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
new wood and the old tester metal parts. It's made about 10
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
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
helps the action of the two positioning screws. It's made from
parts and costs about $20 to make.