Re: [ATM] Can Foucault Results Indicate Astigmatism?

["Richard Schwartz" <richas@earthlink.net>]

> [Original Message]
> From: Jim Burrows <burrjaw@earthlink.net>
> To: <atm@atmlist.net>
> Date: 5/14/2004 5:08:04 PM
> Subject: Re: [ATM] Can Foucault Results Indicate Astigmatism?
>
> At 2004-05-13 22:25 -0500, Kurt Clement wrote:
>
> >I am using TEX and Foucault Test Analysis 1c/Reifke, and they both 
> >indicate that I am at 1/8 or better and 0.96 Strehl, using a three zone 
> >mask.  Included in this message are the TEX results...
>
> Sixtests agrees, surface RMS = 6.2 ± 2.8 nm (2 sigma, assuming .01" sigma 
> readings), .98 Strehl.

So what?   How does your mirror look in a star test?

>
> >How would I interpret this data to indicate the presence or absence of
any 
> >astigmatism?

Your data does not include astigmatism information.   For this, you must
think outside the box.   

> Essentially, the Foucault test is blind to primary astigmatism (it can
see 
> higher order astigmatisms, but they usually don't hurt as much as PA) - 
> you'd have to measure the ROC for different diameters to hair's width
accuracy.

The foucault test is NOT blind to astigmatism.   But Tex and Sextestes and
CELL.EXE and all the rest are blind to astigmatism.   This is because you
are supposed to be using a mirror with a thickness ration of 1:6, and such
a mirror is supposed to be immune to astigmatism.   Ha hah ha!

> >I sat down at the library and read parts of Texereau, edition 1, ...

So, what did Texereau tell you about measuring astigmatism?   It is easily
done with an ordinary foucault tester.   I have posted many times on this,
so I won't boer you.

> About the only quicky PA test available to the amateur is Texereau's
bench 
> star test, p. 85 (edition 2).  It's not very good - non-passing PA is
just 
> barely visible (see http://home.earthlink.net/~burrjaw/atm).

So, if this test fails to reveal what you need to know, why bother to do
it?   Obviously you need to think outside the box on this one.  I repeat: 
it is easy to measure astigmatism with an ordinary foucault tester that has
a pinhole and an accessory set of zonal masks.  I don't know if slit
testers will work on this; that problem is left as an exercise for the
student.

. .. Richard


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