FGS Instrument Handbook for Cycle 11

Overheads

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The assessment of the total spacecraft time needed by a program should take into account the overheads associated with various types of observations. Overheads are incorporated into the RPS2 proposal processing algorithm.

Position Mode Overheads

Table 6.5 lists the estimated overheads associated with guide star and astrometry star acquisitions and slewing of the IFOV in the pickle. Columns 2 and 3 in Table 6.6 list an average overhead for each exposure and the recommended total exposure time (see Table 4.5), both presented as a function of target magnitude. Total spacecraft time is estimated by combining columns 2 and 3. Note that the overheads in Table 6.5 are based on FGS3 characteristics. Slight modifications (a few seconds) may be needed to accommodate FGS1r characteristics.

Table 6.5: Position Mode Overheads

Activity Description	Duration	Comments
Initial guide star acquisition	6 min	Two-guide star FineLock
IFOV slew to target	17 sec + 6 x dist (in arcmin)
Astrometry target acquisition: Search CoarseTrack FineLock Acquisition	5 sec 13 sec or 21 sec varies with V	(Each science target and reference star) For a 15 arcsec search radius. For V < 14 and V > 14 respectively For V<12: ~3 sec duration; For V>16: ~400 sec duration.
Guide star re-acquisition	5 min	Relevant for contiguous multi-orbit visits.

Table 6.6: Position Mode Overheads and Exposure Time vs. Magnitude for FGS3

V Magnitude	Estimated Overhead per exposure (minutes)	Exposure Time1 in seconds
8-12	0.6	20
13	0.7	20
14	2	30
15	3	45
16	8	45
17	8	50

1 For total spacecraft time, add columns 2 and 3.

Transfer Mode Overhead

The overheads for the activities which occur during Transfer Mode exposures are listed in Table 6.7. The guide star and astrometry star acquisition scenarios for Position Mode and Transfer Mode are identical through the CoarseTrack phase.

The average overhead for a Transfer Mode exposure is ~2 minutes.

Table 6.7: Transfer Mode Observing Overheads

Activity Description	Duration	Comments
Initial guide star acquisition	6 min	Two-guide star FineLock
Astrometry target acquisition: Search CoarseTrack	5 sec 13, 21 sec	(Each science target and reference star) For a 15 arcsec search radius. For V < 14 and V > 14 respectively
Time for IFOV to slow and reverse scan direction	15 sec/scan
Guide star re-acquisition	5 min	Relevant for contiguous multi-orbit visits.