Re:ATM Beyond the fan

["Mark D. Holm" <mdholm@telerama.com>]

The problem I see with a Peltier, or any other refrigeration system, is that the thermal
conductivity of glass is pretty poor.  In order to cool the mirror any faster than a fan would do,
you will have to lower the refrigerator temperature considerably below ambient.  That will increase
the temperature gradient in the glass, and make it eventually come out colder than ambient.  As Alan
Alder noted in his S&T article, the biggest thermal gradient, once a fan has been running for a few
minutes, is not likely to be between the mirror and the air, but internal to the mirror.  An
"active" cooler like a Peltier, will not get around this problem.

Now, if you want to precool your mirror during the day, or evening to the expected night
temperature, then a refrigerator would be in order, but you would have to do something to prevent
condensation.

Mark Holm
mdholm@telerama.com