Overheads Example for NIRCam Imaging with FULL Primary Dithers

JWST observing and overhead times are detailed in this example NIRCam imaging program using 7 filters and the FULL 3TIGHT primary dither pattern to cover a contiguous area.

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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 FULL 3TIGHT primary dither pattern to cover a contiguous area.  Each dither step is 58.5" within the pattern and then twice that (117") to return from the last to the first dither position.  Slews of 58.5" are larger than the Visit Splitting Distance for targets at Galactic latitude > 25° (see JWST Slew Times and Overheads).  This is the case we consider here: every dither position is split into a separate visit.  This program is split into 12 visits, each requiring a guide star acquisition and associated overheads.

(Targets at lower Galactic latitude allow dither steps 1 and 2 to be performed within a single visit, but all other dither slews break visits, resulting in 8 visits altogether instead of 12.)

Smart Accounting must be run in APT (see below) to re-analyze the split visits and reduce overheads for the full observing program.

The observing sequence is as follows.  Coordinates (X, Y) are given in arcseconds the NIRCam ideal coordinate system.


ExposureDither positionX-offset (arcsec)Y-offset (arcsec)Dither distance (arcsec)Short wavelength filterLong wavelength filter
11-58-7.5F090WF277W
220058.5F090WF277W
33587.558.5F090WF277W
41-58-7.5117.0F115WF356W
520058.5F115WF356W
63587.558.5F115WF356W
71-58-7.5117.0F150WF444W
820058.5F150WF444W
93587.558.5F150WF444W
101-58-7.5117.0F200WF444W
1120058.5F200WF444W
123587.558.5F200WF444W


Note F444W is repeated, 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 FULL 3TIGHT primary dither pattern depth map for 3 exposures

Depth map given the 3 exposures of the NIRCam FULL 3TIGHT 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 3 exposures are labeled: 1, 2, 3.


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

For targets at Galactic latitude > 25°, the sequence of events is roughly as follows, reading from left to right within each exposure (row).  Each entry gives the time in seconds for operations plus overheads.  Note in this example, every exposure is executed in a separate visit.  The visit slew times are highlighted in yellow; they will be reduced after running Smart Accounting (see next section below):


ExposureVisit slewGS acqMech (filters)OSS scriptExposure overheadsScience (observe)End of visit
118002821904069731105
21742821904069731105
31742821904069731105
42132821904069731105
51742821904069731105
61742821904069731105
72132821904069731105
81742821904069731105
91742821904069731105
102132821904069731105
111742821904069731105
121742821904069731105
TOTAL38313384228048082887721260


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.



Total observing time and overheads

Before running Smart Accounting, APT assumes observing times and overheads given below (in seconds):


ExposureVisit SlewScience (Observe)Instrument OverheadsObservatory OverheadsTotal Charged
118007316865153732
21747316862551846
31747316862551846
42137316862611891
51747316862551846
61747316862551846
72137316862611891
81747316862551846
91747316862551846
102137316862611891
111747316862551846
121747316862551846
TOTAL383187728232333824173


After running Smart Accounting, the slew overheads are significantly reduced:


ExposureVisit slewScience (observe)Instrument overheadsObservatory overheadsTotal charged
11747316862551846
2517316862351703
3517316862351703
4517316862351703
5517316862351703
6517316862351703
7517316862351703
81747316862551846
918007316865153732
10517316862351703
11517316862351703
12517316862351703
TOTAL260787728232314022751


Smart Accounting attempts to group visits into a more efficient observing sequence (that may or may not actually be executed).  The results of these groupings may be found in the Smart Accounting file exported by APT.  In this case, the groups are (9, 12, 3, 6), (8, 2, 5, 11), and (1, 10, 7, 4).  In other words, all filters may be observed at each dither position before moving to the next position, due to the visit splitting.  The initial slew time (1800 seconds) is charged to exposure #9.  The 58.5" slew to the next dither position (174 seconds) is charged to exposures #1 and #8.  All other exposures are charged 51 seconds: the telescope settling time after each visit plus the minimum telescope slew time.

Note this example achieves 39% efficiency: 2.4 hours science observations / 6.3 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% addition to the total observing time and all other overheads.

The total times in every row of this table are reported by APT at the Visit level, since every exposure is split into a new visit.




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Latest updates
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    APT 25.4 overheads (previously APT 25.1.1)