The various books I have acquired suggest building a Foucault testing device. I think I've fashioned a crude knife-edge apparatus and mirror stand. I placed the mirror from my Criterion RV-6 into the holder and after much trial and error discovered the right placement of mirror and tester. They are approximately 100" apart which seems to confirm the 50" etched on the back of the primary.
I haven't added a Barr scale or anything for measuring yet. Do these photos seem to indicate I'm moving in the right direction? I can see bright green specks where the coating is pitted or missing. I also see what looks like a big smudge... and where the mirror cell marked the mirror edge on one side. I'm just pushing against the piece of wood with the razor blade to move it into the light. I had a white light in my first design and then read that white LEDs aren't a good monochromatic light source so I went to green. I need to grind the surface of the green LED flat... it has concentric grooves and they were visible at certain angles...
~ Charlie Stevenson
8" f/5.7 String Telescope - 1st Scope Build; 2nd Place Stellafane 2016 Optical Award for Newtonians 12.5" and Smaller
10" f/4.5 Newtonian (June 2015) mirror refigured by Optic Wave Labs P-V WaveFront 1/14.24, Strehl Ratio 0.993 (Aug '15)
Criterion RV-6 seems to be circa 1973 (June 2015) [For Sale]
Celestron C8-A XLT (January 2015) [For Sale]
Celestron PowerSeeker 70AZ (Christmas 2014)
Aldrich Astronomical Society member since 2015