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Rotator is needed in case ALTAZ (dob) mount is used, for obvious reasons.
Also, perhaps in slitless spectrometry, it is useful. to avoid overlap of spectra with nearby stars.
Otherwise, I can't see the reason for it..
NB:
Now I know I may have just created a lots of anger in art-oriented individuals.. but IMHO, astrophotography is tech-scientific photography. So the orientation has strict limits/requirements: North direction should be UP, West direction should be to RIGHT, always.
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