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Maybe get (or make) an artificial star and record the focuser position at "in-focus", at various positions across the field. If you know the focuser step size, you should get an idea of curvature. You will need to place the artificial star a fair way from the scope to get reasonable accuracy - maybe 50+m - so part of it isn't indoors. A LED torch and a pinhole made from Al foil and with one or a few a tiny pinprick(s) in it will do as the star(s).
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