ATM: Interpreting Foucault Shadowgrams

[Adam Rosenberg <adamr@synopsys.com>]

Ok this is my first time looking at the mirror in the tester and
at the radius of curvature what I see is the knife edge move 
right to left on the outer right edge and left to right at a zone
about 1/3 of the way out from the left center.  No matter how I 
rotate the mirror this continues to happen.  I checked to make sure
everything was parallel.  The image becomes uniformally gray as the
edge moves in a drop more.  Just inside the ROC I get the standard
right to left and outside I get left to right.  Any ideas whats up
with this?  As the knife edge is moved into the beam it does grey
out as expected and looks like the pictures I've seen of a good
spherical surface.  

Thanks,

Adam R.