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John essentially said it all.
I can not add much in terms of technical details but have some practical experience...
I used to have Meade 8" SCT, loved it to death but it did suffer from field curvature..., and coma !
My new scope (Meade 200R) suffers from none of those and to my eyes the field is essentially flat.
I think some of our colleagues who do astro-photography with the new R or ACF optics can comment on its field curvature as it should be visible on large format cameras, those have flat focal plane and may show any curvature that exists, eyes are probably not the best tool as they can accommodate some curvature, worst still the amount of such "accommodation" varies on the day.
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