Space Telescope Science Institute   8.3.1 Sample Orbit Calculation 1:  8.3.3 Sample Orbit Calculation 3:

8.3.2 Sample Orbit Calculation 2:


This example illustrates the impact of short WFC exposures on the useful time in the orbit. We have one orbit to observe a target with WFC in two filters, so the observation consists of two series, each with two identical CR-SPLIT exposures. The ETC has shown that at the minimally accepted signal-to-noise ratio the exposure time must be 540 seconds for each of the filters, so each of the CR-SPLITs must be at least 270 seconds long. For the target declination, we find that the visibility time is 55 minutes. The time budget for the orbit is then as follows:


Table 8.4: Orbit calculation for example 2.
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 exposures in two series
×4 = 8
Includes filter change, camera set-up, and readout
WFC overhead for subsequent exposures in each of the two series
×2.5 = 5
Includes readout
Additional overhead for all but the last exposures in the orbit
5.8 ×3= ~17
Needed to dump the buffer because the next exposure is too short (> 339 seconds) to accommodate the dump time.
Science exposures
×4.5 = 18

Total time for science exposures
18

Total used time in the orbit
54

Comparing with the previous example, we see that although with the adopted minimum exposure times we can squeeze the observation into one orbit, the efficiency of the orbit use is very low because of the large overheads associated with buffer dumps. However, if we increase each of the four exposure times so that they are larger than 339 seconds, we avoid these additional overheads. This would free ~17 minutes of the orbit time for science, which allows us to almost double the science exposure time (35 minutes instead of 18 minutes) and thus significantly improve signal-to-noise.

Similarly, a subarray can be used to readout only a fraction of the detector, allowing more frames to be stored in the buffer before requiring a dump. In this example, using four WFC1-1K subarrays for 4 short (t < 339 seconds) exposures would save 176 seconds in readout time and 1047 seconds in dump time. This frees up ~20 minutes of orbit time to be used for science.


 8.3.1 Sample Orbit Calculation 1:  8.3.3 Sample Orbit Calculation 3:
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