Fine Guidance Sensor Instrument Handbook

6.4

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 APT proposal processing algorithm. They are explicitly pointed out here for illustration.

6.4.1 Pos Mode Overheads

Table 6.5, Pos Mode Overheads lists the estimated overheads associated with guide star acquisitions, the initial configuring (standby to operate) of the FGS for science observing, the time for slewing the IFOV from target to target in the observing plan, the acquisition of target stars, and finally configuring the FGS at the end of the observing sequence (operate to standby).

The acquisition time of an astrometric target depends upon the target’s magnitude. Fainter stars require longer intergration times (FESTIME) of the fine error signal during the walkdown to finelock. For example, the FESTIME used to acquire a V=10 star is 25 msec, while for a V=16 star FESTIME=3.2 seconds. Columns 2 and 3 in Table 6.6, Pos Mode Overheads and Exposure Time vs. Magnitude list an average overhead for each exposure and the recommended total exposure time to be entered on the phase2 proposal (see Table 4.5, Recommended FGS1r Exposure Times) as a function of target magnitude. Total time required for a given exposure is estimated by combining columns 2 and 3. The additional overheads indicated in Table 6.5, Pos Mode Overheads must be included when determining the total time available for collecting science data. Fortunately, APT calculates and includes all of these overheads when processing a phase2 proposal.

Table 6.5: Pos Mode Overheads

Initial guide star acquisition

6 min

Two-guide star FineLock (5 min. for re-Acq)

Initial instrument configuration

4 min.

Incurred at the beginning of every orbit

IFOV slew to target

17 sec + 6 x dist (in arcmin)

for slews between targets in the FOV.

Astrometry target acquisition:

•

•	CoarseTrack

•

FineLock

•

5 sec

•	13, 21 sec

•	varies as V

(Each science target and reference star)

•	For a 10 arcsec search radius.

•	For V < 14 and V > 14 respectively

•	For V<12: ~3 sec; For V>16: ~400 sec

Instrument shut down

2 min.

Incurred at the end of every orbit

Table 6.6: Pos Mode Overheads and Exposure Time vs. Magnitude
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

For total spacecraft time, add columns 2 and 3.

6.4.2 Trans Mode Overhead

The overheads for the activities which occur during Trans mode exposures are listed in Table 6.7, Trans Mode Observing Overheads. The guide star acquisitions, instrumental configurations at the beginning and end of the observing sequence, and astrometry target acquisition activities for Pos mode and Trans mode are identical through the CoarseTrack phase. The FineLock phase in Transfer mode does not depend upon the star’s magnitude. (The star is not really acquired in FineLock. Setting the FineLock flag allows for the transition to fringe scanning under the control of the 486 SSM computer.)

Table 6.7: Trans Mode Observing Overheads