Re: [ATM] Can Foucault Results Indicate Astigmatism?

["Don Ware" <mimas@charter.net>]

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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