Description

Class:

jwst.emicorr.EmiCorrStep

Alias:

emicorr

Overview

The emicorr step corrects for known noise patterns in the raw MIRI data. The majority of the MIRI subarrays have an 390 Hz or other electromagnetic interference (EMI) noise pattern in the raw data. The known frequencies to correct for are in the EMI reference file, under the key frequencies. The effect of these EMI frequencies is to imprint each into the rate images. For short integrations in LRSSLITLESS the correlated noise from this is quite apparent in the rate images. For longer integrations the net effect is to increase the read noise by about 20%.

The process to do the correction is the following (repeated recursively for each discrete EMI frequency desired):

1. Read image data.

2. Make very crude slope image and fixed pattern “super” bias for each integration, ignoring everything (nonlin, saturation, badpix, etc).

3. Subtract scaled slope image and bias from each frame of each integration.

4. Calculate phase of every pixel in the image at the desired EMI frequency (e.g. 390 Hz) relative to the first pixel in the image.

5. Make a binned, phased amplitude (pa) wave from the cleaned data (plot cleaned vs phase, then bin by phase).

6. Measure the phase shift between the binned pa wave and the input reference wave. [1]

7. Use look-up table to get the aligned reference wave value for each pixel (the noise array corresponding to the input image). [1]

8. Subtract the noise array from the input image and return the cleaned result.

[1] (1,2)

Alternately, use the binned pa wave instead of the reference wave to “self-correct”.

The long term plan is a change to the sizes and locations of the subarrays to get the frame times to be in phase with the known noise frequencies like the full frame images. For the previous and near term observations this can be fixed through application of the emicorr step.

An EMICORR reference file can be used to correct for all known noise patterns. The reference file is expected to be in ASDF format, containing the exact frequency numbers, the corresponding 1D array for the phase amplitudes, and a subarray_cases dictionary that contains the frequencies to correct for according to subarray, read pattern, and detector. If there is no reference file found in CRDS, the step has a set of default frequencies and subarray cases for which the correction is applied.

Input

The input file is the _uncal file after the dq_init_step step has been applied, in the in the calwebb_detector1 pipeline.

Output

The output will be a partially-processed RampModel with the corrected data in the SCI extension, meaning, the effect of the EMI frequencies (either the default values or the ones in the reference file) removed. All other extensions will remain the same.