Casse-fractor telescope design

Well, my 4" TV-102 Light Cup is constantly looking for a new way to grow its midget aperture. Having failed at plastic lens design and biological based lens, its aspiration as a budding, albeit illiterate optical dis-signer ;-), came up with this latest innovative telescope
design!
http://members.cox.net/ronby/Photos/Cassefractor.jpg

Although the cartoon mixed in some humor, the fact that the design was drawn on an engineering paper surely suggests the seriousness of this post! What do you guys think? Why hasn't such a design been thought of? Is it possible? If so, it would make an 8" APO extremely portable and hopefully cost a lot less ;-).

Ron B[ee]

Heh heh. Nice! But there are some flaws in the design.

The secondary obstruction is much larger in this case than simply the secondary compared to the objective -- the true secondary obstruction is in effect the size of the hole in the primary mirror relative to the size of that primary which will be disproportionately large for this sort of arrangment, IMHO.

You also have an interesting curve to generate for that primary and secondary since it has to bring to a point focus not parallel incoming rays of light, but already-converging rays, which changes things. You'd need to grind some wacky parabolic elliptical hyperboloidal torus or with a Mobius strip for a secondary. It would be laborious, complicated, EXPENSIVE (!)

Actually, this "cassefractor" has similarities to Houghton-Cassegrain, except that the second mirror has to be convex in order to throw the focus out of the tube. Houghton is a full aperture corrector that consisit of air-spaced positive and negative lens (both same common crown glass), correcting for spherical aberration of spherical primary/secondary. It gives as good image as an SCT, or better, but with only ~1/3 its field curvature. Also, by adjusting the air-space, correction can be nearly perfected in an assembled unit. Alas, the c.obstruction can't go much bellow 30%, just like with any other Cassegrain-like system.

An alternative is Lurie-Houghton, in which the secondary is replaced by a diagonal, for a Newtonian-like configuration.
The system is nearly perfectly color corrected at f/4 and the only aberration is astigmatism (yes, field curvature is also negligible), which is less than 1/10 of the coma of an f/4 Newtonian, and less than 1/5 of the coma in a comparable Mak-Newtonian. For some reason that eludes me, neither system is commercialy available (except as a custom order, which is likely to be - ouch! - expensive.

BTW, the "cassefractor" scheme could be somewhat modified to fold long-focus refractor. Primary flat would be tilted downward, reflecting the light cone to the secondary flat bellow the objective lens; secondary flat would reflect it parallel to the axis to the diagonal. A 6" f/18 achromat could be folded down to less than 30% of its original length.

Ron, you should take a look at the Sept '99 Sky & Tel.
There's an article on a fellow who built a Mersenne telescope, using a 20" f5 parabolic primary, a convex paraboloidal secondary, and a Tele Vue 4" Genesis refractor. The secondary takes the converging light from the primary and turns it back into parallel light. The refractor then views that light, and brings it to a focus, using the refractor's lenses, focuser, and eyepiece. He also built a 30" Mersenne scope utilizing a Tele Vue 140 refractor. The observer can sit in a chair while observing. Pretty neat.