[ATM] Re: 25" mirror possibilities

[vladimir sacek <vladis.2@juno.com>]

"...Is it possible to construct a corrector lens that would go near the
focal plane for an oblate ellipsoid?"

It would be hard to correct so much of spherical aberration with
corrector placed near the focal plane,
and not to have other aberrations induced. One possible solution would be
to have split meniscus
(same convex/concave radii, same glass) in front of the diagonal, to
correct for spherical aberration. 
It would have near perfect achromatism, but would about triple the coma
(I raytraced 25" f/5 mirror w/K=1).
That would put it at the level of  ~f/3.5 parabola, with ~1mm (1 arcmin)
diffraction-limited field. 

Coma can be corrected with another doublet placed near the focal plane. 

Vlad
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