Foucault Testers

These are actually my 2nd and 3rd testers.  The first was a slit tester with a VERY hot light bulb and a short tangent arm.  It was a decent tester, and I learned a lot about testing by using it.  But when my 8" F/3.9 was polished, I built tester the first tester pictured here from new wood and the old tester metal parts.  It's made about 10 mirrors now.  Pieces of brass attached to the carriage ride on a polished stainless pipe.  A dial indicator makes taking readings much less prone to error (it is easy to misread a micrometer in the dark).

tester tester 2 tester 3

Here are some more pictures of slitless tester #1.  They show the slit and mechanism in more detail, and the underside of the carriage.

focault 4 focault 5 focault 6 focault 7

Tester #2 was built as quickly and as cheaply as I dared.  It works quite well.  The carriage has Teflon pads on the bottom that ride on PVC pipe.  The "knife edge" is a piece of brass, prepared by honing on a piece of ground glass as described by Texereau.  I used this tester to work on refiguring two 10" mirrors while the other tester was being used for the 30" F/3.8.

focault 8 focault 9Focault 11
focault 10 focault 12  

This tester worked much more smoothly than I thought, and a little oil always helps the action of the two positioning screws.  It's made from common parts and costs about $20 to make.

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