Description¶
Assumptions¶
It is assumed that the saturation step has already been applied to the input data, so that saturation flags are set in the GROUPDQ array of the input science data.
Algorithm¶
The linearity step applies the “classic” linearity correction adapted from the HST WFC3/IR linearity correction routine, correcting science data values for detector non-linearity. The correction is applied pixel-by-pixel, group-by-group, integration-by-integration within a science exposure.
The correction is represented by an nth-order polynomial for each pixel in the detector, with n+1 arrays of coefficients read from the linearity reference file.
The algorithm for correcting the observed pixel value in each group of an integration is currently of the form:
where \(F\) is the observed counts (in DN), \(c_n\) are the polynomial coefficients, and \(F_\text{c}\) is the corrected counts. There is no limit to the order of the polynomial correction; all coefficients contained in the reference file will be applied.
Upon successful completion of the linearity correction, “cal_step” in the metadata is set to “COMPLETE”.
Special Handling¶
Pixels having at least one correction coefficient equal to NaN will not have the linearity correction applied and the DQ flag “NO_LIN_CORR” is added to the science exposure PIXELDQ array.
Pixels that have the “NO_LIN_CORR” flag set in the DQ array of the linearity reference file will not have the correction applied and the “NO_LIN_CORR” flag is added to the science exposure PIXELDQ array.
Pixel values that have the “SATURATED” flag set in a particular group of the science exposure GROUPDQ array will not have the linearity correction applied to that group. Any groups for that pixel that are not flagged as saturated will be corrected.
The ERR array of the input science exposure is not modified.
The flags from the linearity reference file DQ array are propagated into the PIXELDQ array of the science exposure using a bitwise OR operation.