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I do not think that the field stop is limiting the performance of your RKE 28mm. I calculate it would need a field stop of about 22mm to give it full field with an AFOV of 45 degrees, and the RKE website lists it as having 23.3mm (if the same). Only if you go above about 55 degrees AFOV at about that focal length eyepiece will you need a 2" barrel. At that point I cannot see how you can escape an expensive well corrected eyepiece like a Nagler 31mm which will give you twice the field but with slightly larger exit pupils (~6.5mm). Good luck
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