This primitive subtracts a zero bias from the current frame; unless the data have variance information and were taken using a non-ND mode (i.e. where the bias has not already been subtracted), whereupon a bias frame, if available, is subtracted.

For most instruments there is no bias to subtract so it is something of a placeholder primitive. Its main purpose is to set up CCDPACK for subsequent processing. For instance, CCDPACK will complain if debiassing is not performed before say flat-fielding. The primitive reports a successful bias subtraction and the frames concerned.

• This primitive is suitable for UFTI, IRCAM, INGRID, and Michelle in imaging mode.

• Processing occurs for all frames, and sub-frames therein.

• Where a bias frame is used, it is the most-recent and matching given by $Cal-$>$bias method.

• The observing mode (read type) comes from user header ORAC_DETECTOR_READ_TYPE.

• The bias-subtracted image can be displayed.

• The frame title is propagated.

• Bias-subtracted frame inheriting the frame’s name but with the _db suffix.

CCDPACK: DEBIAS.