ATM Thoughts on scope temperature

["Arjan te Marvelde (EMN)" <Arjan.te.Marvelde@emn.ericsson.se>]

Hello list,

I've been following the thread on mirror cooling/expansion calculation. Being new in 
telescope making, but with a solid physics education, i have a couple of questions about 
this issue.

What seems to me the main issue here is thermal equilibrium, meaning that temperature at 
any place of interest (the optics including the air column you're looking through) is 
stable in time. For the primary this may not be enough: it also should have no internal 
temperature gradients.

Transport of heat can go three ways: conduction, convection and radiation. Conduction 
transports heat through material, and is the main means

Any object facing void would attempt to get into thermal equilibrium with it (i.e. 3 
Kelvin?) by means of radiation. If the object is thermally well enough insulated, it may 
become many degrees cooler than it's environment. I would say that this effect would also 
affect a scope, so i expect the interior to cool down below ambient air teperature (isn't 
this also causing dew?). 

-1- Wouldn't a fan cause turbulent mixing of warm environment air with cool interior air, 
thus causing bad seeing? I can understand using a fan or whatever means to speed up the 
process, but to reach final equilibrium i would say patience is the best mechani
sm.

-2- Does anybody know of some mathematical model for heat exchange in a telescope; this 
would give some more grip on  quantification of the problem, i.e. better means to find 
out which issues matter most.

-3- Isn't the best way of reaching equilibrium very much dependent on the specific scope 
mechanics? I figure that an open-truss OTA cools down less that a conventional tube. Also 
the tube walls will transport heat from environment to interior,  causing turbu
lence.