From: Mark Holm 
To: david_margrave{at}yahoo.com, atm{at}shore.net
Reply-To: Mark Holm 


People have used a water surface as a reference for figuring flats.  I have
never tried it, so can't pass on any personal wisdom.  I think I read a
fairly lengthy account in S&T quite some years ago.

Can't get much cheaper than water.  Of course water isn't really flat, it
has the curvature of the Earth.  That turns out to be small enough for most
purposes.  As I recall, you need to make a Fizeau interferometer setup, but
you really should have one for testing glass flats against each other
anyhow, so it isn't really extra work.  Also, judging from what I have seen
and read, it isn't
really very hard.

Biggest problem with the water flat as I recall is vibration.  You will probably
need to work on a concrete floor directly on the ground, like a basement.

Mark Holm
mdholm{at}telerama.com

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