[ATM] Re: Weird Foucault readings

[Vladimir Sacek <vladis.2@juno.com>]

Bob, we can easily turn the tables around: how about the shame, horor and
fears of talking about Foucault test limitations - and possible
alternatives. No one - except you - is talking about 3-zone mask. You can
go 11-zone mask and still have reading accuracy compromised by the zonal
defocus, although in a lesser degree. But zones can be narrowed only so
much before it starts causing reading difficulties of its own. Also, I
was talking about perfect mirror with perfect zonal defocus structure.
Actual mirrors are certain to be less than perfect. Consequently, their
zonal defocus will be more disturbed (less symmetrical) which doesn't
help reading accuracy.

Fact that Foucault test accuracy has limitations - assuming perfect
setup/application, the core of it, zonal defocus, still remains - doesn't
mean it can't be used to produce excellent mirrors. Afterall, optical
excellence begins at ~0.9 (actual) Strehl, or ~1/6 wave s.a. level. For
any given r.o.c. all it takes is a smooth 1/3 wave surface error from a
perfect parabola.

Regards, Vlad


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