Re: [ATM] undercorrecting?

[Dale Eason <atmpob@yahoo.com>]

I think I remember that one time my mirror dewed up,
but I don't think the back had dew on it, but I am not
sure.  Do others have experince?  

Dale Eason

--- Scott Milligan <starzkey@charter.net> wrote:

> 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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