To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 


This is my 2nd mirror.

Since I am aiming at a sphere (its a 4.25" f9.5), I haven't bothered
with precise quantitative information just yet.  I do note the approximate
knife edge readings, to guide me directionally (is it getting better or
worse).  Although I am pretty good visually, I would not attempt to
parabolize without lots of measurements.  The idea for this mirror was that
a sphere would be good enough.  I plan on using measurements and Sixtests
when I think I am sufficiently close to a sphere (which I may, indeed, be,
given that the large f-ratio does seem to magnify the errors).

I'm not sure I could do more than 3 zones on such a large f-ratio -- the
mirror looks so small.  Being an engineer, I'm game to try.

=Matt


>Delivered-To: mlbrown{at}0
>X-Authentication-Warning: galaga.telerama.com: nobody set sender to
>mdholm{at}telerama.com using -f
>To: atm{at}shore.net
>Subject: Re: ATM Getting less edgy
>Date: Fri, 21 Mar 2003 22:05:58 -0500 (EST)
>From: mdholm{at}telerama.com
>User-Agent: IMP/PHP IMAP webmail program 2.2.6
>X-Scanner: exiscan for exim4 (http://duncanthrax.net/exiscan/)
>*18wZ3E-0001C8-00*MAaqD1su4q.*
>Sender: owner-atm{at}shore.net
>Reply-To: mdholm{at}telerama.com
>X-Report-Errors-to: mikell{at}optonline.net
>
>
>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

--- BBBS/NT v4.00 MP