Re: [ATM] The zonal Foucault test is free of inherent correction

[Michael Peck <mpeck1@ix.netcom.com>]

At 02:57 PM 3/11/2005, Arjan te Marvelde wrote:
>Mike,
>
> > b) He uses rather more zones than is common for ATMs. He recommends a mask
> > width of f/300, which would produce a 15 zone test in an f/5 mirror
> > regardless of mirror diameter.
>
>Or is his f/300 actually ROC/300?

Yes. An f/5 mirror tested at the center of curvature produces an f/10 light 
cone.



>The point of the article was to show that geometric optics approach to
>foucault test gives results that do not correspond with reality, and that
>wave optics does.
>In my opinion this may be the case for the setup he uses (many small mask
>openings, very thin slit), but in setups used generally by ATMs the
>difference might not be as obvious.
>

The basic conclusion he reaches is that while there are significant 
observable phenomena that can only be accounted for with diffraction theory 
zonal measurements aren't actually significantly affected relative to 
predictions of geometric optics. That means it should be safe to use your 
favorite data reduction program, all of which are based on geometry. This 
isn't really a surprising claim: Jim Burrows and Nils Olof have said 
similar things long ago -- not so long ago as Linfoot however.

A generic effect of an extended source is to smear out diffraction effects 
without entirely eliminating them. I'd expect geometric optics to provide a 
better approximation in that case -- at least that seems to be true in 
situations I've analyzed.

Mike Peck

------
Michael Peck
mpeck1@ix.netcom.com

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