The easiest way to learn how to compute total orbit time requests is to work through examples. We provide below two examples. The first example describes a thermal IR observation, with the TWO-chop
pattern. The second example describes a coronagraphic acquisition and subsequent observations.
Observations at long wavelengths will be obtained for target A in NICMOS Camera 2 and 3. The F222M
filter is used in each of the two cameras in turn. The observer requires exposure times of 128 seconds in each exposure, in MULTIACCUM
mode. A good sequence for the target is considered to be STEP8
with NSAMP
=21. The target is extended and the selected chopping throw is one detector width. Note that this changes the time to chop for each camera. The NIC-TWO-CHOP
pattern is used to obtain background measurements.
The declination of the source is -40 degrees, so the visibility period during one orbit is 54 minutes. The orbit requirement is summarized in Table 10.3.
The total time spent on the target is 35.7 minutes, with a visibility period of 54 minutes. There is room for additional exposures with one or more filters.
Note that for multi-filter observations, exposures for all filters can be obtained at each pointing before moving to the subsequent pointing.
If the observation were of a moving target, the slews to the new targets would be taken up in the tracking overhead, and the small angle maneuvers (SAMs) would all take 0.25 minutes, regardless of the camera.
More detailed estimates may also be obtained by building test Phase II proposals; some observers may wish to use this approach for estimating time required for the observations. Not shown in the above example is one parallel memory dump.
The following table shows the overheads for one visit of a coronagraphic observation with two identical visits (acquisitions) in the same orbit with a roll of the spacecraft in between. The overhead associated with the spacecraft roll (between 2-9 minutes, see Table 10.1) is accounted for by the scheduling software; it therefore does not appear in this table, although it needs to be added to the tally for constructing the orbit.
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