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Square stars results from strong four-sided symmetry anywhere in the path.
If you consider that that you actually have a circular objective cut into four smaller quadrants, each one with two straight sides and a curve, hypothetically you can re-arrange them as a square aperture with the curved bits overlapping in the middle. This will give square stars just as surely as shooting through a square objective does.
Only way to get rid of them is either:
- use a scope with no vanes, or
- if you must have vanes, make them curved with a radius equal to the semidiameter of the objective
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