[ATM] is mirror thermal equilibrium important

[Alexis Cousein]

James Lerch wrote:
> ----- Original Message ----- 
> From: "Alexis Cousein" <al at sgi.com>
> 
> 
>> In these parts, that rarely happens, at least with a Dob pulling air
>> (and unfortunately also dust ;) ) from close to the ground. If that
>> air gets close to its dew point, it's usually by having been cooled by
>> the ground, and it will have shed some of its humidity by dewing up
>> the ground, not the scope's mirror :).
> 
> I know with confidence that if you come to Florida for a few days with only fans 
> as your Dew Control system, Dew will shut you down by midnight.  Been there, 
> Done that, Got the Dew Blaster to prove it :)

Quite correct - but the dew hits the secondary first, does it not? That one
*is* small enough to radiate into the sky efficiently enough to dew up (through
the primary acting as a mirror). And fans may not make any air flow around it
(I have exhaust fans, so I have the same potential problem; some others
who have shrouds and only fans at the back pushing air actually have
enough air flowing around the secondary to also help them, but not necessarily
enough).