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.
The infrared-sensitive detectors in JWST science instruments operate very differently from the CCDs that many astronomers are familiar with from ground-based work or HST's ACS and WFC3/UVIS. These IR detectors, similar to those in the Spitzer Space Telescope instruments and HST WFC3/IR, are read out using a non-destructive up-the-ramp readout technique that provides a number of advantages.
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 and the number of integrations to include in the exposure. These are the parameters values that are available in the Exposure Time Calculator (ETC) and the Astronomers Proposal Tool (APT) for specifying exposures.
For NIRSpec, NIRCam, and NIRISS, users will select readout patterns from a menu of available options that are optimized for different types of targets. For MIRI, users have two basic options for readout. See the specific detector articles listed above for more information.
How up-the-ramp readouts work
The up-the-ramp readout (sometimes referred to as MULTIACCUM) 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 the 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 illustrates the components of each up-the- ramp exposure:
- Nframes is the number of frames per group.
- Ngroups is the number of groups per integration.
- Nint is the number of integrations per exposure.
Note that you do not explicitly select a value of Nframes, but rather this parameter is encoded in the definition of the various readout pattern options that can be selected. The specifics available for each instrument and readout mode, however, can vary. Proposers should refer to specific instrument detector pages for details.
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 Ngroups is nominally preferred, primarily to mitigate cosmic rays. However, each saved frame contributes to the total data volume, so a longer readout pattern (with fewer Ngroups) may be desirable (or even required) in certain cases.
Adding multiple integrations to an exposure will also increase the exposure time. Breaking exposures into multiple integrations (Nint > 1) will be most useful for bright sources that would saturate in longer integrations. For faint source observations, Nint = 1 (or at least a low number) may be preferred for each exposure.
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 specify. 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 estimate the impact of multiple dither positions.