The sagitta on a 20" F3.0 mirror is .417 inches. Sagitta is the depth of the curve at the center on the glass. That .417" number is huge for a mirror maker. As in, that's a lot of missing glass! You can see in the photos here that I can get a couple of fingers under the measuring stick I have set across the rim of the glass. This blank was created for us by Newport Glass Works, who also did the Blanchard grinding on the back, and the pre-generation. This one came out right on the numbers, with the generation job clocking in within a half inch of perfect. That saves me a lot of time, because I can start grinding a blank like that with 25-micron instead of #80 or #120. That's many hours avoided on the grinding machine.
This blank weighed in at 38 pounds. You can see the "swirly" marks left by Newport's diamond generator. You need to grind with 25-micron until those are all gone, and then a little more to get rid of sub-surface damage. For this, I ground with about 25 to 35 pounds of weight added on top of the mirror, running the fixed-post grinding machine full out at 60 rpm's.
After grinding and polishing, the mirror was figured by hand using sub-diameter pitch polishers. You can see a photo of the mirror on the test stand with a Couder mask in place. After the mirror was finished, it went to Majestic Coatings and Jeff Decker made it shiny for us. I also fabricated the large elliptical secondary mirror for this project, which Jeff coating as well.
I've heard from my client that this mirror is a top performer, with a terrific star test. On the FigureXP results of the Foucault test, you can see the software gave the mirror a Surface RMS Error of 3.3 nm, with a Strehl ratio calculated at .994. For this particular test series, FigureXP rated the mirror at a bit better than 1/23rd wave, P-V on the wavefront. The larger and faster the mirror, the smaller the surface errors must be to merit a great wave rating like that. You can see from the graph that the highest zone on the mirror is about 11 nm, in the outer area of the mirror. For those like me who prefer inches, that represents a zone that is .0000004 inches too high!