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GRIM II User's Manual
Alan Watson, Mark Hereld, and Bernie Rauscher 15 March 1997


`Jeeves,' I said, `haveyou ever pondered on Life?' `From time to time, sir, in my leisure moments.' `Grim, isn't it, what?' `Grim, sir?' `I mean to say, the di erence between things as they look and things as they are.' |P.G. Wodehouse anticipating the advantages of infrared astronomy in Very Good, Jeeves!

i


Contents
1 Introduction 2 Optics 3 Detector
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 1.1 GRIM II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 User's Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Mailing List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1 1 2

Detector Characteristics . . . Detector Operation . . . . . . Bias and Dark Current . . . . Response Uniformity . . . . . Non-Linearity and Saturation Bad Pixels . . . . . . . . . . . Residual Image . . . . . . . . Peculiarities . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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.5 .5 .6 .6 .6 . 12 . 12 . 14 . . . . . . . . . . . . . .

3 5

4 Operation

4.1 REMARK . . . . . . . . 4.2 MC . . . . . . . . . . . . 4.2.1 Commands . . . 4.2.2 Procedures . . . 4.2.3 Example Session 4.3 Hangs . . . . . . . . . .

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15 15 16 20 23 24

5 Imaging

5.1 Cameras . . . . . . . . . . . . . . . . 5.2 Neutral Density Filters . . . . . . . . 5.3 Filters . . . . . . . . . . . . . . . . . 5.3.1 Transmittances . . . . . . . . 5.3.2 Zero Points and Backgrounds 5.3.3 Broad Band Filters . . . . . . 5.3.4 Narrow Band Filters . . . . . 5.4 Bright Limits . . . . . . . . . . . . . ii

25

25 25 25 25 28 28 35 35


5.5 Flat Field Images . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 5.6 Basic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6 Spectroscopy
6.1 6.2 6.3 6.4

Capabilities Flat Fields Wavelength Absorption

...... ...... Calibration Standards

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39 39 39 41

7 Extinction 8 Standards

8.1 Photometric Standards . . . . . . . . . . . . . . . . . 8.1.1 Elias et al. JH K ,CO, and H2 O Standards . 8.1.2 Carter & Meadows JH K Standards . . . . . 8.1.3 Casali & Hawarden JH K Standards . . . . . 8.1.4 Wainscoat & Cowie K Standards . . . . . . 8.1.5 Manufactured K Standards . . . . . . . . . . 8.1.6 Absolute Calibration . . . . . . . . . . . . . . 8.2 Spectrophotometric Standards . . . . . . . . . . . . 8.2.1 Bohlin Spectrophotometric Standards . . . . 8.2.2 Manufactured Spectrophotometric Standards 8.3 Stellar Colours . . . . . . . . . . . . . . . . . . . . . 8.4 Stellar Spectra . . . . . . . . . . . . . . . . . . . . .
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9 Headers

48

iii


List of Figures
2.1 Optical Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 3.2 3.3 3.4 3.5 3.6 Bias Image (IRAF format 1 second) Bias Image (FITS format 1 second) Flat Field Image in K . . . . . . . . Ratio of Flat Fields Images in J and Non-linearity and Saturation . . . . Bad Pixels . . . . . . . . . . . . . . .
obj4 fexp

4

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4.1 The 4.2 The 5.1 5.2 5.3 5.4 5.5 5.6 5.7

procedure . . . . . . . . . . . . . . . . . . . . . . . . . 22 procedure (for n =5) . . . . . . . . . . . . . . . . . . . 23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 27 30 31 32 33 34

Camera Orientations for Imaging . . . . . . . . . . . Neutral Density Filter Transmittances . . . . . . . . Filter Transmittances . . . . . . . . . . . . . . . . . Filter Transmittances (continued) . . . . . . . . . . . Filter Transmittances (continued) . . . . . . . . . . . Filter Transmittances (continued) . . . . . . . . . . . Broad Band Filter and Atmospheric Transmittances

7.1 Model Atmospheric Transmitances . . . . . . . . . . . . . . . . . 43

iv


List of Tables
4.1 Modes Values and Names . . . . . . . . . . . . . . . . . . . . . . 17 4.2 Scale Values and Names . . . . . . . . . . . . . . . . . . . . . . . 17 4.3 Filter Values and Names . . . . . . . . . . . . . . . . . . . . . . . 18 5.1 5.2 5.3 5.4 Camera Con gurations for Imaging Filter Characteristics . . . . . . . . Filter Zeropoints and Backgrounds Imaging Dome Flat Characteristics . . . a .... .... .... t f=5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 29 29 36

6.1 Wavelength Coverage in m . . . . . . . . . . . . . . . . . . . . . 40 9.1 Header Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

v


Chapter 1

Introduction
1.1 GRIM II
GRIM II is the near infrared camera and low resolution spectrograph in service on the Apache Point Observatory 3.5 meter telescope. GRIM II has a 256 256 NICMOS-3 detector and works between 1.0 m and 2.5 m. In imaging mode it has three pixel scales of about 0.48, 0.24, and 0.11 arcsec/pixel and a large number of broad and narrow band lters. In spectroscopic mode it has three resolutions of about 200, 400, and 800. GRIM II was designed and built byMark Hereld, Bernie Rauscher, Scott Severson, and Bob Loewenstein of the Univeristy of Chicago with engineering support from Dale Sandford, Fred Mrozek, Dave Fischer, and Je Sundwall.
How should GRIM II and APO be acknowledged in publications?

1.2 User's Manual
This manual has two purposes: to provide a basic introduction to near infrared imaging and low-resolution spectroscopy for astronomers familiar with optical CCD imaging and spectroscopy and to describe the speci cs of performing such observations with GRIM II. The authors of this manual are Alan Watson alan@oldp.nmsu.edu Mark Hereld hereld@bucephalus.uchicago.edu Bernie Rauscher B.J.Rauscher@durham.ac.uk Direct and indirect contributions have also been made by Eddie Bergeron Jon Brinkmann Nancy Chanover Karen Gloria Jon Holtzman Mike Ledlow 1


Bob Loewenstein Dan Long James Rhoads Scott Severson This manualis anevolving document the lastest version is available as
ftp://oldp.nmsu.edu/pub/alan/grim/man.ps.Z

Versions are identi ed by the date on the cover. If you nd errors or have suggestions for improvements, please send them to the authors.

1.3 Mailing List
Another source of information on GRIM II is the GRIM II mailing list maintained byMichael Strauss. An archive of mail sent to the list and instructions for subscribing to and sending mail to the list are available from
http://www.astro.princeton.edu/APO/apo35-grim/INDEX.html

2


Chapter 2

Optics
The optical layout of GRIM II is shown in Figure 2.1. All of the optical components are contained in a cryogenic dewar (which is purple and very pretty). The f=10 beam from the telescope enters the dewar and comes to a focus at the slit wheel. After that it passes through the eld lens, collimator lens, grism wheel, two lter wheels, and then the lenses and fold mirrors of one of the three cameras mounted on the camera carousel, before coming to a focus on the detector. The grism wheel contains an aperture stop, the grism, and 3%, 13%, and 25% transmission neutral density lters. The aperture stop is a circular aperture without a central obscuration to block the high-emissivity central hole in the primary or rotating vanes to block the spiders. The lter wheels contain a large number of broad and narrow band lters along with a solid plate known as the `blank-o ' or `dark' lter. The f=5 camera has only one fold mirror but the f=10 and f=20 cameras havetwo. Thus, the image formed bythe f=5 camera is ipped about one axis compared to the images formed by the other two (see x5.1). The throughputs of the cameras is slightly di erent as they eachhave di erent optics. The oblique re ections within GRIM II and from the tertiary mirror induce polarization. Since GRIM II and the tertiary rotate with repect to each other under normal circumstances, there is a photometric modulation of even unpolarized sources. The amplitude of this modulation is though to be in the vicinityof 1% RMS. Polarized sources will also su er from modulations and o sets because of instrumental polarization.

3


Accessory Ring

Slit Wheel

Field Lens

Collimator Lens

Detector

Camera Carousel

Grism and Filter Wheels

Figure 2.1: Optical Layout

4


Chapter 3

Detector
3.1 Detector Characteristics
GRIM II has a 256 256 NICMOS-3 detector. The detector is sensitive from 0.8 m to 2.5 m. The long wavelength cuto lies at the red end of the atmospheric K window and is su ciently short that low backgrounds can be obtained by cooling the detector and optics to 77 K with liquid nitrogen. The device is split into quadrants eachof which has its own ampli er. The quadrants are read simultaneously. The detector has a gain of about 4.7 electrons/DN and an e ective read noise of about 110 electrons. Thus, the read noise is larger than the Poisson noise for exposures of less than about 2500 DN.

3.2 Detector Operation
GRIM II does not have a shutter. Instead, exposures are controlled electronically. GRIM II operates in a `reset, read, read' or `double correlated sampling' mode this mode has lower noise than the `reset, read' mode, whichisanalogous to themodeinwhich CCDs are operated. An exposure begins with a reset, which sets the bias in each pixel. A short time after being reset, the chipisread. The read is non-destructive and merely samples the voltage in each pixel. After a further time the chip is read once more. The GRIM II controller does not allowmultiple rst and second reads. The signal n is the di erence between the rst and second reads. The time between the reset and the rst read is about 0.95 seconds. The time between the two reads is the exposure time t and is t =0:901 + OPENTIME (3.1) in seconds where OPENTIME is a header value. The exposure time is about 0.2 seconds less than the time requested using REMARK or MC. The shortest exposure time possible is 1.0 seconds (corresponding a requested time of 1.2 seconds). 5


3.3 Bias and Dark Current
Between the rst and the second reads, the signal chain bias level changes. Thus, even a short dark exposure has values which are far from zero. To obtain the true signal, one must subtract a bias image. The amountbywhich the bias level changes depends on the exposure time in the sense that longer exposures have more negative bias levels. For this reason, one must construct a separate bias image for each exposure time used. Since bias images can only be constructed from dark exposures, subtracting a bias image also removes the dark current. As the dark current is small, this has a negligible e ect on subsequent corrections for non-linearity. Dark images can be taken using the `dark' or `blank o ' lter. IRAF images obtained with GRIM II have a constant of 10000 DN added to them by the MC. This allows the values to be comfortably represented as 16-bit unsigned integer. FITS images obtained with GRIM II do not have this constant added because BZERO header value can be used to achieve the same ends. The values in short dark exposures are about ;2000 DN for FITS images and 8000 DN for IRAF images. 1 second bias images are shown in Figure 3.1 for IRAF images and Figure 3.2 for FITS images. The bias uctuates, giving rise to the bands seen in the lower rows of each quadrant. The amplitude of the uctuation seems to depend primarily on the exposure, so that the bands largely disappear when two similar exposures are subtracted but remain when two di erent exposures are subtracted. In consequence, the bands largely disappear in the course of the normal processing of ob ject frames (as sky frames are subtracted). Unfortunately they remain in ats (as a low exposures are subtracted from a high exposures) at the few percent level. One approach to this problem which seems to work well is normalizing each row individually so that the central columns of the at have the same mean. Low-level diagonal banding in the lower right quadrant has also been reported.

3.4 Response Uniformity
The response of the detector in GRIM II is fairly uniform compared to many NICMOS-3 detectors. Figure 3.3 shows a at eld image in K . However, the response changes as a function of wavelength Figure 3.4 shows the ratio of at eld images in J and K .

3.5 Non-Linearity and Saturation
Since the in uence of the ARC board does not extend to waiving the laws of physics, the GRIM II detector is by necessity slightly non-linear. The nonlinearity is about 6% over the working range of about 28000 DN and the corrections for non-linearity can be as large as 10%. Furthermore, those unaware 6


Figure 3.1: Bias Image (IRAF format 1 second)

7


Figure 3.2: Bias Image (FITS format 1 second)

8


Figure 3.3: Flat Field Image in K

9


Figure 3.4: Ratio of Flat Fields Images in J and K

10


of the mode in which GRIM II operates can inadvertently expose the chip beyond its working range saturation can occur for signals as low as 13000 DN. Fortunately, the non-linearitycan be characterized and corrected and with forewarning it is not di cult to stay within the working range. The origin of the non-linearity suggests that its characteristics should be stable with time. The non-linearity in a near infrared array occurs because each pixel in the array is e ectively a capacitor. The pixel is read by sensing the voltage across the capacitor. At the start of the exposure, the capacitor is biased. Thereafter, it accumulates charge and the bias decreases. If the capacitance of the pixel were constant, the voltage would be linearly related to the accumulated charge. Unfortunately, the capacitance increases with decreasing bias and the relation between accumulated charge and voltage is sub-linear. Alan Watson and Nancy Chanover investigated and characterized the nonlinearity of GRIM II in June 1996. The full text of their report is available as
ftp://oldp.nmsu.edu/pub/alan/grim/lin.ps.Z

Their main conclusions are sumarized here. The non-linearity can be adequately modelled as a straight line, that is the signal N that would be accumulated by a truly linear detector can be related to the actual signal n by

n =(1 ; 1 N )N

(3.2)

where 1 is a constant. Inverting this expression to correct for the non-linearity is hampered bythe fact that the detector accumulates charge for a time t1 between the reset and the rst read and then a further time t between the rst read and the second read. As noted in x3.2, the actual exposure time is di erent both from the requested exposure time and the OPENTIME header value. The signal n is the di erence between the rst and second reads. (A bias image needs to be subtracted to obtain n.) The value of the signal N that would be given by a perfectly linear detector is

t +2 N = 1 ; (1 ; 42 1 (n(t +2t )t1 )= t) 1 1
1

1=2

t:

(3.3) (3.4) (3.5)

From eight sequences of increasing exposures, they found =2:18 10
;

6

and

t1 =0:95:

Contours of the fractional correction are shown in Figure 3.5 the corrections can be as large as 10%. 11


30000 25000 20000 Signal n
1.10
1.09

1.10

1.09
1.08

1.07
1.06
1.05
1.04

1.08 1.07
1.06
1.05 1.04

15000 10000 5000 0 0 1 2

1.03

1.03

1.02
1.01

1.02
1.01

3

4 5 6 Exposure Time t

7

8

9

10

Figure 3.5: Non-linearity and Saturation The full well of the detector is only about 28000 DN. Since charge accumulates between the reset and the rst read, saturation can occur well before the signal approaches this value. Toavoid saturation, the signal must be kept below the values indicated by the thick line in Figure 3.5. Thus, in a 1 second exposure the signal must be kept below about 13000 DN. (The `signal' in question here is the value in the image after subtracting the bias, which is about ;2000 for FITS images and about 8000 for IRAF images.)

3.6 Bad Pixels
The GRIM II detector is cosmetically excellent compared to many other similar detectors. Figure 3.6 shows an image of the bad pixels constructed by dividing a 15000 DN at eld from a 1500 DN at eld and agging pixels that deviated from the mean by more than 10%. Most of the bad pixels are isolated, although there are three clumps of bad pixels, one of which is fairly close to the center of the detector. The number of bad pixels is expected to increase slowly with time.

3.7 Residual Image
Like all NICMOS-3 detectors, the detector in GRIM II su ers from residual image. This has not been well characterized. The conventional wisdom is that taking a series of bias exposures helps to eliminate a residual image. 12
It might be useful to characterize this.


Figure 3.6: Bad Pixels

13


3.8 Peculiarities
Columns 128 and 256 are o (128 256), (256 128), and pixels (128 i) and (256 i) pixels (128 i + 1) and (256 set down by one row. The values in pixels (128 128), (256 256) in the image are garbage. The values in in the image actually correspond to the values in i + 1) on the detector.

14


Chapter 4

Operation
GRIM II can be controlled by normal users with either the REMARK interface or the MC interface. Both allow the telescope and instrument to be controlled over the Internet and both can automatically copy images to a remote host using FTP.

4.1 REMARK
REMARK runs on a networked Macintosh and provides a remote, graphical interface for controlling both the telescope and instruments. REMARK was written by Bob Loewenstein. The documentation for REMARK can be found on the APO home page (http://www.apo.nmsu.edu). At the time of writing, the documentation describing the operation of GRIM II using REMARK is incomplete.

4.2 MC
MC runs on tycho.apo.nmsu.edu and provides a command line interface for controlling both the telescope and instruments. MC was written by Brian Yanny. The documentation for MC can be found from the APO home page (http://www.apo.nmsu.edu). The advantage of the MC over REMARK is that it can be programmed procedures can be written to perform sequences of tasks such as exposures and o sets. An MC session can be started on tycho with
mcnode

An MC status display can be started on
mcnode -s

tycho

with It is also

in a 80 26 VT100-compatible window (e.g., useful to monitor the hub log on tycho with 15

xterm -geom 80x26).


tail -f /home/apotop/syslog/hub.log

In particular, FTP error messages appear in the hub log.

4.2.1 Commands

The most common MC commands are listed here. The descriptions are somewhat abbreviated and often do not show all of the options consult the MC documentation for more details on these commands and the less common commands.
priority

The priority command sets the priority of this MC session. A priority value of 0 allows only harmless commands a priority value of 1 allows all commands. A new MC session initially has a priorityof 0.

priority

grimmove

The grimmove command moves the slit wheel, grism wheel, lter wheels, and camera carousel in GRIM II. The numerical values of mode, scale, and filter are listed in Tables 4.1, 4.2, and 4.3. These values appear as the MODE, SCALE, and GFILTER header values. Occasionally, a move will fail and need to be repeated. The grimstatus command requests a message describing the status of the grim optical components. The numerical values returned correspond to the mode, scale, and lter values listed in Tables 4.1, 4.2, and 4.3.

mode scale f ilter

grimstatus

inst

The inst command sets the current instrument for the nexpose commands. An instrument value of grim speci es GRIM II. nexpose itime=itime n=n reduce='send>ftp'] The nexpose command takes n exposures eachof itime seconds. As noted in x3.2, the actual exposure time is about 0.2 seconds shorter than the value of itime. If reduce='send>ftp' is speci ed and an FTP connection has been established with the loginftp command, the image is FTP-ed to the remote host.
grimabort

instrument

The

slew

command aborts the current exposure. hh:mm:ss +j-]dd:mm:ss epoch=epoch The slew command executes a slew to the speci ed position. A slew can be aborted with the stop tcc command.
grimabort

16


Mode image grism image grism image image image

Value Name 0 image with slit 1 grism+slit with slit 2 image+slit without slit 3 grism with 3% ND lter 4 image+nd3 with 13% ND lter 5 image+nd13 with 25% ND lter 6 image+nd25 Table 4.1: Modes Values and Names

Scale Value Name f=5 1 f5 f=10 3 f10 f=20 5 f20 f=20 short 13 f20short f=20 long 21 f20long Table 4.2: Scale Values and Names

17


Filter open dark

J H K K Ks Kdar
0

1.08 1.09 1.24 1.28 1.58 1.64 1.70 1.99 2.12 2.17 2.21 2.25 2.26 2.30 2.34

k

m m m m m m m m m m m m m m m

Value Name 13 open 0 dark 1 j 2 h 3 k 4 kprime 5 ks 7 kdark 15 1.08 16 1.09 17 1.24 18 1.28 8 1.58 19 1.64 9 1.70 20 1.99 21 2.12 22 2.17 23 2.21 24 2.25 6 2.26 25 2.30 26 2.34

Table 4.3: Filter Values and Names

18


stop tcc

command aborts a slew. offset x y ty pe abs] The offset command executes an o set of x arcseconds in the x direction and y arcseconds in the y direction. If abs is speci ed, the o set is relative to the last slew, otherwise the o set is relative to the last o set of the given type. The type value can be inst or ob j to specify `instrument' or `ob ject' o sets. Instrument and ob ject o sets are independent. Ob ject o sets have a coordinate system that always has x west and y north. Ob ject o sets are re ected in the RA, DEC, RAOFF and DECOFF header values. Instrument o sets have a coordinate system which rotates with the instrument. At 0 degrees rotation, x is east and y is south. With the f=5 camera, the x and y axes of the coordinate system match the axes of the detector, but with the other cameras the coordinate system is ipped about the x axis. Instrument o sets are re ected in the the X and Y header values.
stop tcc rotate

The

command sets the rotator position angle to angle. object or horizon. The rotation is recorded ROTATION header values. An ob ject rotation of 0 degrees gives the tations shown in Table 5.1 and Figure 5.1. A horizon rotation of 0 d places the slit parallel with the horizon. The

angle type
rotate

type values can be

The in the orienegrees

focus

The focus command sets the telescope focus to f ocus. When an instrument is mounted, the operator normally sets the focus to something reasonable. The imdir command speci es the directory in which images are to be created. The directory must have write permission for all users. Images are created in /export/images by default.

f ocus

imdir

dir

pref ix places places filetype ty pe seq number
diskname

The diskname, places, filetype, and and type of image created. 19

seq

commands specify the name


The le name is pref ix, followed by the sequence number padded with _ zeros to a width of places,followed byeither .hhh or hhd if IRAF images are being written or .fit if FITS images are being written. The filetype command speci es that IRAF or FITS images are created depending on whether type is iraf or fits. The seq command sets the sequence number for the next exposure.
subscribe

The subscribe command determines which messages are printed in this MC session. Values for type are message, monitor, and status. Values of level are 0, 1,and 2,with 0 switching messages o . The MC is quite verbose. It is useful to have two MC session running simultaneously, with one being used only for commands and having all messages turned o and the other being used only for messages. The loginftp command establishes an FTP connection to the directory dir on host host. You will be prompted for a password.

type level

loginftp

hostname username

senddir=

dir

4.2.2 Procedures

MC procedures can be written in the TCL language. Information on TCL is available from http://www.NeoSoft.com/tcl/. Writing MC procedures is not trivial. The basic model is described by Brian Yanny in the MC documentation. A somewhat higher-level model has been suggested byAlan Watson in a message to the GRIM II mailing list on 8 July 1996. Anumber of useful procedures, which can also serve as examples from which to create your own, are available from
ftp://oldp.nmsu.edu/pub/alan/grim/mc.tcl

This le also exists on

tycho.apo.nmsu.edu

as
tycho.apo.nmsu.edu

~visitor1/alan/grim/mc.tcl mcnode

To use these procedures, start an MC session on command and then type:

with the

source ~visitor1/alan/grim/mc.tcl start config loginftp senddir=

hostname username

dir

Because of the way MC works, many of these procedures return before they have completed do not attempt to execute another command until the message stating that the procedure has completed. These procedures will attempt to FTP any exposures they take. The available procedures are: 20


start

This command must be issued before any others. This command should be issued to relinquish control of GRIM II. Query grim for the current mode, camera con guration, and lter and print them in recognizable forms. This command must be issued after the initial start command but before any others. Set the mode according to mode,which can be one of one of names listed in Table 4.1. Occasionally,a move will fail and need to be repeated. Set the scale according to scale, which can be one of names listed in Table 4.2. Occasionally,a move will fail and need to be repeated. Set the lter according to f ilter, which can be one of names listed in Table 4.3. Occasionally,a move will fail and need to be repeated.

end

config

mode

mode

scale

scale

filter

filter

Move the telescope arcsec north, south, east, and west. center xy Move an ob ject that is at pixel location (x y)tothe center of the detector.
exp

arcsec south arcsec east arcsec west arcsec
north

exptime n Take n exposures each of duration exptime. (The procedure actually requests an exposure of 0.2 seconds longer than exptime so that the actual exposure corresponds to the exptime. See x3.2.) obj1 exptime n sk y x skyy Take 2 sequences of n exposures each of duration exptime. The sequences
are ob ject then sky. The ob ject exposures are taken at the current position. The sky exposures are taken at an instrument o set of skyx and skyy arcsec. The telescope is left at the initial position. 21


d 4 1

d

5 skyy 3 0 2 skyx

8

d

6 d

7

Figure 4.1: The
obj4

obj4

procedure

exptime n skyx skyy d Take 8 sequences of n exposures each of duration exptime. The sequences

are sky, ob ject, ob ject, sky, sky, ob ject, ob ject, sky. The ob ject exposures are each centered at the corner of a square of side d arcsec centered on the current position. The sky exposures are each centered at the corner of a square of side d arcsec centered on an instrument o set of skyx and skyy. The telescope is left at the initial position. This is illustrated in Figure 4.1. The telescope starts at position 0, and then takes a sequence of exposures at positions 1, 2, 3, 4, 5, 6, 7, and 8, before nally returning to position 0 again.
home

Return the telescope to its position after the last slew command or the last center, north, south, east, or west procedure. This is useful if an obj1 or obj4 procedure has to be aborted.

fexp

exptime f start f end n Take a focus run of n exposures each of duration exptime with the focus set to values equally spaced between fstart and f