Re: A Schiefspiegler as first telescope (was Re: ATM 3-Vane or 4-Vane Spiders)

["Peter John Smith" <pjifl@bigpond.com.au>]

Regarding the relative difficulty of deep and shallow mirrors.

My experience is that reasonably deep spheres are not too bad to make.  Deep
paraboloids are another matter and are certainly much harder.

I once made a 6 inch F:60 sphere.  It was nasty to test by our traditional
methods because it was so far away.  But the working of the surface was not
particularly hard.

I sometimes wonder if the so called 'difficulty' of making a longer focus
mirror in the F:10  - F:15 range is more in the imagination simply because
it is close to spherical and the testing is so much more sensitive than for
deeper paraboloids.  In other words, the extra work expended on longer focus
mirrors goes into a more accurate surface than required while with deep ones
we do not directly see how bad it really is !  Any comments ???

Peter Smith