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Re: Focault Test

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Posted by Robert May on September 4, 1999 20:34:35 UTC

You need to build a foucault tester as you are unable to hold the knife edge well enough to see what you are doing. Making one out of wood is pretty easy as the critical parts are a smooth rod to slide the slider along and a Dial Indicator (about $8-15) to measure the focal shift of the slider. I sold two of them at RTMC last year for $40 each and made money on them. Use the slitless method for illumination with a led and the tester becomes quite cheap to build. If you don't want to buy an indicator, use a 1/4"X20 threaded rod and mark a dial for 25 marks around. The Ronchi tester gives a good view of the surface shape but won't tell you very well if your curve is right. Turned up edge is easy to get rid of as all it wants is some strong edge work. Turned down edge (Ronchi lines going faster the same way) is a lot harder to get rid of. There are several good designs out on the web for the Foucault Tester.

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