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 total time spent on the target is 35.7 minutes, with a visibility period of 54 minutes. 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 and processing them through APT; 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 (Table 10.4) shows the overheads for one visit of a coronagraphic observation with two identical visits (acquisitions) in adjacent orbits with a roll of the spacecraft between orbits. The overhead associated with the spacecraft roll is accounted for by the scheduling software; it therefore does not appear in this table.
