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Cass Scopes

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Posted by Robert May on February 17, 2003 19:55:39 UTC

The cassegrain scope is a bit more difficult to build than the Newt. However, it isn't that hard to do!
You will need to do some trepanning (fancy name for drilling) on the primary to get a hole in the middle but that is a fairly low speed process and is quite simple to make the tool to do it. If you have a drill press, you just need to make up a drill bit with some plywood and roof flashing to do the hole in the primary.
The rest of the job is to just start grinding optics. You will need 3 pieces of glass - Primary, secondary and a fancy named sphere called a Hindle sphere. A flat may be used in place of the Hindle sphere and the production of a large flat (might as well make it really large so you can both get a good center section or get a flat that can be used for almost anything) so that you can do the double pass method of testing the optics.
Basically if you have gotten to the point where you can make a 1/8th wave paraboloid for a Newt., you probably have the experience and capability to do a very nice cass telescope. It will also really make your astro friends really think that you are doing something amazing.
There is a calculator for Cass scopes on the web (sorry but I don't have the url for it) at two sites and that will tell you what you need for surfaces. For a RC type cass, the primary will be a Hyperbolic surface that is a little bit stronger than a paraboloid (a paraboloid is a -1e surface and the hyperbola is usually about a -1.1 or so which means that it is just about 10% more aspheric than the parabola). The secondary is usually about -4 or so which sounds hard but is actually not all that bad on a small secondary.
I'll note that professional opticians don't like doing anything other than spherical surfaces as surfaces like that need to be handworked and need to have some kind of odd test (they normally only do null tests) with calculating to find out where they are. Companies like Meade and Celestron have made their name by making spherical surfaces work for them - the corrector plates are done by a spherical polish on a stressed corrector plate!
BTW, I'm slowly working on a Cass myself and the slowness is due to the fact that I only work on it in the miror making class when I'm not providing guidance to others.

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