Description

The reset correction is a MIRI step that attempts to correct for the reset anomaly effect. This effect is caused by the non-ideal behavior of the FET upon resetting in the dark causing the initial frames in an integration to be offset from their expected values. Another MIRI effect caused by resetting the detectors is the RSCD effect (see rscd).

Assumptions

The reset correction is a MIRI-specific correction. It will not be applied to data from other instruments.

Background

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 reset anomaly effect fades out by ~group 15 for full array data. It takes a few more groups for the effect to fade away on subarray data. The time constant of the effect seems to be closely related to the group number and not time since reset.

For multiple integration data, the reset anomaly also varies in amplitude for the first few integrations before settling down to a relatively constant correction for integrations greater than the second integration 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.

Algorithm

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. For full frame data, the current implementation uses a reset anomaly reference file, which contains a correction for the first 15 groups for all integrations. The reference file contains two corrections: one for the first integration and a second one for all other integrations. The correction was determined so that the correction is forced to be zero on group 15. 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.

Subarrays

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.