Understanding Exposure Times

All JWST detectors integrate using a non-destructive up-the-ramp sampling technique.  Users specify a readout pattern and the number of groups and integrations, which results in a calculated exposure time for each exposure.   

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Introduction

See also: MIRI Detector Overview, NIRCam Detectors, NIRISS Detector, NIRSpec Detectors

The infrared sensitive detectors in JWST science instruments operate very differently from the CCDs that many astronomers are familiar with.  These IR detectors 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 a 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 values that are available in the Exposure Time Calculator and the Astronomers Proposal Tool 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. Ground-based data processing software can then correct bias drifts using reference pixels, and also 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, while permitting teams to subselect a relatively small set of optimal patterns for observations in flight.

Figure 1 illustrates the components of each up-the- ramp exposure

  1. NFRAMES is the number of frames per group.
  2. NGROUPS is the number of groups per integration.
  3. 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.


Figure 1. Generic up-the-ramp (MULTIACCUM) readout scheme for each exposure.
A generic illustration of the up-the-ramp readout scheme used by all JWST detectors. Each exposure consists of some combination of frames, groups, and integrations. The frames are not selected directly, but encoded into the selected readout pattern chosen by the user. 


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Revised description with more focus on user experience and additional descriptive material.