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This is a useful facility for eliminating flatfield uncertainties. Each observation consists of a pair of exposures, I and II, taken in each of the 2 apertures, L and R. Sky subtraction can be performed via (L-R)I + (R-L)II, and the result is a 2-D sky subtracted frame, which is very useful for extended objects.
Mode A gives a 5" or less slot, which is rather small. Mode B gives a 25" slot, which is well suited to work of this nature, but unfortunately the apertures are 85" apart. This means that the TV cannot see both apertures simultaneously.
For faint object work, where a single 1-D spectra is the required final product, these modes could still be used. Modes C and D are more appropriate particularly since the online software is designed to handle these modes more effectively.
Mode C is a 200" longslit mode used for multiobject work, and very extended objects.
Mode D is a centralised 25" slot offering enough local sky for interpolative sky subtraction. Flatfielding is required.