Re: [ATM] Foucault

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

Dave, probably the best advice at the moment is to read up on the Foucault
test from the various sites on the web.
The test is indeed fairly sensitive - there are other tests that are even
more sensitive - but it is easily controlled by the tester.  If you are
getting a real graying out over the whole mirror's surface, you've been
doing some good work in polishing.  Most people tend to get some turning of
the edge of the mirror's surface for their first effort.
The number of zones in the Couder mask is what you really feel will do the
job well.  5 or 6 is generally good for the average mirror.  The mask will
make things easier to see on the surface of the mirror.  Gentle parabolas
come out of the noise when the mask is used.
The ROC is for the particular zone that you are measuring out.  What you are
trying to do is find the difference in ROC of the various zones which will
tell you what the shape of the mirror is.
The actual position of the micrometer is irrelevant as the exact distance to
the mirror is unknown but it is known that the distance gets longer by some
small number as you measure the parabola from the center region to the edge
region and you want to make sure that you have plenty of room to make those
measurements without having to move the base of the tester which will
invalidate all of the previous measurements.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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