[ATM] TDE Theory

[Frank Ward]

Brad,

There is a site on the web that shows a commerical 
(30-40") scope in France with numerous heating coils in locations
around the edge. The operators adjust a reostat to bring
heat to the edge as they want it to adjust the figure
and bring a cold mirror  to parabola. It was their inexpensive way of
fixing an undercorrected surface.

Also, What do you call a thicker mirror?  The usual 
standard in the low teens is 50mm of Pyrex. If you
are using anything less and possibly plate you can expect
radical temperature swings as testing will quickly show.
Not that you shouldn't do it as many scopes hold thinner
mirrors, but test more frequently and don't consider that
the heat from your hands will spread to the center of the
glass on the bottom, even though your thumbs will do their
work on the top piece.

That's my opinion and like many others around here it is 
rarely humble. *S*


In friendship,

Frank Ward
Atlanta, GA

---- Brad Gilbert <bgilb5 at gmail.com> wrote: 

=============
Houston was hit with a cold front this week, and my working temperatures is
around 60F now. Some how in this cold bout, my TDE has gotten significantly
better. Thus, I came up with a theory of why this is. I noticed the center
of the mirror was significantly warmer than the edges which were still very
cold.

If you imagine all the paths the mirror ever takes on the lap, you will
realize it is basically a gradient with all strokes always going across the
center of both, and gradually to the edge these areas are less worked.

So, my theory is that most of TDE is caused by a thermal differential. In
hot weather, with your hands on the center of the mirror, your hands will
have a cooling action, not a warming action on the mirror. Body temperature
is 98F, but your hands are not close to this high in heat. This lead to the
center glass shrinking, the edges remain warm, and expand. The TDE digs in.
In cold weather, the lap and hands warmer the mirror above ambient
temperature, expanding the center and digging in. Thus creating either a
good edge, or TUE. This probably has less of an effect on thicker mirrors.
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