Sky Subtraction

To be able to do sky subtraction, ORAC-DRneeds to know out of a pair of frames which is the object frame and which is the sky frame. To do this ORAC-DRexamines the FITS headers. If the offset for both right ascension and declination are less than 0.001 arcseconds, then the frame is treated as being on-source. Otherwise, the frame is off-source and is used as a sky frame.

IRIS2does not record telescope offsets for spectroscopy mode, so this method cannot be used. Instead ORAC-DRexamines the aperture used. If aperture A is used then the frame is on-source and the right ascension offset is set to zero, otherwise the frame is off-source and the right ascension offset is set to 26.92 arcseconds.

ESOinstruments are different again as observations are not done in object-sky pairs. Instead they are done in equal-sized blocks of object and sky observations, such that a certain number of object observations are done, followed by an equal number of sky observations. In this case the initial frame in a group is always assumed to be on-source. As with the standard pipeline, an observation is considered to be off-source if its offsets are greater than 0.001 arcseconds.
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Sky subtraction is straightforward - the sky frame is subtracted from the object frame.

For ESOinstruments the corresponding sky frame in a block is subtracted from the respective object frame in a block, such that the same position in each set is considered as an object-sky pair.
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