Mewlon

I have recently acquired a Takahashi Mewlon 250. This design (Dall-Kirkham cassegrain) is known to be a very sharp narrow field instrument. For visual use Takahashi states a usable field diameter of 21mm. When I use a 2"
55mm plossl (46mm dia field) the outer 50% of the field is out of focus. This is mostly due to field curvature since I can sharpen it up by refocusing. I think with a well designed zero power field flattener/corrector with around 150mm backfocus (for 2" eyepieces) this scope could be a superb deep sky performer as well. This would result in a sharp field of about 1 deg. Any thoughts on how to make a suitable lens assembly?

If you know the focal plane curvature, the simplest field flattener would be a single negative (plano-concave) lens just in front of the focal plane (it would have to be installed inside your widest field ep barrel, just in front of the field stop). If the focal plane curvature is R, the lens' radius of curvature is r=R(n-1)/n, with "n" being the refractive index.