Quote:
Originally Posted by Don Pensack
I'm not sure who these eyepieces were intended for.
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Small refractors, Don... they dominate the japanese market. Hence why they were never heavily marketed outside Japan. Of course you'd see off-axis images as blurry defocussed winged seagulls in a big dob, because the field is curved in the opposite direction to that of a small refractor.
Refractor: Petzval curvature = F/3 (approx) - which is strongly convex towards the eyepiece
Newtonian: Petzval curvature = -F - ie concave towards the eyepiece
PS I very much doubt it's astigmatism because if that were the case the primary mirror would have to be warped and this would be painfully evident in EVERY eyepiece you have.