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Re: Dall-Kirkham Primary Mirror Testing

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Posted by Robert May on May 14, 1999 17:32:03 UTC

From the way you stated the question, I suspect that you have made a fixed source Foucault tester? Suggest that you modify it to a moving source tester. This is easy! Just put a Led or opalized lamp near the bottom of the knife edge and have it half obscured by the knife edge. The light is easier to find (the half big spot is brighter) and the test is just as accurate. Also, there's no need for off-axis calculations as everything is on axis. As far as the figuring, you need to go slow with short strokes to get the prolate shape. A hard lap is also usefull in making the prolate shape happen. One of the guys at the mirror making class that I attend made a nice prolate shape that looked and measured just like a very nice paraboloid but the sign of the movement of the tester was the opposite of a paraboloid. With the amount of curvature that you have you may also want to make a subdiameter lap of about 3" in diameter and use that to do the work. Good Luck. BTW, the Dall-Kirkham is an interesting design but you do realize that the coma is worse on that design than a Newt? If you are making it for a planetary scope, then it is a decent design if you have a large magnification with the secondary.

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