> An ATM who lives in my area has made a 40cm tri-schiefenspiegler (sp?) which
> is helium filled. His telescope has a rubber diaphragm or a balloon or
> something like that, which acts as a pressure regulator: As helium diffuses
> out of the tube, the balloon contracts and thus replaces the missing helium.
> He says he has to add helium to the system once in a while, and release some
> of it during hot weather when the helium expands.
I think we have a case study here. Here we have an "ultimate planetary" (at least on paper) : 40cm unobstructed telescope, covered with an optical window and filled with Helium. On the other hand, take an "inferior" Newtonian (of the sime size), with all its problems of secondary obstruction, tube currents, spider diffraction, you name it. Yet, we've all seen awesome images that Don Parker and Japenese guy (Miyazawa ?) routinely produce. Can your friend's super-scope really surpass that work ?
Bratislav