8" Mirror Grinding: At the finish line for the third time
I’m at the final step of polishing a telescope mirror: turning a sphere shaped piece of glass into a parabola shaped piece of glass, called “figuring” or “parabolizing”. I messed up, twice, and since then I have been regrinding my glass back to a sphere for figuring attempt #3. Today, after several months, I finally have a spherical enough surface to start figuring attempt #3!
Here’s both a Ronchi test and a Foucalt test. The Ronchi shows straight lines (yahoo!) and the foucalt shows slight ring shaped surface variations. I don’t know what in my stroke is causing those rings. Maybe I wasn’t varying the stroke length enough?
While pressing my pitch lap into my mirror, I placed the mesh from a bag of lemons in between the two to create regular grooves called microfacets. On a hunch, I stopped using the lemon skin for the last few presses, and the rings seemed to decrease in height (but they were around 1/10 of a wave tall, so not a big deal). Maybe the lemon skin was so tall it stopped the pitch lap from pressing into the surface of the mirror somehow? Or maybe it’s unrelated to lemon bag and the rising temperature made the pitch softer, and that conformed to the surface better. Anyway, my mirror is much smoother now, and it’s time to make a parabola once and for all.