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Mark is right on the money. It would be a shame if someone decided purchasing a mirror based solely on testing method,ie, interferometric testing. Reputations are far, far more important. Mirror making isn't just a science, it's like a skilled tradesman making some other specialty item, such as a world class musical instrument, and art and the skill of the tradesman plays a huge part...if you want something exceptional, rather than the mass produced, although still reasonably good performing optics available today.
An optician with a great reputation will guarantee his work anyway, regardless of certificates.
Interferometry can be abused just as easily as politicians play with numbers and budgets.
Unfortunately, the world has become victim to specifications, since marketing people realised they had a new sales weapon.
I know of a friend who had a professionally made zerodur paraboloid with interferometric data, that was obviously wrong. A quick foucault test or star test to check it revealed the optic wasn't polished quite well enough, and had some zonal aberations. No idea what the actual Strehl was, but no where near the 0.98 they quoted.
Best thing is to take the time, find out and ask people who really is good at their art. This should narrow the list somewhat, and you have freedom to chose from the remaining, hopefully shorter but more acomplished list of mirror maestroes.
Hope your friend finds himself a beautiful optic,
Nick
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