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For the classical ABBA technique, one multiplies each image by -1 and adds
it back to itself after a suitable shift. This method of combining data is
often called double sky subtraction as it effectively moves any residual
sky that remains after the first subtraction. It results in an image
that has one positive spectrum and two negative spectra either side of
the positive spectrum.
In the more complex cases, one combines the individual spectra after suitable
shifts have been applied.