Sky Subtraction

In general the recipes do not sky-subtract in the literal sense of pixel-by-pixel subtraction of a sky frame, or better of some median of jittered sky or even target frames. Such recipes could readily be created if there is a demand. Instead the sky signature is usually accounted for in the flat-fielding. Therefore the mosaics generally have the sky signature removed, but not base level.

The sky varies rapidly in the thermal and mid-infra-red, so dithered pairs, or nodded and dithered pairs respectively are differenced to attempt to remove the sky signature. See the section on chopping.
[_DIFFERENCE_PAIR_]

Another recipe which performs a pixel-by-pixel sky subtraction is SKY_AND_JITTER. It's hardly used and not recommended because it demands a very stable sky, since only one sky frame is observed at the start of the observation; and a region of sky devoid of objects to avoid `holes' appearing in the subtracted target frames.
[_SUBTRACT_SKY_SKY_AND_JITTER_]

Another form of sky subtraction is to remove a representative sky level. This benefits imaging of extended sources whose scale exceeds the dither pattern or even the detector's field of view. The normal procedure is to alternate between dithered integrations on target and a region of sky. The representative statistic is a multiply clipped mean at 2, 2.5, 3, then 3 standard deviations, which effectively gives the mode, and so is not biased by resolved sources. At present the CHOP_SKY_JITTER and EXTENDED_$n\times$$m$ recipes take the average of the modes of bracketted sky frames. For offline processing it would be possible to fit a spline, say, to the modes and provide a better subtraction. Under normal conditions, where the sky level is not varying rapidly or suddenly because of cirrus, the algorithm works well, especially over longer integrations.
[_FORM_SKY_LEVELS_, _NORMALISE_TO_MODE_EXTENDED_, _CLIPPED_STATS_,
_SUBTRACT_SKY_CHOP_SKY_, ESO/_SUBTRACT_SKY_CHOP_SKY_,
_SUBTRACT_SKY_EXTENDED_]