Chopping

In the thermal and mid-infrared regimes the sky is varying so rapidly that normal reduction methods are inappropriate. Instead sky subtraction is achieved either by frequently oscillating the secondary mirror between two beams (mid-infrared), called A and B; or by moving the telescope offsets (thermal) after a short exposure. The generic term is chopping.

Both methods produce frames with the target spectrum on different rows of the detector. The POINT_SOURCE and EXTENDED_SOURCE recipes difference these pairs of frames so that the result has both a positive and negative spectrum, and a background close to zero. The sense of the subtraction is always the same. ORAC-DR subtracts the B beam from the A beam, and the normal sequence is ABBA.
[_SUBTRACT_CHOP_]