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| GGGGGGG HH HH RRRRR SSS |
STScI | GG HH HH RR R SS | STAN - GHRS
Analysis | GG GGG HHHHHH RRRRR SS | Number 14
News | GG GG HH HH RR R SS | July 1996
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CONTENTS:
- GHRS NEWS
+ GHRS Operations
+ Small Science Aperture Flux Discrepancy
+ Personnel Changes
- RECENT HST SAFEMODE EVENT
- APPENDIX: GHRS CONTACTS
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GHRS Operations
---------------
GHRS operations in recent months have been very smooth, with only rare
instances of data loss due to carrousel resets, for example. As we
approach the end of the GHRS mission, GHRS usage will intensify. We
are currently working with the PRESTO group to schedule and execute all
GHRS science programs before the Servicing Mission.
Small Science Aperture Flux Discrepancy
---------------------------------------
The GHRS team recently found that the absolute flux scales
for the same star observed in both apertures were significantly
different. We expect the count rate for a star to drop by about 30%
when it is in the SSA compared to what is seen in the LSA. This is due
to loss of light in the wings of the PSF. Although, the indicated flux should
not change if the data are properly reduced.
We are investigating the source of this discrepancy and will
alter the calibration reference files so that they yield correct fluxes.
We anticipate that this problem will affect very few users since observers
who want reliable fluxes generally use the LSA.
Personnel Changes
-----------------
As STScI prepares for the Servicing Mission, several GHRS group members
will be transferring to the STIS and NICMOS groups. The Contact Scientists
will retain responsibility for their assigned Cycle 6 GHRS programs, so GOs
will not see a decline in user support for their programs.
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Recent HST Safemode Event
--------------------------------
by K. Rudloff and R. Doxsey
HST entered inertial hold safemode at 17:31 UT on Saturday, July 13th.
For reasons that are yet to be understood, one of the four elements of the
on-board quaternion (which represents HST's idea of where it is pointed)
jumped by the equivalent of 7 degrees. The sudden jump caused a loss
of guide star lock just before an earth occultation. As the occultation
ended, a safemode check was performed to determine whether HST was pointed
at the earth. Because the quaternion was in error by 7 degrees, the check
failed and safemode was entered (even though HST was sufficiently clear
of the Earth and there was no risk to the FGSs). The safemode entry
proceeded normally and all Instrument responses were as expected.
An investigation of the problem by flight software staff is in progress.
The discontinuous jump is consistent with a single bit flip in the words
holding the data, but the cause of the bit flip is still unknown. There
is no simple diagnostic which can be run on the computer system without
additional risk, so, for the foreseeable future, one of the on-board tape
recorders will be devoted to recording engineering data. This will provide
a means for diagnosing the problem in the event that it is repeated.
There have been no signs of additional problems since the event on
July 13th, and HST returned to normal science operations on Wednesday
evening, July 17th. STScI staff are actively modifying the schedules to
fit within the constraints of using a single science tape recorder.
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APPENDIX: GHRS contacts
-----------------------
Any questions about the scheduling of your observations should be addressed to
your Program Coordinator. Post-Observation questions can be addressed to your
Contact Scientist.
Analysis, STSDAS or any other questions can also be addressed to help@stsci.edu.
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