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Re: Knife Edge Testing

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Posted by Robert May on May 4, 1999 17:15:52 UTC

The measurements that need to be taken are the ones that measure the distance to the mirror - the first example. This should be measured to .001" or so. For initial work in testing, .005" is plenty good enough for a short FL mirror that has about .1" difference between the center and the edge. There is no need to measure the second example of yours as what you are trying to do is to get two parts (left and right of the center equidistantly) to grey out at the same time and this movement is what does the greying out. The best thing to do is make the Couder Masks (they're not hard - I brought in a set of masks that I made from pizzabox tops to the mirrormaking class that I am part of and everybody is now calling them the pizza masks). The process is to move the tester to a position and see if the greying out is correct. Move the tester in and out until you have the correct position as the mirror section greys out. Absolute position isn't important, just what the difference from the position that the center does it's greying. You are measuring the radius of curvature (in and out, not the angle at which the mirror points to (left and right). Make sure that the edge of the mirror is a larger radius than the center. One of the guys in the class actually got the right dimensions, only got the direction of the measurements backwards - the edge was shorter in radius than the center! I was impressed because he had a 1/33 wave surface. From what you are asking, I suspect that you don't have any books on making a mirror at hand. If so, I would suggest that you find one at the library or buy (Willmann-Bell has several) one. Have you done the calculations so that you know when you have a parabola yet?

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