> I recently corresponded, via email, with the author of the "Christine"
> article in the current Sky and Telescope. He wrote that he obtains much
> better than 0.001" repeatability on the Foucault test - even on center
> zones.
0.001" ? 25 MICRONS ? I have _never_ seen or heard of someone using Foucaut with such a precision. How does he do it ? With such a repeatability you don't need fancy flats and interferometric setups. But it's not the Foucault setup nor "pretty fancy tester" in question - it is YOU who has to judge the position of a very vague zone with precision of a fraction of mm from the distance of several meters ! In his case even more difficult because of the diffraction effects from a Couder mask. Tall order. I'm still somewhat skeptical ...
Bratislav