When the chopper is not used, there are two techniques one uses
in taking IR spectra. There is the classical sequence where one
observes the object at two slit positions. i.e. ABBAABBA... etc., and
there is the more complex case where one has observed the target along
several slit positions.
In the classical case, one simply subtracts frames taken at different
slit positions. So one needs to form A-B, B-A, etc. This simple step
removes the sky and results in an image with two spectra, one
positive and one negative.
In more complex cases, one could build a sky frame from several
spectra, as one does when building the sky frame in imaging. This
results in an image with only one positive spectrum.