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.
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 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.
|Exposure||Dither position||X-offset (arcsec)||Y-offset (arcsec)||Dither distance (arcsec)||Short wavelength filter||Long wavelength filter|
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).
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):
|Exposure||Visit slew||GS acq||Mech (filters)||OSS script||Exposure overheads||Science (observe)||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
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):
|Exposure||Visit Slew||Science (Observe)||Instrument Overheads||Observatory Overheads||Total Charged|
After running Smart Accounting, the slew overheads are significantly reduced:
|Exposure||Visit slew||Science (observe)||Instrument overheads||Observatory overheads||Total charged|
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.