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I think you're totally right as to the principal cause. No central obstruction allows the diffraction patterns of stars to be as pointlike as possible. Other factors also prevent SCTs producing optimal images e.g. thermal issues, the fact that the primary is enclosed and at the bottom of the column of air rather than the top, the fact that the light rays travel 3 times through the tube in SCTs and once only in a refractor. The size of the diffraction pattern from a point source shouldn't have anything to do with focal length or focal ratio.
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