Exposure Timing

Exposure timing is part of the total time required for a JWST proposal. This page explains the model used by JWST's Astronomer’s Proposal Tool (APT) to estimate and account for exposure time.

The primary component of the timing model is the time it takes to execute the exposures being specified. APT models the exposures that will be executed by the onboard scripts. Each exposure specification in APT may represent one or more actual exposures, due to number of exposures, dither points, mosaicking, and other exposure expansion rules. The following applies to the model APT has for the timing of an exposure modeled in APT.

An exposure consists of an exposure setup period in which the scripts are compiled onboard and variables are initialized. Because the detector is reading out constantly, there is a need to synchronize with the detector which on average can be a half a frame read. Then a series of integrations are performed, each one consisting of a group of detector frames, separated by a gap. After each integration, there is a frame reset to clear the detector. Finally, there is a cleanup period.

The exposure timing model is shown in Figure 1.

Photon Collecting Time is detailed in Appendix D. Because not all pixels are read out simultaneously, the timing model shown is a simplification of how much data will be on any one pixel. But the sum of the integration times indicates how much data will be collected on the whole.

Exposing Duration attempts to model the amount of time that the shutter will be open on the whole, and therefore includes the times that frame resets and synchronization are occurring.

Exposure Duration is the total time needed to execute the exposure, and therefore includes all the setup and cleanup activities as well.

 Figure 1. Exposure timing model
Exposure timing module

Setup and cleanup times for each instrument are provided in the Instrument Specific Overheads article.

For each exposure, the Frame synchronization time is averaged to be ½ the FrameReadtime as detailed in Appendix D. The FrameReset time is equal to the FrameReadtime and is accumulated per integration.

Note that for MIRI MRS exposures, the larger value of the long and short integrations is used to determine the FrameResets.

Overhead for fast full-frame reset

ceil (nInts * (frame read time + number of background row resets * 0.00001) +
frame read time * 0.5)

where the number of backround row resets is:
0 for MIRI and FGS
0 for NIRCam, NIRSpec and NIRISS full-frame exposures
2048 for all NIRSpec and NIRISS subarrays
(2048/512/256/16 for NIRCam subarrays, as specified in the table above)

Change log

 Click here to expand...

Version 2 (December 4, 2018)

  1. editorial; removed Table 1 which was out of date and superseded by the Instrument Specific Overheads articles.

Version 1 (June 6, 2017)

  1. Initial version.