A.3 FGS Example

The FGS is used to observe a binary star named Binary01, as well as five reference stars (ref1, ref2, ref3, ref4, ref5). All stars are in the same FGS field of view, and can therefore be observed in one visit (see Section 6.2.2). Stars ref4 and ref5 have magnitude V = 14.6, and all the other targets have 13.0 < V < 14.0. To enable the removal of drift and jitter in the post observation analysis of the data, the Binary01 is observed in POS mode several times (twice in this example) and the reference stars are each observed twice. The desired exposures are listed in Table A.10,

•	The targets have a declination of +42°, which gives 57 minutes of visibility time per orbit (see Table 6.1).

•	The associated overheads are listed in Table A.11.

Therefore, a preliminary observing strategy can be created by the user to fit the observations in one orbit, as shown in Table .

Table A.10: FGS Example: Planned Exposures

Config	Mode	Spectral element	Number of exposures	Time per exposure (minutes)	Notes
FGS	POS	F583W	8	0.6	Two exposures of target Binary01, and two exposures for each of the targets ref1, ref2, and ref3
FGS	POS	F583W	4	1.1	Two exposures for each of the targets ref4 and ref5
FGS	TRANS	F583W	1	13.4	20 scans of 40 sec each (see Table 6.6) for Binary01

Table A.11: FGS Example: Incurred Overheads

Overhead	Overhead time (minutes)	Notes	see Table
Guide star acquisition	6	First orbit in new visit	6.2
POS	1	Per exposure, V < 14	6.7
POS	2	Per exposure, 14 < V < 15	6.7
TRANS	1	Per target	6.7
TRANS	0.2	Per scan	6.7
Instrument Setup	4	Once every orbit	6.8
Instrument Shutdown	3	Once every orbit	6.8

Table A.12: FGS Example: Orbit Planning  

Activity	Duration (minutes)	Elapsed time (minutes)
Orbit 1
Guide star acquisition	6	6
Instrument Setup	4	10
Binary01/POS mode Exp Time	0.6	10.6
Binary01/POS mode Overhead	1	11.6
ref1/POS mode Exp Time	0.6	12.2
ref1/POS mode Overhead	1	13.2
ref2/POS mode Exp Time	0.6	13.8
ref2/POS mode Overhead	1	14.8
ref3/POS mode Exp Time	0.6	15.4
ref3/POS mode Overhead	1	16.4
ref4/POS mode Exp Time	1.1	17.5
ref4/POS mode Overhead	2	19.5
ref5/POS mode Exp Time	1.1	20.6
ref5/POS mode Overhead	2	22.6
Binary01/POS mode Exp Time	0.6	23.2
Binary01/POS mode Overhead	1	24.2
ref1/POS mode Exp Time	0.6	24.8
ref1/POS mode Overhead	1	25.8
ref2/POS mode Exp Time	0.6	26.4
ref2/POS mode Overhead	1	27.4
ref3/POS mode Exp Time	0.6	28
ref3/POS mode Overhead	1	29
ref4/POS mode Exp Time	1.1	30.1
ref4/POS mode Overhead	2	32.1
ref5/POS mode Exp Time	1.1	33.2
ref5/POS mode Overhead	2	35.2
Binary01/TRANS mode Exp Time	13.4	48.6
Binary01/TRANS mode Overhead	1+(20x0.2)	53.6
Instrument Shutdown	3	56.6
Unused	0.4	57.0