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Posted by Daniel Johnson on October 29, 2002 18:09:26 UTC

You can buy Ronchi grids cheaply from Orion (at ). I have one mounted in the barrel of a discarded eyepiece so that I can test the figure of any telescope I come across.
There are many ways to make a Focault tester. Basically you need a pinhole light source and a way to move a knife edge forward and back a known distance, and have the knife edge mounted on a pivot so that it is able to move or rotate across your line of sight a small amount to cut across the image of the pinhole. Consider, for example, having your knife edge screwed onto a a piece of aluminum--the screw serves as a pivot so that your knife edge can move across your line of site. Then attach that piece of aluminum so that it slides firmly against another piece of aluminum, and advance the mobile piece with a 1/4"-20 screw or any other screw of known pitch. By putting a crude dial on the screw, you can know how much the knife edge has advanced.

--Dan Johnson

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