8" Mirror grinding: Parabolization #1
I’m in the home stretch - my goal is to turn this sphere shaped mirror into a parabola shaped mirror.
The final stage of mirror grinding is called either parabolizing, because you’re making a parabola, or figuring, because you have to measure carefully and that involves numbers which are also called figures. You use a stroke that goes up and down fast and side to side slowly in a big zigzag, called a W stroke, to remove a tiny bit of glass from both the center and the edge, as seen in the first picture (from Mel Bartels’ site).
This stage involves removing very small amounts of glass, so parabolizing is very quick. After 15 minutes of a W stroke - which is crazy fast compared to hours and hours it took to get to spherical - my center was slightly deepened, enough to match a parabola! My mirror edge, however, still had very straight bands. The Ronchi test calculator said my edge wasn’t where it should be.
Then I made a mistake: the amateur astronomers in the discord said I had a turned edge, and I needed to return to spherical. A mirror edge that’s too low can only be fixed by grinding the rest of the rest of the mirror down to that height. So I spent anouther two hours undoing all my hard parabolizing work.
But in retrospect, I didn’t need to do that at all - my edge was focusing light too close, so if I wore down the edge it would have focused light at a longer radius of curvature, which is what I wanted! I could have done that! I mistook the error for “turned down edge” when actually the edge wasn’t turned down enough!
Anyway, I’m back at a sphere. Parabolization attempt #2: begin!