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Re: How Does Grinding Work?!?

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Posted by Robert May on June 10, 1999 02:33:48 UTC

The whole thing is to visualize where the weight is resting when the top is hanging over the bottom pieces of glass. Where the force is greatest, the most work is being done.

As far as the comparision between the Ronchi Test and the Foucault Test is that they both test different things. Yes, the Ronchi Test can be used to test for the curvature of a mirror but it is a lot more sensitive to smaller surface errors like zonal errors. The gentle curvature of a line is relativly hard to see in the Ronchi Test. The Foucault test, on the other hand, is very sensitive to curvature errors, especially when a Couder Mask is used. As a result, I test the absolute shape of a mirror with the Foucault Test and then look at the general shape of the mirror with a Ronchi Grating to see if there are any errors in the overall shape in the mirror between the locations that I tested with the Foucault tester. I might also note that with the Ronchi Tester that I have, it is a lot easier to rotate and thus get all of the angles of the mirror under construction.

Lastly, in the older days, a 6" was a decent sized scope and was recommended as a start point. These days, with amateur scopes over a yard in diameter becoming seen at the various conventions, a 10" isn't that big to make. In the mirrormaking group there are 3 6", 4 8" and 3 10" mirrors that are regularlly seen being worked on. One of the 8" mirror grinders is making a binocular telescope. It isn't until you get to about 14-16" that you start having trouble pushing the glass around.

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