Understanding Exposure Times
All JWST detectors integrate using a non-destructive up-the-ramp sampling technique. The exposure time is determined from the users's selection of a Readout Pattern and specification of the number of groups and number of integrations to use.
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But arriving at an exposure time is not as simple as requesting a total time. Rather, the exposure time is derived from the selection of a Readout Pattern and specifying two other parameters, the number of groups per integration and the number of integrations per exposure. These are the parameters values that are available in the Exposure Time Calculator (ETC) and the Astronomers Proposal Tool (APT) for specifying exposures. In these tools, the heading used are Groups/Int and Integrations/Exp.
For NIRSpec, NIRCam, and NIRISS, users will select Readout Patterns from a menu of available options that are optimized for different types of targets. MIRI on the other hand only provides two readout options for science targets. See the instrument-specific detector articles listed above for more information.
How up-the-ramp readouts work
The following is a generic description of the up-the-ramp readout (sometimes referred to as MULTIACCUM) that is the standardized readout sampling for all JWST detectors. In this readout mode, the array is read out non-destructively at intervals defined by the parameters described below during each exposure. Multiple non-destructive frames can be averaged by the onboard flight software into a group and transferred to the solid state recorder for downlinking to the ground. This can be an effective way to reduce the total data volume that needs to be downlinked and mitigate concerns about saturation.
Ground-based data processing software can also correct bias drifts using reference pixels, and use up-the-ramp processing algorithms to reject cosmic rays. This approach is quite flexible since it allows for a large range of Readout Patterns. The instrument teams have pre-selected a relatively small set of optimal patterns for use in making observations on orbit.
Figure 1 provides a generic illustration the components of each up-the- ramp exposure. The user does not choose the number of frames within each group directly, but rather selects various Readout Pattern options that define different numbers of frames per group, different numbers of Groups/Int and different numbers of Integrations/Exp. The specific details available for each instrument and readout mode may differ depending on assumptions regarding use of reset frames (or not), whether a frame is dropped (or not) between groups, and how these frames get used by the data processing software, which may be different between near-IR and mid-IR instruments. Proposers should refer to specific instrument detector pages for more detailed information.
APT limits the total number of frames in any one exposure to 196608 frames per detector. The total number of frames in an exposure is Integrations/Exp * Groups/Int * Frames/Group. For NIR detectors, Frames/Group includes coadded frames, but not dropped frames. For mid-IR detectors, Frames/Group is 1 for both FASTR1 and SLOWR1 Readout Patterns. Very long time-series observations with a small subarray can encounter this limit. In that case, use multiple exposures to cover the desired time interval.
See also: JWST Recommended Observing Strategies
Given the variables in play, there are multiple combinations of parameters that will result in the same reported exposure time. So how does one choose? There are no hard and fast rules to be applied to all cases. However, here are some guidelines to consider. The detailed instrument strategy articles may be helpful for specific cases.
For a given integration time, a larger number of Groups/Int is nominally preferred, primarily to mitigate cosmic rays. (This assuming that saturation is not a concern.) However, each saved frame contributes to the total data volume, so observers may want to consider a readout pattern that reduces Groups/Int while still preserving total integration time.
Adding more Integrations/Exp will also increase the exposure time. Breaking exposures into multiple integrations will be most useful for bright sources that would otherwise saturate in longer integrations. For faint source observations, a larger number of Integrations/Exp and/or simply more exposures may be preferred to achieve a given exposure time.
Finally, recall that it is recommended that most datasets obtained with JWST instruments be dithered. Each selected dither position is a separate exposure, so the total exposure time you will get depends on the number of dither steps. You should note that by default the Exposure Time Calculator is providing information on individual exposures, so you can specify the number of exposures to be the number of dithers expected in order to estimate the impact of multiple dither positions in APT.