Re: [ATM] Which diameter gets worked for a given stroke direction?

[Jim Burrows <burrjaw@earthlink.net>]

At 2005-02-24 13:36 -0600, Mike Lockwood wrote:

>This brings up a question - what is the minimum amount of astigmatism 
>(cylinder) that the Focault test can dependably detect?  What amount 
>causes a detectable tilt in the knife edge shadow as it passes the center 
>of the mirror?  I've never seen either of these quantified. (I'm hoping to 
>spark a discussion here.  If it has been quantified, I apologize for 
>wasting the bandwidth.)

I don't think astigmatism can be "quantified" with Foucault, but, as Tex 
sez, p. 85, it can be detected "... if the worker is quite familar with 
[the shadows], especially for certain orientations of the defect with 
respect to the knife edge."  If the axis of the astigmatism is at 45° to 
the knife edge, the shadow has opposite bulges on top and bottom.  Tex 
recommends a bench star test.  If you visit my web site in the ATM section, 
there's simulated images of both whole-mirror Foucault and the bench star 
test on a mirror with just passing primary astigmatism (maybe help workers 
to become familar...).

>However, many amateurs can't claim this.  What's the worst-case scenario 
>here?  Someone on the list makes a mirror and it tests to 0.95 strehl with 
>the Focault test on multiple axes.  Assuming reasonable testing accuracy, 
>what is the worst-case amount of astigmatism that can be present in the mirror?

If it's primary astigmatism, it's invisible to multi-axis Foucault, so the 
worst-case is horrible.

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)  

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