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