wavelength Calibrations
The GHRS has two spectrum calibration lamps, although only lamp SC2 now operates.
Lamp SC1 should not be specified under any circumstances. Both are platinum­neon
hollow cathode lamps manufactured by Westinghouse, providing a rich array of emis­
sion lines throughout the ultraviolet region that the GHRS observes. The lines are bright
enough so that a 30 second exposure will yield a good comparison spectrum at almost
any wavelength, although longer times are needed at some echelle settings. The lamps
are designated SC1 and SC2 and are selected by usingWAVE as the target specification
with SC1 or SC2 as the aperture. Each lamp, in fact, has its own aperture, offset from
the two science apertures of the GHRS. SC2 forms its spectrum at the samey deflection
as the SSA, but displaced inx (the direction of dispersion). SC1 is nearly aligned with
the SSA in x but differs in y by about 130 deflection steps. The lamp apertures are each
67 microns square and they form Gaussian­shaped images with FWHM = 1.1 diode
widths.
A wavelength calibration exposure made with the star in the SSA may be contaminated
by the stellar spectrum. This is because the SSA has no shutter and the fact that the SC2
aperture is in line with the SSA. This contamination is rarely a serious problem, how­
ever, because it is possible to subtract the stellar component. Also, such exposures are
usually used to only confirm the zero­point of the spectrum and not to obtain a full
wavelength solution.
For a comprehensive listing of the platinum lines, see Reader et al. (1990).
8.6.6.1 Aperture Offsets
The light from the spectrum calibration lamps does not enter the spectrograph along the
same path that starlight takes. This introduces a wavelength shift that must be corrected
for in solving for the wavelength solution. The data reduction software incorporates cor­
rections that were determined during pre­flight ground testing (new values are being
measured in Cycle 4).

Reference Tables for Instrument Performance
99 GHRS Instrument Handbook 5.0
8.6.6.2 Thermal Effects
The image formed by the Digicons is also affected by the thermal environment within
the GHRS, which in turn is influenced by whatever electronics happen to be on or off in
the GHRS and the other instruments. The temperature inside the GHRS can be moni­
tored and the image motion calibrated. This correction is also provided for in the data
reduction software. However, this correction is applied only once to a given Exposure
Logsheet line. We recommend that the exposure times for individual Exposure Logsheet
lines be kept shorter than about one hour as long as you do not encounter problems with
using too much on­board memory (see Section4.6 on page 47).
8.6.6.3 Geomagnetic Effects
As for overall image stability, geomagnetic effects influence wavelength stability. Long
exposures should be divided into units of about 5 minutes each, the time over which the
wavelength scale does not change measurably.

Reference Tables for Instrument Performance
100 GHRS Instrument Handbook 5.0

GHRS Instrument Handbook 5.0 101
Chapter 9 GHRS Bibliography
9.1 Ultraviolet Extinction 102
9.2 GHRS­Related Technical Papers 102
9.3 GHRS Scientific Papers 104
9.4 Acknowledgments 109

GHRS Bibliography
102 GHRS Instrument Handbook 5.0
We have tried to make this Handbook a comprehensive guide to using the Goddard
High Resolution Spectrograph, but some of the best information on the instrument and
the uses to which it can be put can be found in the open literature. Here we provide three
lists. The first provides additional information on interstellar reddening in the ultravio­
let. The next is technically oriented, and gives papers that provide detailed information
on specific aspects of the GHRS. The final list is of scientific papers that have used
GHRS data.
9.1 Ultraviolet Extinction
``Ultraviolet Photometry from the Orbiting Astronomical Observatory. II. Interstellar
Extinction.''
Bless, R.C., and Savage, B.D. 1972, ApJ, 171, 293--308.
``Studies of Ultraviolet Interstellar Extinction with the Sky­survey Telescope of the TD­1
Satellite.''
Nandy, K., Thompson, G.I., Jamar, C., Monfils, A., and Wilson, R. 1976, A&A, 51,
63--69.
``Empirical Effective Temperatures and Bolometric Corrections for Early­Type Stars.''
Code, A.D., Davis, J., Bless, R.C., and Hanbury Brown, R. 1976, ApJ, 203, 417--
434.
``Interstellar Extinction in the UV''
Seaton, M.J. 1979, MNRAS, 187, 73P--76P.
``Observed Properties of Interstellar Dust''
Savage, B.D., and Mathis, J.S. 1979, ARA&A, 17, 73--112.
9.2 GHRS­Related Technical Papers
``Ultraviolet High­Resolution Spectroscopy from the Space Telescope.''
Ebbets, D.C., Brandt, J.C., and the HRS Investigation Definition Team 1983, PASP,
95, 543--549.
``Wavelengths and Intensities of a Platinum/Neon Hollow Cathode Lamp in the Region
1100--4000 å''
Reader, J., Acquista, N., Sansonetti, C.J., and Sansonetti, J.E. 1990, ApJS, 72,
831--866.
``Status of the Goddard High Resolution Spectrograph in May 1991.''
Ebbets, D.C., Brandt, J., Heap, S. 1991, in The First Year of HST Observations,
edited by A.L. Kinney and J.C. Blades, p. 110­122,

GHRS Instrument Handbook 5.0 103
GHRS Bibliography
``Scattered Light in the Echelle Modes of the Goddard High Resolution Spectrograph
Aboard the Hubble Space Telescope. I. Analysis of Prelaunch Calibration Data.''
Cardelli, J.A., Ebbets, D.C., and Savage, B.D. 1990, 365, 789--802.
``Scattered Light in the Echelle Modes of the Goddard High Resolution Spectrograph
Aboard the Hubble Space Telescope. II. Analysis of Inflight Spectroscopic Observa­
tions.''
Cardelli, J.A., Ebbets, D.C., and Savage, B.D. 1993, ApJ, 413, 401--415.
``Resolution and Noise Properties of the Goddard High Resolution Spectrograph''
Gilliland, R.L., Morris, S.L., Weymann, R.J., Ebbets, D.C., and Lindler, D.J. 1992,
PASP, 104, 367--382.
This last paper is especially recommended for its discussion of the deconvolution of
the effects of the Point Spread Function (PSF) and Line Spread Function (LSF) of
HST and the GHRS.
``Final Report of the Science Verification Program for the Goddard High Resolution
Spectrograph for the Hubble Space Telescope''
Ebbets, D.C. 1992, prepared for NASA/Goddard Space Flight Center by Ball Aero­
space Systems Group.
This is a technical document prepared by Ball to fulfill a contractual requirement. It
provides a detailed description of the tests and calibrations performed during the
Science Verification phase that occurred immediately after the launch ofHST. We
cite it here for completeness, but a General Observer should usually be able to get
the information that he or she needs from thisHandbook or by consulting us.

GHRS Bibliography
104 GHRS Instrument Handbook 5.0
9.3 GHRS Scientific Papers
A number of GHRS­related papers are concentrated in three special volumes whose
contents will not be itemized here:
. The First Year of HST Observations, 1991, edited by A.L. Kinney and J.C. Blades,
and published by STScI.
. Astrophysical Journal Letters, volume 377, number 1, 1991.
. Science with the Hubble Space Telescope, 1992, edited by P. Benvenuti and E.
Schreier, and published by ESO.
1992:
``The Abundance of Boron in Three Halo Stars''
Duncan, D.K., Lambert, D.L., and Lemke, M. 1992, ApJ, 401, 584--595.
``Ultraviolet Observations of the Gas Phase Abundances in the Diffuse Clouds T
oward
Zeta Ophiuchi at 3.5 Kilometers per Second Resolution''
Savage, B.D., Cardelli, J.A., and Sofia, U.J. 1992, ApJ, 401, 706--723.
``Fractionation of CO in the Diffuse Clouds Toward Zeta Ophiuchi''
Sheffer, Y., Federman, S.R., Lambert, D.L., and Cardelli, J.A. 1992, ApJ, 397, 482--
491.
``Highly Ionized Atoms Toward HD 93521.''
Spitzer, L., and Fitzpatrick, E.L. 1992, ApJ, 391, L41--L44.
``Ultraviolet and Optical Spectral Morphology of Melnick 42 and Radcliffe 136a in
30 Doradus''
Walborn, N.R., Ebbets, D.C., Parker, J.W., Nichols­Bohlin, J., and White, R.L. 1992,
ApJ, 393, L13--L16.
``Detection of a Proton Beam During the Impulsive Phase of a Stellar Flare''
Woodgate, B.E., Robinson, R.D., Carpenter, K.G., Maran, S.P., and Shore, S.N.
1992, ApJ, 397, L95--­L98.
1993:
``Interstellar Mg II and C IV Absorption Toward Mrk 205 by NGC 4319: An `Optically­
Thick' QSO Absorption System''
Bowen D.V., and Blades, J.C. ApJ, 403, L55--L58.
``Observations of 3C 273 with the Goddard High Resolution Spectrograph on the Hub­
ble Space Telescope''
Brandt, J.C., et al. 1993, AJ, 105, 831--846.

GHRS Instrument Handbook 5.0 105
GHRS Bibliography
``The Galactic Halo and Local Intergalactic Medium toward PKS 2155--304''
Bruweiler, F.C., Boggess, A., Norman, D.J., Grady, C.A., Urry, C.M., and Kondo, Y.
1993, ApJ, 409, 199--204.
``Ultraviolet Transitions of Low Condensation Temperature Heavy Elements and New
Data for Interstellar Arsenic, Selenium, Tellurium, and Lead''
Cardelli, J.A., Federman, S.R., Lambert, D., and Theodosiou, C.E. 1993, ApJ, 416,
L41--L44.
``Abundance of Interstellar Carbon Toward Zeta Ophiuchi''
Cardelli, J.A., Mathis, J.S., Ebbets, D.C., and Savage, B.D. 1993, ApJ, 403, L17--
L20.
``Detection of Boron, Cobalt, and other Weak Interstellar Lines toward z Ophiuchi''
Federman, S.R., Sheffer, Lambert, D.L., and Gilliland, R.L. 1993, ApJ, 413, L51--
L54.
``Quantitative Spectroscopy of K647 --- the PNN of Ps1 in the Globular Cluster M15''
Heber, U., Dreizler, S., and Werner, K. 1993, Acta Astron., 43, 337--342.
``The Interstellar Abundances of Tin and Four Other Heavy Elements''
Hobbs, L.M., Welty, D.E., Morton, D.C., Spitzer, L., and York, D.G. 1993, ApJ, 411,
750--755.
``Time­Series Observations of O Stars. III. IUE and HST Spectroscopy of z Ophiuchi
and Implications for the `Photospheric Connection'''
Howarth, I.D. et al., 1993, ApJ, 417, 338--346.
``Hubble Space Telescope Spectra of the Phase­Modulated Wind in the SMC O+WR
Binary R31''
Hutchings, J.B., Morris, S.C., and Bianchi, L. 1993, ApJ, 410, 803--807.
``Deceleration of Interstellar Hydrogen at the Heliospheric Interface''
Lallement, R., Bertaux, J.­L., and Clarke, J.T. 1993, Science, 260, 1095--1098.
Provides a good illustration of geocoronal Ly­a with the LSA and Echelle­A.
``High Resolution UV Stellar Spectroscopy with the HST/GHRS, Challenges and
Opportunities for Atomic Physics''
Leckrone, D.S., Johansson, S., Wahlgren, G.M., and Adelman, S.J. 1993, Physica
Scripta, T47, 149--156.
``Goddard High Resolution Spectrograph Observations of the Local Interstellar
Medium and the Deuterium/Hydrogen Ratio Along the Line of Sight Toward Capella''
Linsky, J.L., Brown, A., Gayley, K., Diplas, A., Savage, B.D., Ayres, T.R., Landsman,
W., Shore, S.N., and Heap, S.R. 1993, ApJ, 402, 694--709.
``The Boron Abundance of Procyon''
Lemke, M., Lambert, D.L., and Edvardsson, B. 1993, PASP, 105, 468--475.

GHRS Bibliography
106 GHRS Instrument Handbook 5.0
``Detection of [O II]l2471 from the Io Plasma Torus''
McGrath, M.A., Feldman, P.D., Strobel, D.F., Moos, H.W., and Ballester, G.E. 1993,
ApJ, 415, L55--L58.
``A Search for Proton Beams During Flares on AU Microscopii''
Robinson, R.D., Carpenter, K.G., Woodgate, B.E., and Maran, S.P. 1993, ApJ, 414,
872--876.
``Observations of the Gaseous Galactic Halo Toward 3C273 with the Goddard High
Resolution Spectrograph''
Savage, B.D., Lu, L., Weymann, R.J., and Morris, S.L. 1993, ApJ, 404, 124--143.
``Goddard High Resolution Spectrograph Observations of Narrow Discrete Stellar Wind
Absorption Features in the Ultraviolet Spectrum of the O7.5III Starz Persei''
Shore, S.N., Altner, B., Bolton, C.T., Cardelli, J.A., and Ebbets, D.C. 1993, ApJ, 411,
864--868.
``The Early Ultraviolet Spectral Evolution of Nova Cygni 1992''
Shore, S.N., Sonneborn, G., Starrfield, S., Gonzalez­Riestra, R., and Ake, T.B.
1993, AJ, 106, 2408--2428.
``High­Resolution Ultraviolet Observations of the Interstellar Diffuse Clouds toward
µ Columbae''
Sofia, U.J., Savage, B.D., and Cardelli, J.A. 1993, ApJ, 413, 251--267.
``Composition of Interstellar Clouds in the Disk and Halo. I. HD 93521''
Spitzer, L., and Fitzpatrick, E.L. 1993, ApJ,409, 299--318.
1994:
``Interstellar and Intergalactic Magnesium and Sodium Absorption toward SN 1993J''
Bowen, D.V., Roth, K.C., Blades, J.C., and Meyer, D.M. 1994, ApJ, 420, L71--L74.
``Interstellar Detection of the Intersystem Line Si II] l 2335 toward z Ophiuchi''
Cardelli, J.A., Sofia, U.J., Savage, B.D., Keenan, F.P., and Dufton, P.L. 1994, ApJ,
420, L29--L32.
``GHRS Observations of Cool, Low­Gravity Stars. I. The Far­Ultraviolet Spectrum of a
Orionis (M2Iab)''
Carpenter, K.G., Robinson, R.D., Wahlgren, G.M., Linsky, J.L., and Brown, A. 1994,
ApJ, in press.
``Composition of Interstellar Clouds in the Disk and Halo. II.g 2 Velorum''
Fitzpatrick, E.L., and Spitzer, L. 1994, ApJ, in press.
``Search for CO Absorption Bands in IUE Far­Ultraviolet Spectra of Cool Stars''
Gessner, S.E., Carpenter, K.G., and Robinson, R.D. 1994, AJ, 107, 747--750.

GHRS Instrument Handbook 5.0 107
GHRS Bibliography
``Comparison of New Experimental and Astrophysical f­values for Some Ru II Lines,
Observed in HST Spectra ofc Lupi''
Johansson, S.G., et al. 1994, ApJ, 421, 809--815.
``Is There Primordial Gas in IZw 18?''
Kunth, D., Lequeux, J., Sargent, W.L.W., and Viallefond, F. 1994, A&A, 282, 709--
716.
``Interstellar Clouds toward Sirius and Local Cloud Ionization. I. GHRS Observations
of Sirius A''
Lallement, R., Bertin, P., Ferlet, R., Vidal­Madjar, A., and Bertaux, J.L. 1994, A&A, in
press.
``Interstellar Carbon Monoxide toward z Ophiuchi.''
Lambert, D.L., Sheffer, Y., Gilliland, R.L., and Federman, S.R. 1994, ApJ, 420, 756--
771.
An especially good discussion of how to achieve very high signal­to­noise with the
GHRS.
``High Velocity Plasma in the Transition Region of Au Microscopii: Evidence for Mag­
netic Reconnection and saturated Heating During Quiescent and Flaring Condi­
tions''
Linsky, J.L., and Wood, B.E. 1994, ApJ, in press.
``Probing the Galactic Disk and Halo. I. The NGC 3783 Sight Line''
Lu, L., Savage, B.D., and Sembach, K.R. 1994, ApJ, in press.
``Observing Stellar Coronae with the Goddard High Resolution Spectrograph.
I. The dMe Star AU Microscopii''
Maran, S.P., Robinson, R.D., Shore, S.N., Brosius, J.W., Carpenter, K.G., Wood­
gate, B.E., Linsky, J.L., Brown, A., Byrne, P.B., Kundu, M.R., White, S., Brandt,
J.C., Shine, R.A., and Walter, F.M. 1994, ApJ, 421, 800--808.
``Highly Ionized Gas Absorption in the Disk and Halo toward HD 167756 at 3.5 Kilo­
meters per Second Resolution''
Savage, B.D., Sembach, K.R., and Cardelli, J.A., 1994, ApJ, 420, 183--196.
``Al III, Si IV, and C IV Absorption toward z Ophiuchi: Evidence for Photoionized and
Collisionally Ionized Gas''
Sembach, K.R., Savage, B.D., and Jenkins, E.B. 1994, ApJ, 421, 585--599.
``A Search for Chromospheric Emission in A­type Stars Using the Goddard High Reso­
lution Spectrograph''
Simon, T., Landsman, W.B., and Gilliland, R.L. 1994, ApJ, in press.
``The Abundant Elements in Interstellar Dust''
Sofia, U.J., Cardelli, J.A., and Savage, B.D. 1994, ApJ, in press.

GHRS Bibliography
108 GHRS Instrument Handbook 5.0
``High­Resolution Spectra of Jupiter's Northern Auroral Ultraviolet Emission with the
Hubble Space Telescope''
Trafton, L.M., Gerard, J.C., Munhoven, G., and Waite, J.H. 1994, ApJ, 421, 816--
827.
``A Weak Diffuse Interstellar Band in the Far­Ultraviolet Spectrum of z Ophiuchi?''
Tripp, T.M., Cardelli, J.A., and Savage, B.D. 1994, AJ, 107, 645--650.
``G191--B2B: Accurate Abundances for Nitrogen, Silicon, and Iron from GHRS Obser­
vations''
Vidal­Madjar, A., et al. 1994, A&A, in press.

GHRS Instrument Handbook 5.0 109
GHRS Bibliography
9.4 Acknowledgments
It is easier to write a document like this for an instrument that has already been operat­
ing for several years, so we owe a debt to Dennis Ebbets and Doug Duncan, who com­
piled earlier versions of this Handbook when much less was known. Others who have
contributed to the success of the GHRS, especially in the technical areas that this docu­
ment treats, include D. Lindler, E. Malumuth, S. Shore, G. Wahlgren (and others at
Goddard Space Flight Center), and R. Gilliland and J. Skapik at STScI. The GHRS
Investigation Definition Team (IDT) is also thanked for their help and for the quality of
the instrument that they have provided to the astronomical community.

GHRS Bibliography
110 GHRS Instrument Handbook 5.0

GHRS Instrument Handbook 5.0 111
Glossary of T
erms and
Abbreviations
Here we provide definitions and explanations of technical terms and abbreviations used
in the text. The usual abbreviations found in HST­related documents (e.g., WFPC2,
FGS) are not repeated here.
Blaze Function
The efficiency of an echelle grating drops sharply as one moves away from blaze center.
The shape of the response function is virtually the same for the different orders and this
function is known as the Ripple Function (see Section8.3.3 on page 91).
Corner diodes
The detector area of the Digicons is laid out into specific diodes, each of which acts as
an independent detector. There are 500 science diodes, each of which is skinny but tall,
four focus diodes (see below), and four corner diodes. The corner diodes are large rect­
angles (0.1 â 1 mm) of detector area above and below the science diodes and are used
for measuring background.
Cycles
Proposals to use HST are solicited and reviewed on roughly an annual basis. However,
because HST's properties changed fundamentally when COSTAR and WFPC2 were
installed, Cycle 3 was defined to end at the time of the Servicing Mission. Cycle 4 began
at the end of SMOV. Cycle 5 is due to begin in mid 1995.

Glossary of Terms and Abbreviations
112 GHRS Instrument Handbook 5.0
CVZ
Continuous Viewing Zones. The inclined orbit of HST allows for uninterrupted obser­
vations of objects in some declination ranges at certain times. See theCall for Proposals
for further information.
DEFCAL
Short for Deflection Calibration. All GHRS acquisitions begin with aDEFCAL, which
measures the instantaneous location of the images on the onboard spectrum lamps and
then compares that location to the nominal coordinates stored in the onboard database.
The differences can range over several deflection steps in response to thermal and mag­
netic drifts. The offsets are applied to the database coordinates of the science apertures
to provide an updated estimate of their location. In practice, only observations of lamp
SC2 are used because of the loss of Side 1 and because using SC2 decreases the time
interval between the DEFCAL and the target locate phase of the acquisition.
Focus diodes
See Chapter 7 to see how the diodes in the GHRS Digicons are configured. At both ends
of the array of 500 science diodes are two focus diodes. The focus diodes are smaller
than the science diodes and are square. The image of the LSA is deflected to the focus
diodes to generate MAPs and IMAGEs. The focus diodes are 25 microns square.
GIMP
Geomagnetically­induced image motion problem. This problem underlies our recom­
mendation to have no single exposure be longer than about 5 minutes in length. See
Section 8.6.5 on page 98.
GSC
Guide Star Catalog, the list used to find stars upon which the Fine Guidance Sensors can
lock to control the pointing of HST.
LSA
Large Science Aperture. This is a square opening at the front of the GHRS that is used
to acquire stars and for some science observations. Its dimensions were 2.00 arcsec
square before COSTAR is installed and 1.74 arcsec square afterwards. The name used
for the LSA will continue to be ``2.0''.
OSS
Observation Support System, which is the facility located at STScI for real­time interac­
tion between the ground and theHST spacecraft.

GHRS Instrument Handbook 5.0 113
Glossary of Terms and Abbreviations
Phase I
A Phase I proposal forHST includes just the information need by the Telescope Alloca­
tion Committee (TAC) and STScI to judge scientific merit and technical feasibility. In
addition to the scientific justification, you are asked to provide a list of the targets that
you wish to observe and a brief description of the observations themselves. We recom­
mend adding comments to provide a clearer explanation of what you intend, even if they
are not required.
Phase II
The Phase II proposal is written once the Phase I proposal has been accepted for the
HST science program. The Phase II proposal includes all the detailed specifications that
are needed to turn your science program into the commands that the spacecraft will exe­
cute. As with Phase I, we recommend the liberal use of comments to help ensure that
your goals will be achieved.
Ripple Function
See Blaze Function
SAA
South Atlantic Anomaly. A region lying over southeastern South America where the
earth's radiation belts dip low, leading to high particle background rates for satellites in
Low Earth Orbit. GHRS observations are suspended during passage through the SAA.
Side 1, Side 2
GHRS is split into two ``sides,'' one for the short­wavelength detector (D1) and one for
the long­wavelength detector (D2). The sides operate independently but depend on each
other for communication with the spacecraft. The installation of the GHRS Repair Kit
during the HST Servicing Mission has meant that all GHRS communications are now
through Side 2. Moreover, Side 2 now solely controls the grating carrousel and LSA
shutter.
SMOV
Servicing Mission Orbital Verification, the period of time immediately after the Servic­
ing Mission in which the basic capabilities of the telescope and instruments are verified.
SPYBAL
SPectrum Y BALance. ASPYBAL consists of a quick observation of the spectrum cali­
bration lamp SC2 at a standard wavelength setting to ensure that the spectrum is prop­
erly centered on the diodes in the cross­dispersion direction. They position at this
standard wavelength is compared to a stored value and the difference is applied to the
observations made with the proposal configuration until anotherSPYBAL is done. A
SPYBAL is normally done before each new use of a different spectrum element, such as

Glossary of Terms and Abbreviations
114 GHRS Instrument Handbook 5.0
a grating. The resultant spectrum is provided to the observer and can be used to improve
the default wavelength calibration.
SSA
Small Science Aperture. The nominal (pre­COSTAR) size was 0.25 arcsec square, but
after COSTAR it is 0.22 arcsec square. The name for this aperture will continue to be
``0.25''.
STEIS
Space Telescope Electronic Information Service. This service provides on­line news,
information, and documents via anonymous ftp. To use it, ftp to stsci.edu (Internet node
130.167.1.2) and login with usernameanonymous, using your last name as password.
Use get to transfer the README file in the entry directory; this will provide a general
explanation of how to access STEIS information. For more details, consult the User
Support Branch.
STEP­PATT
STEP­PATT is the pattern of operations undertaken in anACCUM. A typical STEP­
PATT defines the relative proportions of time spent accumulating on the science diodes
versus time with the background diodes. See Section8.4 on page 92.
STEP­TIME
STEP­TIME is the exposure time for the smallest unit of an exposure. For example, dur­
ing an acquisition, STEP­TIME is the amount of time spent at each dwell point while
executing a spiral search pattern. During anACCUM, the detector integrates for aSTEP­
TIME before reading the diodes and adding their contents to the memory. A unit of
STEP­TIME is spent executing each portion of aSTEP­PATT, for example.

GHRS Instrument Handbook 5.0 115
Glossary of Terms and Abbreviations

GHRS Instrument Handbook 5.0 115
Index

116 GHRS Instrument Handbook 5.0
A
Accumulation Mode 13, 19, 34, 43--45
CENSOR 44
details 95--96
CENSOR, see also noise rejection
COMB 15, 44
DOPPLER 44
FP­SPLIT 14, 30, 43, 44
high signal­to­noise methods 107
parameter summary 44
STEP­PATT 15, 44, 92, 114
STEP­TIME 44
substepping 19
acquisition parameters
ACQ/PEAKUP, see acquisition parameters,PEAKUP
BRIGHT and FAINT limits 17, 26, 37, 41
calculating 68
BRIGHT=RETURN 17, 25, 34, 37, 41, 71
BRIGHT=RETURN compared to specifying limits 37
DEC­OFF 40
ETA­OFF 40
EXTENDED 39
LOCATE 17, 41
LOCATE=EXTENDED 8
MAP 37, 40, 41, 42
MAP=ALL­POINTS 40
PEAKUP 15, 34, 38, 40, 41
STEP­TIME 71
RA­OFF 40
SEARCH­SIZE 36, 37, 40
Special Requirements 41
STEP­TIME 71
for PEAKUP 71
maximum permitted 38
STEP­TIME as COMMENT 41
summary 40
XI­OFF 40
acquisitions 15, 17, 24, 34, 35--41, 68--80
early 16, 18, 25, 35, 36
extended objects 39, 76
faint objects 39
DARK TIME 39, 80
Side 2 acquisition for Side 1 observing 8, 38
using FOS 8, 39
flux measurement 17
initial pointing 35
interactive 16, 18, 24, 35
offseting
and guide stars 40
offsetting 36, 39
onboard 17, 36, 40
parameter summary 40
real time, see acquisitions, interactive
reddening 68
see also targets
spiral search 36, 37
targets, see targets
apertures 12
angular scale 9
LSA 9, 12, 17, 18, 28, 34, 35, 36, 38, 41, 42, 85, 1
12
shutter 12, 28
nomenclature 9
physical locations 58
physical size 60
plate scale 9
SSA 9, 12, 17, 28, 34, 38, 41, 42, 114
B
background
echelle interorder 19
geocoronal Lyman­a 80
LSA shutter 12
measurement 92
scattered light 87, 88
echelles 103
bandpass 87, 88
blaze function 91, 111
C
calibrations
precision and accuracy 46
wavelength 34, 47, 98
aperture offsets 98
contamination from SSA 98
geomagnetic effects 99
lamps 98
line widths 98
tables 102
thermal effects 99
carrousel 14
CENSOR
see noise rejection
Continuous Viewing Zone 16, 112
conventions
text 7
units 8
coordinated observations 47
cosmic rays 95
CVZ, see Continuous Viewing Zone
Cycle 5 111
D
DARK TIME Special Requirement 39, 80
data quality 43
deadtime correction
see paired pulse correction
deconvolution 103
DEFCAL 112
detectors 60
count rate linearity 98
D1 14
D2 14
dark count 95
Digicon, see Digicon
granularity 14
image stability 98
sampling 15

GHRS Instrument Handbook 5.0 117
Digicon 14
paired pulse correction 71
diodes
corner 28, 92
focus 42, 43, 112
science 92
diodes, corner 111
Doppler shift 19, 44, 46
E
early acquisition, see acquisitions, early
e­mail addresses 2
exposure time calculation 26
extinction, ultraviolet 93, 102
F
fluxes, variable 18
FLYLIM
see noise rejection
FOC 18, 24, 35, 36, 43
G
geocoronal Lyman­a 27, 80, 105
geomagnetically­induced image motion 47, 98, 112
GHRS
as slitless spectrograph 43
paired pulse correction 71
photometric precision 13
Point Spread Function 94
resolving power 12, 83, 103
scientific papers 104
STScI contacts 2
technical papers 102
time resolution 13
useful wavelength range 12
granularity 14
gratings
cross­dispersers 60
Ech­A 87
Ech­B 88
echelle 12, 14, 60
bandpass and sensitivity 87
blaze function 91, 111
resolving power 83
scattered light 87
wavelength format 89
first­order 12, 14, 60, 85
resolving power 83
useful ranges 83
holographic 14
GSC 35, 112
H
HST memory capacity 13, 16, 47
I
IMAGE Mode 16, 20, 40, 42--43
as slitless spectrograph 43
critically sampling 42
DELTA­X, DELTA­Y 42
maximum STEP­TIME 43
NX, NY 42
parameter summary 42
PRECISION 43
interactive acquisitions, see acquisitions, interactive
L
Large Science Aperture, see apertures, LSA
Line Spread Function (LSF) 103
M
MAP, see acquisition parameters,MAP
mirrors 36, 40, 42, 60
A1 14, 25
A2 14
attenuated 14
Cam­A 14
Cam­B 14
effective areas 78
N1 14
N2 14
normal 14
moving targes, see targets, moving
N
NO GAP Special Requirement 34
noise rejection
CENSOR 27, 95
FLYLIM 9, 27, 45, 97
noise, fixed­pattern 30
O
OSCAN Mode 20, 45
see also parameters for Accumulation Mode
OSS 112
P
paired pulse correction 71
peakup, see acquisition parameters,PEAKUP
Phase I 22, 113
acquisitions 24
Phase II 22, 113
photometric precision 13
plate scale 9
Point Spread Function 94, 103
R
RAPID Mode 13, 16, 20, 45, 46
SAMPLE­TIME 20, 46
Rapid Readout Mode, see RAPID Mode
reddening 27, 93, 102
effect on acquisition count rates 68
resolution, time 13
resolving power 12, 83
ripple function
see blaze function

118 GHRS Instrument Handbook 5.0
S
SAA, see South Atlantic Anomaly
sampling 15
scattered light 28, 87, 88
SEARCH­SIZE, see acquisition parameters,SEARCH­
SIZE
sensitivity 26
sensitivity function 85
Side 1 8, 12, 113
Side 2 12, 113
signal­to­noise estimation 29
slitless spectrograph mode 43
Small Science Aperture, see apertures, SSA
South Atlantic Anomaly 16, 19, 27, 95, 113
spectrograph temperature 47
Spectrum Y Balance, seeSPYBAL
spiral search, see acquisitions, spiral search
SPYBAL 19, 113
STEIS 10, 114
STEP­TIME 114
substepping 92
T
targets
bright problems 25
complex 38
coordinates 18
crowded fields 25, 35
extended 8, 18, 26
faint 26, 39
moving 18, 25, 35
nearby neighbors 18
offsetting 36, 39
point source 36
variable 18, 26, 35
Targets of Opportunity 47
telephone numbers 2
time resolution 13
W
wavelength tables 102
WFPC2 18, 24, 35, 36
WSCAN Mode 20, 45
see also parameters for Accumulation Mode
WAVE­STEP 45