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 believe it was his experience that the problem of diffusion and pressure changes inside the telescope could not be solved simply by making the telescope as air-tight as possible.
Jaakko Oksa