[ATM] Re: ATM Digest, Vol 20, Issue 10

[Hugues.Laroche@ses-astra.com]

Hello Guy and Jim,

Subject: Re: [ATM] refiguring a 20 inch Pyrex f/2 cassegrain

>>Has anybody ever tried refiguring a 20 inch Pyrex f/2
>>(or thereabouts) Cassegrain? I understand that the
>
>  You'll have to think about what test to use - 
>  Foucault's hopeless at that f/.

Relatively funny, but Ronchi works well with such
deep mirrors, if you take it as an improved wire
test. The nulled zone is measurable as the radius
of the "bifurcation" points: inside this radius, you
have the typical Ronchi pattern outside focus, and 
vice-versa. These bifurcation points are actually
the intersections of the straight line and circle of
the greek letter "phi" (classical look of the wire test).
The advantage over the wire test is purely practical:
as there are many lines, no need of careful alignment etc...
So the lpi of the grating as in principle no 
importance here.

If you play with RonWin, you will see more clearly 
what I mean.

On the other hand this test does not give much information
on the surface status, so it must be completed with smthing
else.

Hugues
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