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Re: Foucault Testing 8" F/4.5

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Posted by Robert May on February 3, 2000 18:51:34 UTC

Sounds like you have a severely rolled down edge if you're talking about the last section. The relative distance from the center gives the difference in Center Of Curvature of the various zones. You should get to a point when traveling out where the left and right sides dim equally. Take as the point for the comparision about 1/4" in from the edge as the edges themselves do generate a nice bright diffraction of light on the left side. If you made a mask, you will also notice that all of the edges of the mask do tend to be very bright. Again this is diffraction at work. You do indicate that the Ronchi Test indicates a good surface which is kind of in disagreement with what the results you're indicating for the Foucault test. Or are you trying to get the diffraction image to work like the rest of the mirror? As far as the slit is concerned, I prefer the slitless method as you don't have to worry about a slit width and can use a .003"-.005" slit if you desire and you don't have to worry about alignment of the slit to the KE.

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