So, above is the FigureXP graph of the surface error of my 6" F4 mirror when I put it aside to fiddle with my tester (see post below). Stellafane Mirror Class was last Saturday, so I took the mirror with me in this condition. Dave Kelley, one of the resident testing and figuring gurus, took a look at it. His beautifully machined aluminum testing apparatus is of course vastly more precise and sophisticated than my makeshift plywood rig. For starters, his test bench includes an exqusite (better than 1/20 wave PV, I've been told) 16" flat for autocollimation testing. More importantly, he has a professional career’s worth of experience interpreting what he sees at the knife edge, and an apparently intuitive knowledge of what to do strokewise in response.
Through the day and with frequent testing, Dave had me work with various strokes and various amounts of suppression of the middle of the lap—which was accomplished by pressing with a square of paper between the mirror and the lap and later by actually shaving away at the lap in the middle with a razor blade. I should have made better notes; I don't remember precisely all the variations in strokes used. At first I was completely mystified because I assumed that the goal was to move the mirror toward a paraboloid. But after a while I understood that Dave wanted me to go back to a sphere and start over with the parabolizing.
I still wasn't sure why he was having me do what I was doing. When I got home it occurred to me to have Figure XP display the graph again, but this time in reference to a sphere rather than a paraboloid. This is easy—the conic constant of a paraboloid is -1; that of a sphere is 0, and this is something you enter when setting up the file, so all I had to go was go back to the setup page and change that one field. Below is the result, showing the mirror’s departure from sphericity. It's easy to see why he wanted me to work the outer zones but not the middle, and I think the strokes he prescribed were chosen to avoid turning down the edge.
The result at the end of the day was very near a sphere, but with annular rings of error showing in the foucault test, as you can see below. I can see them a little in the Ronchi test, and more so in the Foucault, but they're pretty small errors.
The kicker, though, was that Dave thinks many of my problems are due to the lap, which he deems too hard and insufficiently off-center. So my next project is to make a new lap. I hope to find time to do this well before the next (and last for this year) class, so I can work the mirror a bit before then, first to try to smooth out those rings, and then to start parabolizing.
Note: I had to exaggerate the contrast in photoshop to make those rings visible onscreen. They were somewhat more obvious to my eye at the KE. Also, those black blotches are artifacts of dust in the camera optics.
And a final note: At the meeting after the class, I once again was voted into (associate) membership in the Springfield Telescope Makers. I can't begin to express how great it feels to be once more, after so many years away from the venerable group, welcomed into the fold. To any members who happen upon this site: THANK YOU VERY MUCH.