Re: [ATM] Ball bearing for triangles

["A. Suijkerbuijk" <a.suykerbuyk@wxs.nl>]

Hi Tom,

Personally I still don't use fans, I simply don't like the idea of blowing
dust on the mirror. But the Netherlands has a humid climate and not much
difference between day and night temperatures.
Nevertheless overcorrection can be seen during cooling, so the theory works
in this case. What interests me most is: can the cooling of a mirror be
predicted? Is there a certain balance between back and front side?
The program "cool" uses just the thickness of the mirror, which puzzles me.
I would say two mirrors of the same thickness have not the same cooling
rate, the larger one will cool faster. In other words: numbers like surface
and volume of the mirror play a certain role.

Regards,
Adrie Suijkerbuijk
----- Original Message ----- 
From: "lou krajci" <loukrajci@comcast.net>
To: <atm@atmlist.net>
Sent: Tuesday, September 07, 2004 5:21 PM
Subject: Re: [ATM] Ball bearing for triangles


> >From: "A. Suijkerbuijk" <a.suykerbuyk@wxs.nl>
>
> >"Classical theory" (books, articles
> >etc.) says that the back side cools faster since there is no aluminum on
> it.
> >Perhaps in the case you mentioned (very thin mirror) things are
different.
> >Perhaps the mirror will reach the air temperature very fast, after which
> the
> >aluminum side radiates most. In this case the mirror will cool below the
> air
> >temperature? Or perhaps the meniscus plays a certain role.
>
> Yes, the aluminized side of the mirror will not radiate as much heat as
the
> back side of the mirror...but remember that the aluminized side faces the
> cold sky, so it has a tendency to cool off...and the back side faces the
> earth, which is much warmer than the cold sky...so the back side may stay
> warmer because it's facing/exposed to a different thermal environment than
> the front side.
>
> This difference in environment is one way to make a cloud detector for
your
> robotic observatory.  Sandwich a thermocouple between two plates of
> aluminum.  One plate faces up to sky, the other faces toward ground.  On a
> clear night the upper plate will be colder than lower plate.  When heavy
> clouds move in, they reflect IR energy back down and upper plate warms up
a
> bit.  Measure this temp. difference and you can set an alarm to close the
> observatory, or wake the astronomer, because heavy clouds are overhead.
>
> Getting back to the mirror...this is why I advocate multiple fans on both
> sides of mirror to help keep thermal equilibrium.
>
> Tom Krajci
> Albuquerque, New Mexico
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