Quote:
Originally Posted by Wavytone
Frank,
No, a visual observation of the moon or Jupiter is not a meaningful test. Sure it is satisfying to know you are getting an image, but it tells you nothing about how good the figure is, nor what you should do to correct it.
You could make these measurements with the mirror in a telescope pointed at a bright star, with a knife edge at focus and no eyepiece you can do something similar to a Foucault test - however its very awkward to do this in practice, and atmospheric turbulence makes it impossible most of the time on mirrors more than 6" or perhaps 8".
You should be setting up the mirror in a place where you have still air and using one of the various tests described in the ATM books - a Foucault test, Ronchi, Ross null test, Dall null test, caustic test or perhaps interferometry. Plenty of sources on the net describe these.
These tests will tell you how good the figure is,and also show you where the surface is high or low.
My mirror making days were long ago... Maybe one day I'll try again.
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thank you for the info. It passes using my foucault tester, just. I will try your knife edge idea, Cheers Frank