[ATM] Foucault blind to astigmatism?

[flex at commspeed.net (Bill Kelley)]

Dale,
In skimming thro the recent posts I didn't notice any
comments on this. I look for astigmatism both with
Foucault and Ronchi and find it easy to see. One of my 
students found astigmatism in his mirror recently with 
Ronchi set at 2 bars outside of focus. The tilt alerted him.
At focus it was easy to see. We switched to Foucault
to observe the typical "comma" shape.
Bill



> Thanks Don,
> The reason I asked was that much of the distortion for
> a mirror in the horizontal position as you described
> would be due to astigmatism.  The Foucault test is
> blind to astigmatism.  So saying that the mirror
> tested fine under these conditions does not confirm
> that the cell did not induce astigmatism distortion.
> 
> 
> Dale