MadSci Network: Physics |
Hi Howard,
You've given a nice list of most of the complications involved in
calculating the temperature of the corrector plate. Here are a few more:
The actual radiative temperature of the night sky is much warmer than 3
Kelvin, because the atmosphere is opaque to most infrared radiation. Thus
the corrector plate mostly "sees" the air, not deep space. On a clear
night, the radiative temperature of the air is about 250 K, but if there
are clouds, it can be considerably warmer, because the clouds are less
transparent and the plate doesn't "see" as far into the cold upper
atmosphere.
If the wind is blowing, the air will carry heat to or from the telescope
enormously better than it does on a windless night. Air is such a poor
conductor of heat that effectively all the heat it carries is transported
by convection, not conduction. It's the actual bulk motion of the air
which carries the heat around, and a little bit of wind makes a huge
difference.
From the beautiful pictures on your website, you must have a clock drive on
your telescope. The motor for the clock drive undoubtedly generates waste
heat, which is probably a substantial factor in warming the telescope, and
therefore warms up the corrector plate.
The result of all these complications is that it's nearly impossible to
calculate when the corrector plate will get condensation. Generally
speaking, the most important factor should be the weather. On clear, moist
nights with no wind you're most likely to have problems with
condensation.
Sorry I couldn't give you a nice, simple calculated answer. I do have a
suggestion for you, though: Wrap the outside of your telescope, near the
corrector plate, with an electric blanket or a strip of heat tape (used to
keep pipes from freezing), and use a dimmer switch (or the control on the
electric blanket, if it goes low enough) to adjust the heat until the
corrector plate stays dry. Don't warm it up any more than necessary,
because if it gets too warm you'll set up convection patterns in the air
which will blur the images (like looking over an asphalt road on a hot
day). It shouldn't take much heat to keep the dew from forming. You can
also buy commercial heaters specifically designed to go on your telescope.
I did a quick web search at www.google.com with keywords "dew" and
"telescope" and found lots of examples. I've never tried any of them, but
I'll bet they work at least as well as my heat tape arrangement.
Try the links in the MadSci Library for more information on Physics.