[ATM] Phase Contrast and Foucault image puzzle

[Dale Eason <atmpob@yahoo.com>]

http://home.comcast.net/~doeason/phase_and_foucault.jpg

100k image.

I just learned how to make a phase contrast image of a
mirror.  Texerau calls it the Lyot test in his book. 
The above image has the phase contrast on the left and
the Foucault image of the same mirror on the right.

The fouculat image presents a puzzle to me.  It has a
very strange surface.  This mirror was made by a local
ATM 60 years ago and it has been in storage or on a
shelf ever since.  I think he was 17 when he made it
and made and used a machine to make it.  It is
uncoated Pyrex 10 inch F5.  It has been cored part way
through.  The surface looks very smooth with no hint
of what is seen here except for 2 small scratches and
the finger prints close to the edge.

Has anyone any idea what has caused the groves and
winding features seen in the Foucault image?  None of
the features visible on the back show up in Foucault
image.

Dale Eason

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