[ATM] Foucault blind to astigmatism?

[burrjaw at earthlink.net (Jim Burrows)]

At 2004-02-03 16:21 -0700, Bill Kelley wrote:

>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.

It is true that whole-mirror KE (let's not call it Foucault) can see 
primary astigmatism (PA) if its axis isn't lined up with the knife edge 
(see the image on top of http://home.earthlink.net/~burrjaw/atm, an 8"f/6 
mirror with 100 nm of PA at 45?).  But lined up or not, the Foucault test 
(with mask and measurements) can't evaluate the PA.  Finally, Foucault done 
on multiple diameters of the mirror can see higher order astigmatisms - but 
they don't hurt the image as much as PA.

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