MadSci Network: Physics
Query:

Re: How can I predict when dew will form on a corrector plate.

Date: Tue Jan 4 00:10:44 2005
Posted By: Steven Levin, Research Scientist, Astrophysics
Area of science: Physics
ID: 1103849022.Ph
Message:

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.

-Steve Levin

__________________________________________________________
DISCLAIMER: Just because I work for JPL/NASA/Caltech doesn't mean anything I say is in any way official. This is just me talking, not NASA, JPL, or Caltech.


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