[ATM] Foucault blind to astigmatism?

[jlerch1 at tampabay.rr.com (James Lerch)]

----- Original Message ----- 
From: "Vladimir Galogaza" <vladimir.galogaza@zg.tel.hr>
> Jim,
>
> Even if it is true that:
>
> >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! I have never seen an actual shadowgram that will come close
> in  sharpnes or details to the simulated  ones like those from Diffract.

Vladimir,

Have a look at this image:
http://lerch.no-ip.com/atm/2ndTry/Axis_A_15/Raw_Zone7rep3.bmp

This is Charlie's infamous 12.5" F/4.5 mirror with its 165nm of primary
astigmatism.

While inverted when compared to Jim Burrow's Simulated (and un-related) image,
they share a striking comparison.

What catches my eye is how the "Bridge" of light is different between the top of
the shadow gram and the bottom.  During our ATM mirror lab class, I look for
this regularly, and when seen interferometry confirms that at least a 1/4 wave
or more of astigmatism is present.

James