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GHRS Instrument Handbook 4.1 103
Addendum 1 Advisory:
Potential Data Corruption
A1.1 The Problem: Incorrect Doppler Compensation 104
A1.2 How to Tell if Data Are Affected 105
A1.3 Fixing the Problem 105

Advisory: Potential Data Corruption
Advisory: Potential Data Corruption
Advisory: Potential Data Corruption
GHRS Instrument Handbook 4.1 104
This Addendum to Version 4.0 of the GHRS Instrument Handbook describes a problem
that has been discovered in the flight software for the Goddard High Resolution Spec­
trograph, how that problem may affect GHRS observations that already exist, how you
can tell if data in your possession is corrupted, and how we plan to fix the problem. This
discussion pertains not just Cycle 4 but to current observations and all previously
obtained GHRS data.
A1.1 The Problem: Incorrect Doppler Compensation
HST orbits the earth at a speed of 7.6 km s
­1 , meaning that spectra obtained on opposite
sides of the orbit of an object inHST's orbital plane could be shifted by as much as 15
km s ­1 . That is significantly more than the resolution of the GHRS in echelle mode
(about 3 km s ­1 ) and a substantial fraction of the resolution when using the medium res­
olution gratings (about 12 km s ­1 ). For this reason the flight software that controls the
operation of the GHRS includes a means of active compensation of the motion of the
spectrum over the course of an orbit. This is done with theDOPPLER parameter that
you specify on an Exposure Logsheet line with anACCUM. The default is for the doppler
compensation to be on while GHRS is observing an e
xternal target (such as a star) and
off while on an internal target (such as aWAVE exposure).
On a few occasions over the past year we have noticed instances in which a subspectrum
of a star (for example, one part of anFP­SPLIT) was obviously corrupted because all
or part of that subspectrum was obtained with an incorrect doppler compensation. While
these few instances were obvious (they were grossly different than the normal spectra
obtained immediately before or after and resembled the spectrum of a double­lined
binary), the source of the problem was not so obvious, and it is only in the past month
that the underlying cause has been determined, thanks to the effort of Harry Garner at
Ball Aerospace, in Boulder.
The doppler correction consists of a component that depends on the position of the
object being observed relative to HST's orbital plane at the time of observation. That
component is sensibly constant over the course of an observation. The variable compo­
nent depends on the sine of the phase angle ofHST in its orbit, and it is symmetric
(except for sign) in the four quadrants of the orbit. A new value of the doppler compen­
sation is calculated every minute, and this sine component is determined by looking in a
pre­loaded table of values. The actual compensation is effected as a change in the mag­
netic deflection of the spectrum within the detector, and there is no motion of the carrou­
sel. The index for the look­up table is computed from the number of minutes since the
start of the quarter of the orbit in whichHST is currently circling the earth, relative to
the direction to the object.
The problem arises under certain circumstances near the earth's terminator, where the
doppler compensation is usually greatest (for an object in the anti­solar direction). The
original flight software was written based on an incorrect value of the orbital period, a
value that was slightly too small. On occasion the software finds itself looking for a
position in the sine table that is beyond the bounds of the table. A value of zero is
returned, and the doppler compensation is then wrong.

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GHRS Instrument Handbook 4.1 105
The incorrectly shifted portion of the spectrum can only last for one or two minutes at
the most, so corruption of a spectrum will be most flagrant when a v
ery bright star is
being observed in a short exposure, and that star happens to lie in or near HST's orbital
plane. However, the problem can corrupt other observations in subtler ways, and we
encourage and advise you to carefully determine if your data might be compromised
before proceeding with a detailed analysis.
It may be possible, in principle, to reconstruct a corrupted spectrum, b
ut we recommend
that suspect data be discarded. In almost all cases such loss should not compromise your
science goals; if it does we recommend that you consult us.
A1.2 How to Tell if Data Are Affected
J. Eisenhamer at STScI has added a parameter with the taskobsum (which is in stsdas
under calhrs) that will identify potentially corrupted GHRS spectra, based on informa­
tion in the headers. To obtain this updated software:
. Go to your home directory for iraf (usually~/iraf).
. Logon to STEIS (ftp to stsci.edu and enter anonymous as the username and
your last name as a password).
. Enter ``cd instrument_news/ghrs/misc'' to descend to the directory where
this information resides.
. Enter ``mget *obsum*'' to pull over to your computer the files you will need.
These files will beREADME.obsum and obsum.tar.
. Enter ``quit'' to exit STEIS.
. Examine the README.obsum file, which will provide a detailed recipe on how you
can update your local version of obsum to provide information on the doppler cor­
rection for individual spectra. The new help file for obsum also has useful informa­
tion.
If you run into problems, please contact us.
A1.3 Fixing the Problem
In the short­term, this doppler compensation problem will be fixed by changing the
table that provides the values of the sine function so as to be large enough to obviate the
problem for the current orbital period or any reasonable value of it. That should occur
by the end of March, 1993. Thus observations obtained in April, 1993, and thereafter
should be free of this problem. This fix is likely to work as a long­term solution as well,
although that is being investigated.

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