Overheads Example for NIRCam Imaging with INTRAMODULE Primary Dithers
JWST observing and overhead times are detailed in this example NIRCam imaging program using 7 filters and 4 INTRAMODULE primary dither positions to cover the short wavelength detector gaps.
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.
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.
|Exposure||Dither position||X-offset (arcsec)||Y-offset (arcsec)||Dither distance (arcsec)||Short wavelength filter||Long wavelength filter|
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).
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:
|Exposure||Visit slew||GS acq||SAMs (dither + GS)||Mech|
|OSS script||Detector overheads||Science |
|End of visit|
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
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)
|Exposure||Visit Slew||Science (Observe)||Instrument Overheads||Observatory Overheads||Total Charged|
Note this example achieves 59% efficiency: 3.2 hours science observations / 5.5 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.