The reset step is not currently applied to MIRI exposures, but will likely be reinstated in a future build.
The reset correction is a MIRI-specific correction. It is assumed that the input science data have NOT had the zero group (or bias) subtracted. The reset correction should not remove the bias signal from the science exposure, therefore the reset correction for the first group is defined to be zero.
For MIRI exposures, the initial groups in each integration suffer from two effects related to the resetting of the detectors. The first effect is that the first few groups after a reset do not fall on the expected linear accumulation of signal. The most significant deviations ocurr in groups 1 and 2. This behavior is relatively uniform detector-wide. The second effect, on the other hand, is the appearance of significant extra spatial structure in these initial groups, before fading out in later groups.
The time constant associated with the reset anomaly is roughly a minute so for full array data the effect has faded out by ~group 20. On subarray data, where the read time depends on the size of the subarray, the reset anomaly affects more groups in an integration.
For multiple integration data, the reset anomaly also varies in amplitude for the first set of integrations before settling down to a relatively constant correction for integrations greater than four for full array data. Because of the shorter readout time, the subarray data requires a few more integrations before the effect is relatively stable from integration to integration.
The reset correction step applies the reset reference file. The reset reference file contains an integration dependent correction for the first N groups, where N is defined by the reset correction reference file.
The format of the reset reference file is NCols X NRows X NGroups X NInts. The current implementation uses a reset anomaly reference file for full array data containing a correction for the first 30 groups for integrations 1-4. The reference file was determined so that the correction is forced to be zero on the last group for each integration. For each integration in the input science data, the reset corrections are subtracted, group-by-group, integration-by- integration. If the input science data contains more groups than the reset correction, then correction for those groups is zero. If the input science data contains more integrations than the reset correction then the correction corresponding to the last intergration in the reset file is used.
There is a single, NCols X NRowss, DQ flag image for all the integrations. The reset DQ flag array are combined with the science PIXELDQ array using numpy’s bitwise_or function. The ERR arrays of the science data are currently not modified at all.
The reset correction is subarray-dependent, therefore this step makes no attempt to extract subarrays from the reset reference file to match input subarrays. It instead relies on the presence of matching subarray reset reference files in the CRDS. In addition, the number of NGROUPS and NINTS for subarray data varies from the full array data as well as from each other.