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Re: Focal Length

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Posted by Robert May on December 5, 1999 01:13:56 UTC

I pulled out my Ingalls book and found I was in the middle of refractor work and then pulled out the Willmann-Bell edition and found the test. Unfortunatly, you didn't read too well as the "spit test" is for the Radius of Curvature rather than the Focal Length. To fix the problem, you go back to rough grinding and put the mirror on the bottom so that you open up the Radius of Curvature to where you want it. You can do the "spit test" at any time by merely spraying water on the mirror so that it is a complete sheen on the glass and then you will be able to get a reflection off of the surface. Getting the mirror back out will be about 1/2 of the work that you spent getting it there. The Radius of Curvature is what you're going to do the Foucault test at and the finished telescope will be the Focal Length long which incidently is 1/2 the length of the Radius of Curvature.

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