To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Matt,

I didn't read, or have forgotten your earlier posts.  Are you experienced,
or fairly new at mirror making?

I notice that you didn't mention any quantitative testing, just qualitative
Foucault & Ronchi.  Now a few very talented mirror makers manage to
make good or
even excellent mirrors with a minimum, or perhaps even no quantitative testing.
 In order to do this they almost always have to result to some sort of null
test.

For me, reading Foucault shadows and Ronchi bands is a difficult and not at
all quantitative process.  Although your Foucaultgram is excellent, I can't
tell if one of those high zones is higher than the other, or whether your
turned edge is
only a little down, or enough to cause a big problem.  My approach, one I
have mentioned a couple of times here before, is to do the standard
quantitative Foucault test using typically two, or more Couder masks to
test 8 - 10 zones. Old timers would have scoffed at this approach in large
part because they didn't
want to take the time to plow through all the analysis by hand.  Now we
have wonderful computer programs like Sixtests to do that for us and plot
out calibrated graphs of the error curves.  There is still the tedium of
actually making the measurements, and typing them into the computer, but
that is a fairly
easy and straight forward task.  (Well sometimes judging the shadows in the
couder mask openings takes some judgement.)

My own take on it is that the quantitative graph of surface shape is so
valuable, that the tedium of testing is well justified.

Now perhaps you are well experienced, and are one of those visual-spatially
talented folks who can make good judgements of Foucault shadows without
needing to resort to measurement. On the other hand, I suspect that the
ranks of those talented folks are thinner even than many who consider
themselves to be so talented think.  If you are new to the game, and even
if not, I suggest that you
give multi zone quantitative Foucault testing a try.  Even if it shows only that
your qualitative impression is basically correct, that will be useful
confirmation.  If it shows that those zones that look so nasty are only
1/20 wave high, you may want to proceed to parabolizing without further
ado.

Mark Holm
mdholm{at}telerama.com

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