RE: [ATM] Can Foucault Results Indicate Astigmatism?

["Thomas" <tomjan@ozemail.com.au>]

But that's why most books tell you to rotate the mirror under testing,
so that if there is astig present it will show it's ugly head!

And yes it won't tell how much it is.....


Clear skies,
Thomas Janstrom
http://www.tjanstrom.com
http://www.norsewines.com.au
"Your nobody untill you've been ignored by your seventh cranio-facial
nerve."


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Don Ware
Sent: Sunday, 16 May 2004 9:19 PM
To: atm@atmlist.net
Subject: Re: [ATM] Can Foucault Results Indicate Astigmatism?


A friend sent me his 16" f/4.4 mirror for testing after he had measured
it with his foucault tester using a Couder screen.  His tests indicated
a better than 1/4 wave figure.

I tested the mirror using Robo-foucault, which showed quite a poor
figure.

I then tested the mirror using a Fizeau/Ceravolo interferometer.  Fringe
analysis using FringeXP showed significant astigmatism, (Z4/Z5 =
0.82/-0.60). The surface error pattern followed the mirror when it was
rotated, so definitely in the glass, not a test stand artifact.

Orienting the foucault tester so the test axis was parallel to the null
between the astigmatic lobes gave a result similar to what my friend had
achieved.  Orienting so the test axis passed through the astigmatic
lobes gave my original result.

Conclusion:  A series of  foucault tests along several axes can show
that astigmatism is present, but without quantitative values.

Don Ware
http://webpages.charter.net/dware1/


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