Re: [ATM] Does glass retain moisture?

[mdholm@telerama.com]

Yes, it might cool down a little slower if the back is open to radiate to the
enviroment, and the envionment is significantly colder than the mirror.  Several
observations:

1.  Usually, it is the front of the mirror that faces the sky, and that is the
really cold portion of the environment.  The sky might have an effective
radiation temperature 200K or more below the mirror temperature (I don't
remember the effective radiation temperature of the night sky, but, on a clear
night, it is closer to absolute zero than to Earth temperature.  It depends some
on altitude and the amount of water vapor.)  So, changing the infrared
emissivity/absorbtion of the back of the mirror isn't going to have as big an
effect as changing the front would.  Of course, the front is already low
emissivity aluminum and that turns out to be good from the thermal point of view.

2. Usually tube currents are caused when the mirror is warmer than the air in
the tube.  Usually, one wants the mirror to cool faster, not slower.  (See the
article by Brian Greer in May, 2004 Sky and Telescope.)  A mirror colder that
the air could equally cause tube currents, but this seems to be a fairly
uncommon situation.

3. It has been observed that making the outside of a telescope and/or
accessories with a shiny surface will indeed keep them a bit warmer, and retard
dewing.  There was a discussion of this in a Sky and Telescope article a few
years ago.  One useful idea was making a shiny shroud around a Telrad to make it
more dew resistant.  Making the outside of refractor/sct dew caps shiny might
help fend off dew a little longer.  A possible easy source of shiny coating is
chrome "Super Monokote" sold for covering model airplanes.  It is an iron on
film.  It has the disadvantage that the metallic layer is under a clear plastic
layer.  This isn't quite as good from the emissivity point of view, but the
plastic does provide protection for the thin metal film.

4. Although one wants a miror to fall to near air temperature a quickly as
possible, one doesn't want it to fall below air tmperature, even a little, and
it may be best to be slightly above air temperature and run a fan to blow off
the "tube" currents (really mirror boundary layer currents).  Why?  Dew.  A
mirror colder than the air will dew up even before the air cools to the dew
point.  A mirror slightly warmer than the air will fend off dew longer.  If
enough other things in the local environment cool faster than the mirror (grass,
trees, etc.) they may soak up the dew, leaving the mirror high and dry.

Mark Holm
mdholm@telerama.com

Quoting "Lawrence D. Lopez" <lopez@mv.mv.com>:

> Wait one cotton picking second:
> What would happen if you coated both sides of the mirror with aluminum
> ?
> Would something magical happen ?
> Wouldn't the mirror radiate less and thus cool down slower with the
> subsquent characteristics of fewer tube currents and less dewing ?
> Not that dewing is a problem.
> 
> Just a really weird idea
> 
>     Larry

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