I have been thinking about building an off axis scope for quite a few years now, but still have not got around to it,the best designs i have seen
are the buchroeder tri-shiefspiegler which consists of a concave primary and tertiary, and
a convex secondary, all 3-mirrors have the same
R.C.,The secondary and tertiary are spheres,and the primary is about 50% corrected parabola(if i
remember correctly).
Jose Sasian also has a good design, i have not
studied the various off axis designs lately, but i do recall that surface smoothness is a very important factor with off axis scopes, i was thinking of trying submerged polishing with barnasite(the real stuff with thorium oxide)to
produce an extremely smooth surface which would be
practically free of microripple.
Search the web using key words like buchroeder
,shiefspiegler,off axis telescopes,etc. good luck!