Overheads Example for NIRCam Imaging with FULLBOX Primary Dithers

JWST observing and overhead times are detailed in this example NIRCam imaging program using 7 filters and 4 INTRAMODULEBOX primary dither positions to cover the short wavelength detector gaps.

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The Astronomer's Proposal Tool, APT, charges time and overheads to NIRCam observing programs as outlined in this overview and detailed in the visit overheads timing model, including NIRCam overheads. This page gives a detailed breakdown of all overheads for an example NIRCam imaging program.

Overhead times given on this page are subject to change. The values listed here are as given in APT 25.4.



Observing program

In this example, we obtain NIRCam imaging with 7 filters and the FULLBOX 6TIGHT primary dither pattern to cover a contiguous area.  Each dither step is < 25", reducing slew times and overheads. All dither positions are executed before changing to the next filter pair, as detailed in the observing sequence below.  Coordinates (X, Y) are given in arcseconds in the NIRCam ideal coordinate system. These observations can all be performed within a single visit for most fixed targets (those with visit splitting distances >30"); that is the case we consider here.


ExposureDither positionX-offset (arcsec)Y-offset (arcsec)Dither distance (arcsec)Short wavelength filterLong wavelength filter
11−24.25.50F090WF277W
2224.45.310.802F090WF277W
330.15.424.3F090WF277W
4424.25.524.3F090WF277W
5524.45.310.802F090WF277W
660.15.424.3F090WF277W
1124.25.524.3F115WF356W
2224.45.310.802F115WF356W
330.15.424.3F115WF356W
4424.25.524.3F115WF356W
5524.45.310.802F115WF356W
660.15.424.3F115WF356W
1124.25.524.3F150WF444W
2224.45.310.802F150WF444W
33-0.15.424.3F150WF444W
4424.25.524.3F150WF444W
5524.45.310.802F150WF444W
660.15.424.3F150WF444W
1124.25.524.3F200WF444W
2224.45.310.802F200WF444W
330.15.424.3F200WF444W
4424.25.524.3F200WF444W
5524.45.310.802F200WF444W
660.15.424.3F200WF444W


Note F444W is repeated in the last filter change, yielding 7 filters in all.  We observe with both modules and full subarrays on all 10 detectors, yielding the exposure map for the short wavelength channel shown in Figure 1 (and long wavelength exposures not shown here).

Figure 1. NIRCam FULLBOX 6TIGHT primary dither pattern with 6 exposures

Depth map given the 6 exposures of the NIRCam FULLBOX 6TIGHT primary dither pattern in the NIRCam ideal coordinate system. Color represents depth (number of exposures) in the short wavelength channel. (Depth in the long wavelength channel is not shown here.) The 6 exposures are executed in order clockwise, beginning at the bottom left of 2 rows of 3 pointings each.


Sequence of events

Each exposure consists of 7 groups of the MEDIUM8 readout pattern.  APT reports each exposure time in two different ways:

  • Science: 731 seconds (rounded up from 730.1) – exposure time for each pixel
  • Exposure time (consistent with the ETC): 740.8 seconds – for the full detector, including one additional frame time for all pixels to complete their exposures

The sequence of events is roughly as follows, reading from left to right within each exposure.  Each entry gives the time in seconds for operations plus overheads charged by APT:


ExposureVisit slewGS acqSAMs (dither + GS) Mech
(filters)
OSS scriptDetector overheadsScience
(observe)
End of visit
118002821904069731
247.9869731
353.2269731
453.2269731
547.9869731
653.2269731
753.221504069731
847.9869731
953.2269731
1053.2269731
1147.9869731
1253.2269731
1353.221504069731
1447.9869731
1553.2269731
1653.2269731

17

47.9869731
1853.2269731
1953.221304069731
2047.9869731
2153.2269731
2253.2269731
2347.9869731
2453.2269731

105

TOTAL18002821183620160165617544105


Note the totals in the last line are rounded up by APT to integer seconds.

The operations consist of the following (with terms in bold as given in APT):

  • Visit Slew: 30 minutes initial slew to target; approximate average applied uniformly to all programs
  • GS Acq: guide star acquisition
  • Small Angle Maneuvers (SAMs): time to dither, settle, and reacquire the guide star (see JWST Slew Times and Overheads)
  • Mechanism movements (filter changes), including 20 seconds for each pair of pupil or filter wheel that moves (see NIRCam Overheads)
  • Exposure detector overheads and Science observation:
    • Setup: 30 seconds
    • Wait for next readout cycle: 5.37 s on average (half a cycle)
    • Reset all pixels: 10.74 s (for full detector)
    • Science exposure
    • Cleanup: 22 seconds
  • End of Visit: Cleanup activities
Most of these operations are categorized as Instrument Overheads by APT.  The initial visit Slew is counted separately, as is the Science observing time.

Note the overheads for the final dither position are significantly higher than the rest because the slew distance is > 20" (see JWST Slew Times and Overheads).



Total observing time and overheads

The final observing times and overheads are given below (in seconds, rounded up for each total)


ExposureVisit slewScience (observe)Instrument overheadsObservatory overheadsTotal charged
118007315814983610
2731117136984
3731122137990
4731122137990
5731117136984
6731122137990
77313121671210
8731117136984
9731122137990
10731122137990
11731117136984
12731122137990
137313121671210
14731117136984
15731122137990
16731122137990
17731117136984
18731122137990
197312921641187
20731117136984
21731122137990
22731122137990
23731117136984
247312271531112
TOTAL1800175444006373627086


Note this example achieves 65% efficiency: 4.9 hours science observations / 7.5 total hours charged to the program. Efficiency could be improved by increasing the exposure times. All overheads would remain the same, except for the Observatory Overheads, which would increase. Observatory Overheads are a 16% surcharge on the total observing time and all other overheads.

The total times in the bottom row of this table are reported by APT at the Visit level.




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