Bias variation

The bias in the ISAAC short-wavelength camera depends upon the detected flux. Thus in target frames there is a residual bias not fully corrected by bias subtraction evident as two steepening ramps downwards to rows 1 and 513. An ISAAC-variant recipe corrects for this as follows. First within a flat-fielded frame they locate sources and mask them (as described in earlier) Then it forms a one-dimensional profile by masking objects collapsing along rows using the median, from which it subtracts a clipped mean of the profile to form a new profile of the bias variations. The bias-variation profile is then subtracted from each row of the original flat-fielded frame.
[ISAAC/_BIAS_CORRECT_GROUP_]

NACO has alternating positive and negative signals in its columns, most noticeable in longer exposures. The same filter as ISAAC, except it collapses along columns and does not mask objects, is applied to the flat-fielded frames.
[NACO/_BIAS_CORRECT_GROUP_, _REMOVE_COLUMN_ROW_STRUCTURE_]