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

[bchilders at everestkc.net (buck childers)]

----- Original Message ----- 
From: "Guy Brandenburg" <gfbrandenburg@yahoo.com>
To: <atm@atmlist.net>
Sent: Tuesday, February 24, 2004 8:22 PM
Subject: [ATM] Working with plate glass - the figure changes like the
weather


> 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.
>

Just goes to show you how much heat is generated due to friction. With a
4.25 f/9.6 there's not going to be much difference from the center to
the edge.  Would be easy to overdo things especially if you're basing
any work on a bad reading.    I was told long ago not to test just after
doing lap work.  Always let the mirror fully cool.  You're just getting
false readings otherwise.  This is true with pyrex too although the
effect is more dramatic with plate glass.
-Buck