ATM astigmic mirror.

["Jerry B. Hillman" <JBHillman@ev1.net>]

Hi all,
I modified my Foucault tester this morning and ran more tests on the mirror.
Since the mirror is spherical, it should null simutaneously if the KE is at
COC.  It doesn't.  There is an hourglass shaped area through the center of
the mirror, offset to one side by 1.5", that nulls sooner than the rest of
the mirror.  There is about 2/1000" inch difference between the focal
lengths of these two areas.
Until I correct this, I am fighting a losing battle.  Now that I have seen
it, and mapped it on the back of the mirror with a sharpie, I can do
something constructive about it.
This is the reason I have kept the mirror at a sphere.  If the Foucault
testing equipment is sensitive enough, you can see astigmatism.
Thanks all,
Jerry