Space Telescope Science Institute   10.2 NICMOS Exposure Overheads Chapter 11:  Techniques for Dithering, Background Measurement and Mapping

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 in the Thermal Regime Using a Chop Pattern and MULTIACCUM

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.

Table 10.3: Orbit Determination for Observations of Target A 
Action
Time (minutes)
Explanation
Orbit 1
Initial Guide Star Acquisition
6
Needed at start of observation of new target
19 seconds setup at beginning of each orbit
Science exposure, NIC2 F222M
2.2
128 seconds exposure time on target
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)
0.9
Move off-target, allow for filter wheel motion
Science exposure, NIC2 F222M
2.2
128 seconds exposure time on background
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)
0.9
Move on-target, allow for filter wheel motion
Science exposure, NIC2 F222M
2.2
128 seconds exposure time on target
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)
0.9
Move off-target, allow for filter wheel motion
Science exposure, NIC2 F222M
2.2
128 seconds exposure time on background
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (from NIC2 to NIC3) + Reconfigure Instrument
6.1
move on-target in NIC3 plus instrument reconfiguration (change focus from NIC2 to NIC3), and filter wheel motion
Science exposure, NIC3 F222M
2.2
128 seconds exposure time on target
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)
1.1
Move off-target, allow for filter wheel motion
Science exposure, NIC3 F222M
2.2
128 seconds exposure time on background
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)
1.1
Move on-target, allow for filter wheel motion
Science exposure, NIC3 F222M
2.2
128 seconds exposure time on target
4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)
1.1
Move off-target, allow for filter wheel motion
Science exposure, NIC3 F222M
2.2
128 seconds exposure time on background
4 seconds for MULTIACCUM overhead

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.

Coronagraphic Overhead Example

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.


 10.2 NICMOS Exposure Overheads Chapter 11:  Techniques for Dithering, Background Measurement and Mapping
Space Telescope Science Institute
http://www.stsci.edu
Voice: (410) 338-1082
help@stsci.edu