It performs a null debiassing, bad-pixel masking, then differences each pair of frames. Once all pairs have been so processed, these are then treated like sky flats; normalised frames are combined pixel by pixel using the median. Details of each flat are filed in the index of flats for future selection and use of the flat. See the “Notes” for further details.

• The bad-pixel mask applied is $ORAC_DATA_CAL/bpm.

• Each dark-subtracted frame has thresholds applied beyond which pixels are flagged as bad. The lower limit is 5 standard deviations below the mode, but constrained to the range $-$100 to 1. The upper limit is 1000 above the saturation limit for the detector in the mode used.

• Intermediate frames are deleted.

• Sub-arrays are supported.

• The created flat field in flat_$<$filter$>$_$<$group_number$>$ for the first or only cycle, and flat_$<$filter$>$_$<$group_number$>$_c$<$cycle_number$>$ for subsequent recipe cycles. Token $<$filter$>$ is the filter name, $<$group_number$>$ is the frame number of the group, and $<$cycle_number$>$ is the number of the cycle, counting from one.

• The flats are filed in $ORAC_DATA_OUT/index.flat.

NUMBER = INTEGER

USEVAR = LOGICAL

• The processing engines are from the Starlink packages Ccdpack and Kappa.

• Uses the Starlink NDF format.

• History is recorded within the data files.

• The title of the data is propagated through the intermediate file to the flat.

• Error propagation is controlled by the USEVAR parameter.