Chopping

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

Both methods produce frames with the target image at different positions on the detector. The aforementioned recipes difference these pairs of frames, so that the result has both a positive and negative image, and a background level 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. For the thermal data, the chopped beam is only notional, but the same terminology and subtraction sense is used.
[_DIFFERENCE_PAIR_, _DIFFERENCE_PAIR_SIMPLE_,
_DIFFERENCE_CHOP_BEAMS_]

If the telescope is further offset (nodded), the final mosaic of the differenced frames can have two positive and two negative representations of the source. In practice the thermal reductions register and co-add the nodded frames to compensate for flat-field errors in IRCAM.