Re: [ATM] is mirror thermal equilibrium important

[Alexis Cousein <al@sgi.com>]

Jack Swaton @ Starry Host wrote:
> I think this is a more pragmatic definition of thermal equilibrium in the
> light of heavy dew:
> If you're dewed up you don't observe, no matter what the figure is!

But you only get dew when a mirror gets cooled *below* ambient
temperature; there's no way to force air below its dew point by
warming it. Of course, if the air is close to its dew point without
the need for cooling it, you can't prevent dewing, but you can't
prevent the formation of mist all around you either ;).

Mirrors are bad radiators of heat into the night sky, so supercooling
is slow. So any continued air flow around the mirror will usually
suffice to prevent dewing - unless you're blowing warmer air onto
the mirror (which can be forced below its dew point in contact
with the mirror).

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 :).
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