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SPACE TELESCOPE SCIENCE INSTITUTE
15 March 2005
Denise Taylor
FGS Tutorial
Unlike the other instruments on HST, the FGSes are interferometers. Each FGS
has a star selector field with a 5” square instantaneous field-of-view (IFOV) that can
be moved anywhere in the pickle. The IFOV can perform multiple scans across a target (TRANS mode), or can record wave interference at a particular target position
(POS mode).
FGS proposals have become more straightforward than they were years ago.
Most of the engineering parameters are gone.
General Characteristics of Proposals
1. Orbit structure: one alignment per orbit, one orbit per visit.
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SPACE TELESCOPE SCIENCE INSTITUTE
15 March 2005
Denise Taylor
2. Most common Special Requirements:
♦ ORIENT - needed to determine scan direction in TRANS mode; needed to
keep all targets in pickle in POS mode.
♦ BETWEEN - used for monitoring and parallax observations.
3. Pointing:
♦ TRANS mode observations only contain one target, usually at POS TARG
(0,0) position. HST does not move during observation; the IFOV does the
scanning.
♦ POS mode observations need multiple, reference targets. Alignment is pointed
with a POS TARG on one target, and all exposures must be included in a SEQ
NON-INT requirement. All reference targets must have -REF at the end of
their names. This forces the single-alignment structure and prevents the telescope from moving during the observation. This applies for mixed POS and
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SPACE TELESCOPE SCIENCE INSTITUTE
15 March 2005
Denise Taylor
TRANS mode observations, also. An alternate method for setting up POS
mode pointing is to place SAME POS AS requirement on each exposure that
does not have the POS TARG specification. Then, the -REF designation is not
necessary for the reference target names.
4. Most common prep problems:
♦ Lack of guide stars due to ORIENT, BETWEEN and only 2 FGSes available.
Roughly 25% of all FGS observations are performed on single guide stars.
♦ Targets out of the pickle due to bad ORIENT or no ORIENT requirement. You
MUST check this in FOV before setting to flight ready. Most PIs have no way
of checking this. Adjust or add the ORIENT requirement as necessary. All
POS mode observations MUST have an ORIENT requirement.