Re: [ATM] Rough grinding, focault test

[Mark Holm <mdholm@telerama.com>]

I am not opposed to a beginner starting Foucault testing a little before
getting fully polished out.  But, he shouldn't start to do any attempt
at correcting, except in the grossest way, until fully polished out.

Why do I say this?  A true beginner has never done Foucault testing at
all.  Unless he has experienced help at hand, learning to read the
Foucault shadows is going to be a fairly steep part of the learning
curve.  You can read Foucault shadows on a partly polished mirror.  It
may seem like a waste of time to the experienced, but a beginner is
going to waste both time and mental energy anyhow when he starts
Foucault testing, until he gets some practice at it.  Does it matter a
lot at what stage the time is wasted?

Now, in general, it would be a big waste of time to start trying to do
any correcting until the glass is fully polished.  The only exception
might be a very large defect that is probably the result of something
fairly badly off about the polishing stroke/technique.  A major over or
under correction, k << -1 or k >> 0, might indicate a need to shorten or
lengthen stroke or work more TOT or MOT.  Similarly, a really deep hole
or very badly turned edge might indicate a need for a technique change.
 Still, the main goal is to remove glass until the pits are gone, and in
doing so, any niceties of figure are going to be erased many times over.

-- 
Mark Holm
mdholm@telerama.com

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