Worse news: My mirror is not only not a sphere, it isn’t even fully polished. For the first time, I got the glass properly clean (lots of running water and soap while soft wiping with thoroughly cleaned wet fingertips, rinsing with distilled water, drip-dried) and gave it the laser test. I shone a red laser pointed down onto the polished surface, and observed two spots: A bright one from the back surface of the glass, and a fainter (but still quite distinct) spot on the front surface. This second spot had something of a sparkling appearance, suggesting to me that there are still a lot of microscopic pits remaining. Damn.
So while I puzzle this one out, I’ve been working on building a slitless foucault tester. I’m basing it upon a design from the Stellafane website, and it is coming along nicely. I’m adding the final parts to the stage to link it to the base properly, and once that’s done I can start the actual guts of the system: The knife-edge (using an old blunt blade from a Stanley knife), an LED light source, and some way to hold the bits together. Picture will follow in a later post.