NICMOS Instrument Handbook for Cycle 11

Orbit Use Determination

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The easiest way to learn how to compute total orbit time requests is to work through examples. We provide below two examples which should be representative of the many types of observations. The first example describes a thermal IR observation, with the TWO-chop pattern. The second example describes a coronagraphic acquisition and subsequent observations.

Observations in the Thermal Regime Using a Chop Pattern and MULTIACCUM

Observations at long wavelengths will be obtained for target A in all three NICMOS cameras. The F187N filter is used in each of the three 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 59 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 18 seconds setup at beginning of each orbit
Science exposure, NIC1 F187N	2.5	128 s exposure time on target, 4 s for MULTIACCUM overhead, 16 s for filter wheel motion
Small Angle Maneuver (chop)	0.8	Move off-target, allow for filter wheel motion
Science exposure, NIC1 F187N	2.2	128 seconds exposure time off target 4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)	0.8	Move back on-target, allow for filter wheel motion
Science exposure, NIC1 F187N	2.2	128 seconds exposure time on target 4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)	0.8	Move off-target, allow for filter wheel motion
Science exposure, NIC1 F187N	2.2	128 seconds exposure time off target 4 seconds for MULTIACCUM overhead
Small Angle Maneuver (from NIC1 to NIC2) + Reconfigure Instrument	1.1	move on-target in NIC2 plus instrument reconfiguration (change focus from NIC1 to NIC2), and filter wheel motion.
Science exposure, NIC2 F187N	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 F187N	2.2	128 seconds exposure time off target 4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)	0.9	Move on-target, allow for filter wheel motion
Science exposure, NIC2 F187N	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 F187N	2.2	128 seconds exposure time off target 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 F187N	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 F187N	2.2	128 seconds exposure time off target 4 seconds for MULTIACCUM overhead
Small Angle Maneuver (chop)	1.1	Move on-target, allow for filter wheel motion
Science exposure, NIC3 F187N	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 F187N	2.2	128 seconds exposure time off target 4 seconds for MULTIACCUM overhead
DUMP NIC buffer	~3.0	Terminal dump, could go in occultation.

 

The total time spent on the target is  51.3 minutes, with a visibility period of 59 minutes. Thus, there is room for a slight increase in exposure time without having the observations go into a second orbit. Note that 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.

Note that more detailed estimates may also be obtained by building test RPS2 Phase II proposals; some observers may wish to use this approach for estimating time required for the observations. Not shown in the above example are three parallel memory dumps.

Coronagraphic Overhead Example

The following table lists the overheads for each 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 is accounted for by the scheduling software; it therefore does not appear in this table

• Target: HR4796
• Declination: -39 degrees
• Visibility: 55 minutes.

  Table 10.4: Time Estimator Example for NICMOS Coronagraphy - Visit 01

  Activity	Duration	Elapsed Time
  Orbit 1
  GS ACQ	6m	6m
  NIC2-ACQ	2 X 0.284s, F171M
  Overhead	2m 57s	9m
  Dark (to remove persistence)	1 x 14.28s
  Overhead	1 x (7 + 25 * 0.6s) + 16s filter change	10m
  F160W	1 x 191.96s
  Overhead	1 x 4s + 16s filter change	13.5m
  F160W	1 x 255.96s
  Overhead	1 x 4s	18m
  F160W	1 x 223.97s
  Overhead	1 x 4s	22m
  Buffer dump	2m	24m