Overheads Example for NIRCam Imaging with INTRAMODULEBOX 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 exposures at 4 INTRAMODULE primary dither positions with 7 filters.  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
11−2.93.1
F090WF277W
223.12.96.003F090WF277W
332.93.16.003F090WF277W
443.12.96.003F090WF277W
512.9-3.16.003F115WF356W
623.12.96.003F115WF356W
732.93.16.003F115WF356W
843.12.96.003F115WF356W
912.93.16.003F150WF444W
1023.12.96.003F150WF444W
1132.93.16.003F150WF444W
1243.12.96.003F150WF444W
1312.93.16.003F200WF444W
1423.12.96.003F200WF444W
1532.93.110.61F200WF444W
1643.12.931.82F200WF444W


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 INTRAMODULEBOX primary dither pattern depth map for 4 exposures

Depth map given 4 exposures of the NIRCam INTRAMODULEBOX 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 4 exposures are executed in order clockwise, beginning at bottom left.


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
11800282
1904069731
2

48.0

69731
3

48.0

69731
4

48.0

69731
5

48.01504069731
6

48.0

69731
7

48.0

69731
8

48.0

69731
9

48.01504069731
10

48.0

69731
11

48.0

69731
12

48.0

69731
13

48.01304069731
14

48.0

69731
15

48.0

69731
16

48.0

69731105
TOTAL1800282720620160110411696105


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
2
731117136984
3
731117136984
4
731117136984
5
7313071661204
6
731117136984
7
731117136984
8
731117136984
9
7313071661204
10
731117136984
11
731117136984
12
731117136984
13
7312871631181
14
731117136984
15
731117136984
16
7312221521105
TOTAL1800116962991263819125


Note this example achieves 61% efficiency: 3.2 hours science observations / 5.3 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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