Here are some pictures of my "Double Foucault" tester, the reason for my calling it a double Foucault tester is quite obvious when you look at the pictures, basically what I did was a couple of years ago I built my first tester as per Texereau, and found it difficult and imprecise in use, so I decided to keep the main stage and build a better version on this. This second stage does not use a conventional slit or pin hole and knife edge, but a single edge slit and the continuation of this edge is also the knife edge. This has two advantages over the traditional Foucault tester in that the light source moves with the knife edge and that the slit is ALWAYS parallel with the knife edge. The light source I used, which is housed inside in the aluminum angle, is the light globe from a Mini Maglite. I will in the near future add a schematic of the light source/knife edge housing to clarify matters some what.
This picture shows the front of the tester, here you can see the combination slit/knife edge arrangement.
Here you can see the side of my tester, you can see all I did was to build a second tester on top of the old one, this gives me a course adjustment stage (the lower older one) and a fine adjustment stage which is micrometer operated.
Here you can see the set up ready to test my 4.25" Cassegrainian primary. Incidentally the bench that the mirror and tester is set up on is also my grinding bench.
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