Re: [ATM] undercorrecting?

["Scott Milligan" <starzkey@charter.net>]

On the subject of emissivity, I would remind the discussion that highly
reflective materials such as metals actually exhibit very low emissivity in
the visible and infrared spectral regions, and hence are somewhat
"reluctant" to give up heat via radiation mechanisms. On the other hand,
metals generally exhibit high thermal conductivity, and hence readily give
up heat via conduction or convection.  

Whether telescope mirrors cool primarily by convection or radiation (i.e.
which mechanism dominates in situ for the problem at hand) I couldn't say,
but perhaps an experiment that compares the rate at which a mirror cools
while facing a clear dark sky with the rate obtained facing the inside of
one's mirror box cover that had been covered with aluminum foil could "shed
some light" on that question.  But that may not be the question that was
originally asked.

Please excuse me if I went off on a tangent,


Scott Milligan
 
 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Mike Lockwood
Sent: Friday, September 01, 2006 3:42 PM
To: atm@atmlist.net
Subject: Re: [ATM] undercorrecting?

Mitch,

Mitch wrote:
> As I am still the young kid on the block, could someone please explain 
> a bit about what "emissivity" is, or just point me to some simple, 
> clear text on the net.

Excellent question.  Surprised no one else has asked it.

Emissivity is a measure of how easily a material gives up heat by emission
of radiation.  Put a warm piece of metal outside on a cold winter night when
there's no breeze, and most of the cooling is likely through radiated heat.
(Better yet, put it in the cold vacuum of space - then ALL cooling is
through emission.)  The material emits photons which have a wavelengths in
the infrared region of the spectrum.  By giving off energy like this, the
material cools.  Higher emissivity means the material can more easily give
off the photons.

The word emission is used to refer to something (radiation, air
pollution) that is emitted, and thus the word emissivity.  For more on
emission of energy, go Google "blackbody radiation".

It's different if you blow cold air across warm objects.  Then the air
itself gets warmed by the material, and heat is transferred from the metal
to the air molecules directly.  This is *convective* cooling, or cooling
done by moving air.  It's different.

The question that is being kicked around is - if a coated mirror is facing
the sky, does the emitted heat cool the front face of the mirror enough to
cause it to warp significantly?  The mirror is looking at the cold vacuum of
space, so there is a large temperature difference between it and space, so a
significant amount of infrared radiation can be emitted, cooling the front
of the mirror more than the back or sides.  (This is why metal can get
colder than air temperature on a clear night - it keeps emitting radiation
into the very cold sky.)

 From your later message:
> The point is, I may be overcomplicating things, but with such small 
> tolerances of temperature difference how can we be sure even 4 fans, 
> each blowing at the mirror 90 degrees apart is enough?

We can't be sure.  I typically run the fans for a while to get the mirror
temperature close to ambient, and then shut them off.  Then over time the
heat in the mirror will redistribute itself on its own so that the mirror is
all about the same temperature.

Boy am I posting too much today.

	Mike L.


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