ATM Foucault Tester: use of Moire patterns in place of micrometer

[Richard Schwartz <richas@idt.net>]

I am trying to find a way to replace the micrometer in a foucault tester
with two ronchi gratings by observing the Moire patterns.  Very small
relative motions of the gratings result in large changes of the
displayed pattern.

The principal is not unlike a vernier scale.  Imagine a ronchi screen
which is 90% black.  Imagine that it is a 90% reduction of a ruling that
is printed on paper and is also 90% black.  Then a white stripe will
appear where the two patterns line up, and motion of the white stripe
will be ten times the relative motion of the two patterns.   

Problem: a "white" stripe that is 90% black is not very bright.

I am continuing to experiment with these ideas in an effort to make some
kind of gizmo that will make foucault testing easier and make the
readings less subject to mistakes (like how many full turns of the
leadscrew).