There was a telescope design I heard of that created a collimated beam by using a special curved secondary. The scope I think was called an Afocul Mersenne. There was a fellow in Texas Clyde Bone, that essentially "relayed" the afocal image down to the bottom of his 20" dob, then re-imaged the parallel light with a refractor. His main concern was being able to sit down while viewing.
I recall hearing there was some unusual effects with this design, one of them was a lot of field curvature.
I have tried some simple relay experiments, and found it was possible for a newtonian to give a correct view image !
If Vladimir is still around, he could give you some concrete info on any pitfalls to this idea.
Dick Jacobson said:
I am building a periscope accessory that will attach to the side of a big Newtonian to lower the eyepiece height. Does anyone know if this has been done before? I've heard of a periscopic Newtonian that was spotted many years ago at the Texas Star Party.