Re: [ATM] [atm_free] RE: data and musings on thin mirror

[Stephen Koehler <s.c.koehler@gmail.com>]

Jerry,

> Now I estimate how many bands could fit on the mirror if they were all the
> size of the three bands in the center spherical area. I would say 6.

I have never seen this before, but it makes sense.  This is a very
useful test!  Thanks for explaining it.

I think the test for basic correction can be done without a need to
know the absolute distance inside or outside of focus that the Ronchi
image was taken, as long as you know the line spacing on the grating. 
 For a given focal ratio you are aiming for and for a given number of
lines spanning the Ronchi image across the horizontal diameter, there
should be a fixed line-ratio for the inner zone versus the outer zone.
 The ratio changes, but it can be calculated using simple geometry.  I
will come up with the formula, and try it out on some simulated
examples.

Has anyone else used this form of the test?  With it, it seems that
just a Ronchi test at COC could get you fairly close to figuring a
good mirror.  It tells you edge condition, smoothness, and overall
correction.  I should add that it is also important to test for
astigmatism when the mirror is still at a sphere.

--
Steve Koehler
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