ATM Expansion calc.

[dosc@CSWNET.COM (Raymond A. Desmarais Jr.)]

Hi All,
tom said
>
>However, a thinner mirror has less material to cool...so all other things
>being equal...it will cool faster than a thicker blank...and give you good
>images sooner than a thicker blank.  (Also, the warm mirror produces tube
>currents, in addition to the distorted figure from a non-uniform temp
>profile in the mirror.)
>
>My advice still stands:  Strive for the lowest thermal mass, the highest
>thermal conductivity, and the best use of active cooling (fans) so that your
>telescope has the "fastest thermal response time" possible.  That way, less
>time will be spent having the image spoiled by a warped figure or tube
>currents.

Having used mirrors that cool almost instantly and knowing about figuring
and testing.
The less material to cool the better.
The air layer above a mirror, when the mirror is not cooled off to ambient
is much worse than all other things .

Getting the optic to ambient temperature is the key, fans and good cell
design help .
When a mirror is in a transition of changing temp, the thick areas are slower 
to respond.
Any area on the mirror that is not at ambient temp. will create turbulance
on the air layer above the mirror. This is bad for what is often called seeing.
One night in texas a 18" lightweight plate glass mirror was being used at
over 1000 power under great seeing. All of the other telescope users in the
field said the seeing was bad.
The optics they used were not cooled to ambient, some were 7 deg F or more
from ambient.
How many of you have measured to keep track of the temp. of various sections
of your mirror in relationship to ambient temp.?
Everyone can make a difference with your scope.
Try to cool off your mirror as soon as possible, anyway you can.
The focal length is stable at any soaked temp.
don't wait for the movie
RAY