JWST Instrument Overheads
The JWST instrument overhead times are adopted by the Astronomer's Proposal Tool for each overhead activity,
The Astronomers Proposal Tool (APT) charges users times and overheads for their observing programs (see overview and summary) to provide a resource accounting for each visit as well as for the proposal as a whole. Instrument overheads are charged for operations performed during the use of each instrument. The actual overheads applied in APT may be adjusted over time as more is learned about real operations, but representative values as of early Cycle 2 (APT 2023.3.4) are shown for reference in Table 1. Further details for each instrument are provided in Instrument Specific Overheads. Details for your observing program are available by exporting the times report from APT.
Table 1. Typical instrument overheads (in seconds) for common operations
| Operation | NIRCam imaging | NIRSpec MSA | NIRISS WFSS | MIRI imaging | FGS |
|---|---|---|---|---|---|
| Onboard Script System (OSS) compilation (per exposure) | 21 | 24 | 18 | 22 | 14 |
| Exposure overheada (full frame, single integration) | 69 | 56 (IRS2 readout) | 35 | 44 | 31 |
| Filter/grism changeb | 112 (module B) 186 (ALL) | 110 | 44 or 64 | 50 | N/A |
| Visit overhead and cleanupc | 96 | 215 | 84 | 90 | 72 |
a Exposure overhead is overhead charged ("Exp Ovhd" in the APT times report), not the exposure time itself. Exposure overhead is for the first exposure in a sequence of identical exposures (e.g., a dither pattern). NIRCam, NIRspec and MIRI have now implemented data acq reuse, so the timing is reduced for all exposures after the first.
b Filter/grism change generally assumes all filter/grating/pupil wheels will need to be moved. NIRSpec MSA configuration is not included. NIRISS WFSS is a special case—the assumption is that both the filter and pupil need to move for the initial grism exposure (64 s), but then only the pupil changes for the subsequent direct image and grism exposures (44 s).
c Visit overhead includes visit cleanup and is charged in addition to the 284 s to acquire a guide star at the beginning of each visit. All of the values include 72 s of general visit overhead, with the remainder being instrument-specific end-of-visit activities.