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Orbit Use Determination

The easiest way to learn how to compute total orbit time requests is to work through a few examples. We provide below seven examples of how to use the information in Table 9.1 to determine your orbit requirements. The overhead examples cover the following cases:

Example 1: Coronographic observation, with the two-chop pattern.
Example 2: Polarimetric observation, with the four-chop pattern.
Example 3: Emission-line mapping of an extended object, with the SQUARE-WAVE-DITH pattern.
Example 4: General observation with switch of camera and pattern.
Example 5: Mapping of an extended object with the -spiral-dith-chop pattern. Unless otherwise specified, we assume in the examples that the target declination is 50 degrees, which implies that its visibility period during one orbit is 56 minutes (see the Phase 1 Proposal Instructions).

Example 1: A Two-Chop Pattern Using Multi-Accum and the Coronograph

The proposed science is to observe the extended nebulosity around an IR-bright source at

= 2.1 microns. Since the source would saturate the cameras, the observer decides to hide it behind the coronographic spot in NIC2. Coronographic observations require an onboard acquisition of the target under the coronographic spot (see Chapter 5). The two acquisition exposures, which are needed to locate the target in the Camera 2 field of view are assumed here to require 10 seconds integration each in the selected filter (F207M for this observation). The total overhead for reading the two images, calculating the positions of the target and the coronograph spot, and moving the target under the spot is 170 seconds. The long wavelength observations are affected by non-negligible background, and the observer chooses the TWO-CHOP pattern with a chopping throw of 19 arcsec for background subtraction. The overhead for slewing between the target and each of the two backgrounds is 36 seconds, and the same guide stars can be maintained for such small slews (Table 9.1). The sequence for the TWO-CHOP pattern is: target-background-target-background, with the two background fields positioned on opposite sides of the target (see Chapter 10).

The observer wishes to use the F207M filter in MULTIACCUM mode for the observation, and wants exposure times of 15 minutes at each pointing, for a total exposure time target+background of 60 (i.e., 15 x 4) minutes, half of which is spent on the target. A good sequence for this time would be STEP64 with NSAMP=22, a total time per exposure of 896 seconds. The orbit requirements are summarized in Table 9.2 below:

Table 9.2: Orbit Determination for Example 1

Action

Time (minutes)

Explanation

Orbit 1

Initial Guide Star Acquisition

7.1

Needed at start of observation of new target

Target Acquisition
(under the coronographic spot)

2.8

121 seconds overheads
10 seconds for first acquisition exposure
10 seconds for second acquisition exposure
29 seconds slew to coronagraph

Science exposure, NIC2 F207M

15.4

896 seconds exposure time on target
27 seconds for MULTIACCUM overhead

Science exposure, NIC2 F207M

16.0

36 seconds chopping slew
896 seconds exposure time on background
27 seconds for MULTIACCUM ovrhead

Orbit 2

Guide star re-acquisition

5.3

Start of new orbit

Science exposure, NIC2 F207M

15.4

896 seconds exposure time on target
27 seconds for MULTIACCUM overhead

Science exposure, NIC2 F207M

16.0

36 seconds chopping slew
896 seconds exposure time on background
27 seconds for MULTIACCUM overhead

Table 9.2: Orbit Determination for Example 1
Action	Time (minutes)	Explanation
Orbit 1
Initial Guide Star Acquisition	7.1	Needed at start of observation of new target
Target Acquisition (under the coronographic spot)	2.8	121 seconds overheads 10 seconds for first acquisition exposure 10 seconds for second acquisition exposure 29 seconds slew to coronagraph
Science exposure, NIC2 F207M	15.4	896 seconds exposure time on target 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F207M	16.0	36 seconds chopping slew 896 seconds exposure time on background 27 seconds for MULTIACCUM ovrhead
Orbit 2
Guide star re-acquisition	5.3	Start of new orbit
Science exposure, NIC2 F207M	15.4	896 seconds exposure time on target 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F207M	16.0	36 seconds chopping slew 896 seconds exposure time on background 27 seconds for MULTIACCUM overhead

Te total time spent on the target is (7.1+2.7+15.4+16.0) = 41.2 minutes, with a visibility period of 56 minutes. Thus, the third and fourth exposures in the F207M filter must be done during the second orbit, following the guide star re-acquisition and the instrument set-up, since NICMOS exposures cannot be paused across orbits, and the third exposure would not fit in what remains of the first visibility window. The user requests two orbits to accomplish the proposed science observation. (The observer should develop their program by including additional exposures through the same or other appropriate filters to make more efficient use of what is left of the orbits).

Example 2: Polarization Observations Using a Chop Pattern and MULTI-ACCUM

Polarimetric observations at long wavelengths will be obtained for target A. The NIC1 filters POL0S, POL120S, and POL240S will provide information on the three Stokes parameters. The observer requires exposure times of 20 minutes in each polarizer, in MULTIACCUM mode. A good sequence for this time would be STEP128 with NSAMP=19, a total time per exposure of 1280 seconds.

The declination of the source is 30 degrees, so the visibility period during one orbit is 53 minutes. The orbit requirement is summarized in Table 9.3 below.

Table 9.3: Orbit Determination for Example 2

Action

Time (minutes)

Explanation

Orbit 1

Initial Guide Star Acquisition

7.1

Needed at start of observation of new target

Science exposure, NIC1 POL0S

21.8

1280 seconds exposure time on target
27 seconds for MULTIACCUM overhead

Science exposure, NIC1 POL120S

21.8

1280 seconds exposure time on target
27 seconds for MULTIACCUM overhead

Orbit 2

Guide Star re-acquisition

5.3

Start of new orbit

Science exposure, NIC1 POL240S

21.8

1208 seconds exposure time on target
27 seconds for MULTIACCUM overhead

Table 9.3: Orbit Determination for Example 2
Action	Time (minutes)	Explanation
Orbit 1
Initial Guide Star Acquisition	7.1	Needed at start of observation of new target
Science exposure, NIC1 POL0S	21.8	1280 seconds exposure time on target 27 seconds for MULTIACCUM overhead
Science exposure, NIC1 POL120S	21.8	1280 seconds exposure time on target 27 seconds for MULTIACCUM overhead
Orbit 2
Guide Star re-acquisition	5.3	Start of new orbit
Science exposure, NIC1 POL240S	21.8	1208 seconds exposure time on target 27 seconds for MULTIACCUM overhead

Example 3: Making a Map using SQUARE-WAVE-DITH

The proposer wants to map an extended region in the light of [FeII] (

1.644 microns). The choice is to use the F164N (line) and F166N (continuum) filters of NIC1, MULTIACCUM mode, and the SQUARE-WAVE-DITH pattern. The selected dithering step is 4 arcsec, with a total of 3 positions. The slewing overhead between each position will be 26 seconds. The requested exposure time at each position is 10 minutes in each filter, implying that the exposure time for the observation is: T = (10 x 2 x 3) = 60 minutes, exclusive of overheads. The orbit requirements are summarized in Table 9.4. A good sequence for this time would be STEP128 with NSAMP=14, a total time per exposure of 640 seconds.

Table 9.4: Orbit Determination for Example 3
Action	Time (minutes)	Explanation
Orbit 1
Initial Guide Star Acquisition	7.1	Needed at start of observation of new target
Science exposure, NIC1 F164N	11.2	640 seconds exposure time on position # 1 27 seconds for MULTIACCUM overhead
Science exposure, NIC1 F164N	11.5	27 seconds for dither 640 seconds exposure time on position # 2 27 seconds for MULTIACCUM overhead
Science exposure, NIC1 F164N	11.5	25 seconds dither 640 seconds exposure time on position # 3 27 seconds for MULTIACCUM overhead
Science exposure, NIC1 F166N	11.5	640 seconds exposure time on position # 1 27 seconds for MULTIACCUM overhead 25 seconds dither
Orbit 2
Guide Star re-acquisition	5.3	Start of new orbit
Science exposure, NIC1 F166N	11.1	640 seconds exposure time on position # 2 27 seconds for MULTIACCUM overhead
Science exposure, NIC1 F166N	11.5	25 seconds for dither 640 seconds exposure time on position # 3 27 seconds for MULTIACCUM overhead

Example 4: Changing Cameras and Pattern

The proposed science is to observe a target taking:

Exposures in the NIC1 F108N filter, using the MULTIACCUM mode and the XSTRIP-DITH pattern, with 5 arcsecs dithering step and 4 positions; the exposure time is T=900 seconds at each position. A good sequence for this time would be STEP64 with NSAMP=22, a total time per exposure of 896 seconds.

Exposures in the NIC2 F205W filter, using the ACCUM mode with NREAD=1. The TWO-CHOP pattern with chopping throw of 150 arcsecs and guide star re-acquisition on the target (see Example 1) is selected to remove the background. The exposure time is T=250 seconds at each pointing, implying a total on-source exposure time of 500 seconds. The closest TPG time is 251.188 seconds and 0.598 seconds must be added to get 251.786 seconds total exposure time. Table 9.5 lists the orbit requirements.

Table 9.5: Orbit Determination for Example 4
Action	Time (minutes)	Explanation
Orbit 1
Initial Guide Star Acquisition	7.1	Needed at start of observation of new target
Science exposure, NIC1 F108N	15.4	896 seconds exposure time on position # 1 27 seconds for MULTIACCUM overhead
Science exposure, NIC1 F108N	15.8	26 seconds for dither 896 seconds exposure time on position # 2 27 seconds for MULTIACCUM overhead
Orbit 2
Science exposure, NIC1 F108N	15.4	26 seconds dither 896 seconds exposure time on position # 3 27 seconds for MULTIACCUM overhead
Guide Star re-acquisition	5.3	Start of new orbit
Science exposure, NIC1 F108N	15.8	26 seconds dither 896 seconds exposure time on position # 4 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F205W	6.2	(39+51) seconds for camera change + set-up 252 seconds exposure on target 31 seconds ACCUM overhead
Science exposure, NIC2 F205W	5.9	74 seconds chop 252 seconds exposure on background # 1 29 seconds ACCUM overhead
Orbit 3
Guide Star re-acquisition	5.3	Start of new orbit
Science exposure, NIC2 F205W	4.7	252 seconds exposure on target 29 seconds ACCUM overhead
Science exposure, NIC2 F205W	5.9	74 seconds chop 252 seconds exposure time on background # 2 29 seconds ACCUM overhead

After the initial guide star acquisition and the instrument set-up of 12 seconds, the first science exposure of 896 seconds in the first filter (F108N) is obtained on the first pointing (position #1). The telescope slew to reach position #2 requires 26 seconds, and the exposure in the F108N filter is repeated up to the forth position. For the second part of the observation, the switch from NIC1 to NIC2 requires 90 seconds. The on-target exposure time requested in the F205W filter is T=500 seconds, split between the two target visits of the TWO-CHOP pattern. After the first exposure on the target and after the camera read-out, the telescope slews to the background in 74 seconds. The subsequent slew, back onto the target, is followed by a guide star re-acquisition of 6 minutes. The sequence continues, alternating target and background, until the pattern is completed.

Example 5: Another Example of Mapping

The observer wants to map an extended object, for instance a galaxy, with NIC2 and the F187W filter. The wavelength chosen is long enough that the observation may be affected by background emission, and the SPIRAL-DITH-CHOP is selected for background removal. The dithering step is set equal to the size of NIC2, 19.2 arcsec, and the chopping throw is set to 180 arcsec. The SPIRAL-DITH-CHOP pattern will start at the center of the target and move outward. The galaxy has a bright core and relatively faint extended emission, so MULTIACCUM mode will be used. The proposed exposure time is 3 minutes per pointing, and the galaxy will be covered with 9 different pointings. A good sequence for this time would be STEP32 with NSAMP=13, a total time per exposure of 192 seconds. The orbit requirements are summarized in Table 9.6.

Table 9.6: Orbit Determination for Example 5
Action	Time (minutes)	Explanation
Orbit 1
Initial Guide Star Acquisition	7.1	Needed at start of observation of new target
Science exposure, NIC2 F187W	3.7	192 seconds exposure time on target position # 1 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	78 seconds chop 192 seconds exposure time on background # 1 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	11.2	72 seconds chop 383 seconds guide star re-acquisition 192 seconds exposure time on target position # 2 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	78 seconds chop 192 seconds exposure time on background # 2 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	11.3	76 seconds chop 383 seconds guide star re-acquisition 192 seconds exposure time on target position # 3 27 seconds for MULTIACCUM overhead
Orbit 2
Guide Star re-acquisition	5.3	Start of new orbit
Science exposure, NIC2 F187W	5.0	77 seconds chop 192 seconds exposure time on background # 3 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	3.7	192 seconds exposure time on target position # 4 27 seconds for MULTIACCUM overhead
DUMP buffer	1.0	Partial buffer dump
Science exposure, NIC2 F187W	5.0	78 seconds chop 192 seconds exposure time on background # 4 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	11.4	80 seconds chop 383 seconds guide star re-acquisition 192 seconds exposure time on target position # 5 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	77 seconds chop 192 seconds exposure time on background # 5 27 seconds for MULTIACCUM read out
Guide Star re-acquisition	5.3	Start of new orbit
Science exposure, NIC2 F187W	11.3	76 seconds chop 383 seconds guide star reacq 192 seconds exposure time on background # 6 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	77 seconds chop 192 seconds exposure time on background # 6 27 seconds for MULTIACCUM overhead
Orbit 3
Science exposure, NIC2 F187W	3.7	192 seconds exposure time on target position # 7 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	77 seconds chop 192 seconds exposure time on background # 7 27 seconds for MULTIACCUM read out
DUMP buffer	1.3	Partial buffer dump
Science exposure, NIC2 F187W	11.2	72 seconds chop 383 seconds guide star re-acquisition 192 seconds exposure time on target position # 8 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	77 seconds chop 192 seconds exposure time on background # 8 27 seconds for MULTIACCUM read out
Science exposure, NIC2 F187W	11.2	72 seconds chop 383 seconds guide star reacq 192 seconds exposure time on background # 9 27 seconds for MULTIACCUM overhead
Science exposure, NIC2 F187W	5.0	77 seconds chop 192 seconds exposure time on target position # 9 27 seconds for MULTIACCUM overhead

The size of the chop is determined by the distance between the pointings; the time for the slew between the target and the background is always 77 seconds, while that for the slew between the background and the target is about 72 seconds. Note that guide star reacquisitions at the start of a visibility period can hide some time in occultation. Thus the observer is charged more time (383 s) for reacquisitions that occur during a visibility period.

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stevens@stsci.edu

Orbit Use Determination

Example 1: A Two-Chop Pattern Using Multi-Accum and the Coronograph

Table 9.2: Orbit Determination for Example 1

Action

Time (minutes)

Explanation

Example 2: Polarization Observations Using a Chop Pattern and MULTI-ACCUM

Table 9.3: Orbit Determination for Example 2

Action

Time (minutes)

Explanation

Example 3: Making a Map using SQUARE-WAVE-DITH

Table 9.4: Orbit Determination for Example 3

Action

Time (minutes)

Explanation

Example 4: Changing Cameras and Pattern

Table 9.5: Orbit Determination for Example 4

Action

Time (minutes)

Explanation

Example 5: Another Example of Mapping

Table 9.6: Orbit Determination for Example 5

Action

Time (minutes)

Explanation