Re: [ATM] Day and Night Foucault

["Bob May" <rmay@nethere.com>]

The readings can be affected by all kinds of things.  If you're
using a slitless tester, you want to do your readings at the
darker edge of the brightness of the zones.  Also helpful is to
have a reference gray level in the brightess o9f the mask to
assist in getting the right shade of gray.  Also ehlpful is to
have a mask with only the one zone of interest in it or, to a
lesser degree, all of the odd zones in one mask and the even
zones in the other mask. This helps with the confusion that the
eye gets into when the full formal Couder mask is used..
It also helps to do a zone several times, coming in from either
side of the null and verify that you get the same results each
time.  Getting a zone better than two thou is often difficult for
many.  In addition, having a flat black background helps in
finding the same level of brightness for each side of the zone as
you don't havve to mentally take that out of the equaton.  A
rough back will cause differing levels of reflection to also mar
the accuracy of the test which is why many want to have the back
fine ground all over so as to  provide an even illumination of
the zones from the back surface of the glass.
Bob May
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