Space Telescope Science Institute   8.3 Orbit Use Determination Examples  8.3.2 Sample Orbit Calculation 2:

8.3.1 Sample Orbit Calculation 1:


Consider a target to be imaged with WFC in a given filter in one orbit. Using the Exposure Time Calculator (ETC), we find that we need 2400 seconds of exposure time to reach the desired level of signal-to-noise ratio. Given that the observation must be split into a series of two exposures by CR-SPLIT (CR-SPLIT=2), we map the overheads and the science exposure times onto the orbit as follows:


Table 8.3: Orbit calculation for example 1.
Action
Time (minutes)
Explanation
Orbit 1
Initial guide-star acquisition
6
Needed at start of observation of a new target
WFC overhead for the first exposure
4.0
Includes filter change, camera set-up, and readout
First science exposure
20.0

WFC overhead for the subsequent science exposure in the series
2.5
Includes readout
The next science exposure in the series
20.0

Total time for science exposures
40.0

Total used time in the orbit
52.6

Thus, the two WFC exposures totaling 2400 seconds make full use of the typically available time in one orbit. The exposure times can be adjusted if the actual target visibility time differs from the derived total used time.


 8.3 Orbit Use Determination Examples  8.3.2 Sample Orbit Calculation 2:
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