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Re: Alumnized Mirror Failure

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Posted by Robert May on September 9, 1999 19:02:44 UTC

Astigmitism can be seen if you rotate the mirror on the tester and then do a foucault test. The results will be different, especially if you teset at each position several times (and if somebody else does the test also). Rotate about 30deg. between tests and you will see a different amount of correction or even difference in the focusing point. On the testing with a Ronchi grating at a star, rotating the grating will give a different amount of banding as you rotate through 360 degrees although I generally don't put as much weight to it as it is more subtle. Many people don't rotate enough when doing parabolizing strokes and thus end up with some of that nasty error. I might also note that EPs can be astigmatic also so that may be the problem, especially if the star is off of center. The last thing that can cause astigmatism is the secondary isn't flat but rather some large radius curve. Have fun tracking it down.

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