8" Parabolizing attempt #2: Abort...
My local astronomy club has a foucalt tester, and today I tested that mirror after 30 seconds of parabolizing. It revealed a big turned down edge (yahoo…) and some slight zones in the center of the mirror. I knew those zones existed - heck, you can even see the slight non-sphericalness in the second image of my “close enough to spherical!” celebration - but the radius of curvature changes rapidly where one zone meets another instead of smoothly blending. I was willing to call that close enough, but according to my local astronomy club’s resident professional optician, that non-smooth curvature would throw light all over the place and was bad enough I should return to a sphere and smooth that out. AAAAARGH
For reference, a mirror is usually considered good enough if its shape is within 1/4 of a wavelength of light (usually green light, wavelength 500 nanometers) - of an idealized parabola. This optician is a perfectionist thinks there’s no reason an amateur telescope mirror can’t get to within 1/10 of a wavelength. I appreciate his optimism but man I don’t want to return to sphere a second time.
…I’m going to return to sphere a second time. I saw the zones using my current testing setup - I just ignored them. This is possible. I can do this.
Total grinding time: 26 hours