Space Telescope Science Institute  Appendix A: Orbit Calculation Examples A.2 FGS

A.1 ACS

Suppose that you wish to use the ACS to observe a target at a declination of +35 degrees, so that there are 54 minutes of visibility time per orbit (see Table 6.1). Your desired exposures are listed in Table A.1, and the associated overheads are listed in Table A.2. In Table A.3 you can see how the observations fit into a total of 2 orbits.


Table A.1: ACS Example: Planned Exposures
Config
Mode
Spectral
Element
Number of Exp
Time per Exp [m]
Notes
ACS/WFC
IMAGING
F606W
4
8.5
Pattern (see Section 5.4) with an offset of 25" between positions
ACS/HRC
IMAGING
F814W
2
6
CR-SPLIT
ACS/HRC
IMAGING
F606W
2
6
CR-SPLIT
ACS/HRC
IMAGING
F435W
2
7
CR-SPLIT

Table A.2: ACS Example: Incurred Overheads
Overhead
Overhead Time [m]
Notes
see
Table
Guide star acquisition
6
First orbit in new visit
Guide star re-acquisition
5
Per orbit after first orbit
WFC exposure (first)
4
First exposure in a series of identical exposures
WFC exposure (not first)
2.5
Per exposure after the first exposure in a series
HRC exposure (first)
2.5
First exposure in a series of identical exposures
HRC exposure (not first)
1
Per exposure after the first exposure in a series
SAM of 25"
0.67
Small Angle Maneuver between positions in the pattern

Table A.3: ACS Example: Orbit Planning
Activity
Duration [m]
Elapsed Time [m]
Orbit 1


Guide star acquisition
6
6
WFC/F606W Exp Time
4x8.5
40
WFC/F606W Overhead
4+(3x2.5)+(3x0.67)
53.5
Unused time
1
54.5
Orbit 2


Guide star re-acquisition
5
5.5
HRC/F814W Exp Time
2x6
17.5
HRC/F814W Overhead
2.5+1
21
HRC/F606W Exp Time
2x6
33
HRC/F606W Overhead
2.5+1
36.5
HRC/F435W Exp Time
2x7
50.5
HRC/F435W Overhead
2.5+1
54
Unused time
0.
54.5
Appendix A: Orbit Calculation Examples A.2 FGS
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