[ATM] Working with plate glass - the figure changes like the weather

[gfbrandenburg at yahoo.com (Guy Brandenburg)]

One of the folks in our mirror-making class has been
struggling - and I do mean that literally - with a
small (4.25", f/9.6) mirror that he got from Newport
Glass. The weird thing is the extent to which the
figure changes almost completely once it has cooled
down. I had read about this, but had never seen it for
myself. 

He and I will spend time figuring it, and we will
think it is just about perfect when we look at it
under the Wrong - Key grating (get it?) or the Foo -
Koe knife edge. Then we look at it the next week - and
it's totally different. Last time, the gently bowed
lines as seen under the ronchigram turned out to be
much sharper one week later. Last Friday, it looked
very, very good immediately after testing. Then by
Monday afternoon - with the mirror sitting in a
tupperware bowl in the same workshop, same
temperature, same everything - the lines were once
again much more bowed than they had been at the start
of the weekend. 

A bit bemused, I decided to cast a new lap, press it,
do some more figuring, 1/3 COC, only about twice
around the post (so to speak), and rinse it in cold
water and put it on the stand. Bang! tons of turned
edge! OK, press some more, try 1/3 COC again, 1 time
around, then rinse in warmish water. Turned edge all
gone! Ronchi lines look perfect! (not straight, but
bowed slightly outwards inside the Center of
Curvature, and bowed slightly inwards outside of the
COC).

I decide not to go any further, and I put the mirror
back in its tupperware bowl. On a hunch, I return this
afternoon (Tuesday), and put the mirror back on the
testing stand, when nobody has touched the glass since
then. Guess what - the turned edge is back! Plus, the
lines are now slightly bowed in the **wrong**
direction - in other words, it is now a slightly
oblate spheroid. I decided not to touch it, just to
see what happens to it between now and next Friday.
(Plus, I am employed as a teacher and am supposed to
be grading papers, and in addition to that, there was
supposed to be a drumming lesson located in 'our'
workshop this afternoon.)

The moral of this story is that if I can ever help it,
I will never try to figure a plate glass mirror again
unless there is a really, really good reason (like a
nearly free 16-inch plate glass disk). It's too much
of a pain trying to figure out what effect the
'weather' of the local heat of figuring (and so on) is
going to cause. 

Guy Brandenburg
NCA
Washington DC

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Guy  BrandenburgWashington, DChttp://home.earthlink.net/~gfbranden/GFB_Home_Page.html

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