Re: ATM Foucault test accuracy

["Bill T." <twentiethwave@hotmail.com>]

Michael Spooner wrote:

> One thing I've wondered about is if the background is not uniform behind
the
> mirror

I usually test mirrors in their cells-to-be.   I started using 1" diameter
Teflon discs for mirror support points.  On the very  next mirror I had a
long, puzzling fight with one of the middle zones, until I discovered that a
white Teflon pad was right behind that Couder mask window, on one side of
the mirror only.  It was "boosting" my perceived brightness of the window!
Cost me several days of tail chasing.  I learned to dab such offending pads
with a large, black Magic Marker.  Am I the only one this kind of stuff
happens to?

Bill T.