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Large, long focal length SCTs - Off Axis Guiders OAG (or the more up market) or On Axis Guiders (ONAG - beam splitter using a cold mirror - so IR goes to the guide camera and visible light to the main camera) which means the gude camera sees exactly what the man imager sees - so easier to find stars and less coma).
With a separate scope - mirror flex or droop in the SCT becomes impossible to isolate out - so its like a case of horrible differential flexure - running most of your shots.
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