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Near Infrared Camera and Multi-Object Spectrometer Instrument Handbook for Cycle 17 > Chapter 10: Overheads andOrbit Time Determination > 10.3 Orbit Use Determination

10.3 Orbit Use Determination
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.
10.3.1
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 57 minutes. The orbit requirement is summarized in Table 10.3.
4 seconds for MULTIACCUM overhead
4 seconds for MULTIACCUM overhead
4 seconds for MULTIACCUM overhead
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (from NIC2 to NIC3) + Reconfigure Instrument
move on-target in NIC3 plus instrument reconfiguration (change focus from NIC2 to NIC3), and filter wheel motion
4 seconds for MULTIACCUM overhead
4 seconds for MULTIACCUM overhead
4 seconds for MULTIACCUM overhead
4 seconds for MULTIACCUM overhead
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.
Coronagraphic Overhead Example
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.
2 0.284 s F171M acquisition exposures
25 0.6 s dark exposure to remove persistence
1 191.96 s science exposure
Repeat 2 without filter change
 

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