> Jaakko Oksa writes :
>
> > An ATM who lives in my area has made a 40cm tri-schiefenspiegler (sp?)
which
> > is helium filled.
>
> 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 ?
I was able to locate a four page article about the telescope in an old magazine. The atm in question wanted to make the best possible planetary telescope for the local environment. The telescope has thin zerodur optics and the tube is made of thin aluminium. He says this kind of construction was chosen so that the instrument would reach thermal equilibrium as fast as possible. He designed the optical configuration with his own software (consisting of 75,000 lines of fortran:-), and used some kind of a home made laser interferometer to test them. He made the windows out of BK7 optical glass. The article does not give any quantitative information about the performance of the telescope, only things like being able to see 20 craterlets on the bottom of lunar crater Plato (is that good?) and being able to see stars as diffraction patterns almost every night. Also, it says that with a ccd camera, star images are often only .5" in diameter, which is why fainter stars can be recorded than with his old 60cm cassegrain. Unfortunately there are very few images in the article, and not being a lunar observer, I can't say whether they are good or not :-)
Jaakko Oksa