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Should all telescopes be gold?

Started by jacobson, 02/18/2007 07:36PM
Posted 02/18/2007 07:36PM Opening Post
I have often noticed while observing that any black object becomes extremely cold and accumulates dew or frost. I suppose this is due to blackbody radiation - black objects emit as well as absorb light very efficiently. This leads me to believe that black is the worst possible color for telescopes, finders, binoculars, eyepieces, and anything else that needs to remain free of dew. Since we're talking mainly about infrared light, and infrared mirrors are usually coated with gold, does this mean that all our equipment should be gold plated or covered with gold leaf?

Has anyone done an experiment measuring temperatures of different colored objects at night? You could take a few tin cans, paint one black, another white, another left naturally reflective. Put a thermometer under each one and see what happens.
Posted 02/18/2007 08:28PM #1
>>>Since we're talking mainly about infrared light, and infrared mirrors are usually coated with gold, does this mean that all our equipment should be gold plated or covered with gold leaf?
---

It only seems like it is covered with gold when you buy it. smile

Bigger problem with scopes than dew is getting them cooled down. Of course IMHO, an even bigger problem is running it them because you can't see them.

White is a nice color for a scope.

Jon
Posted 02/18/2007 08:36PM #2
I'm no physicist, but black body radiation really doesn't have anything to do with the color black, does it?

I like white telescopes, too.

Randy Roy
Posted 02/19/2007 12:18AM #3
Hello:
Many metals are good reflectors/poor emitters in the thermal infrared. Gold, silver, and aluminum are the stanard metals used in coatings for IR telescopes. Today, silver is often used instead of gold since it has a lower emissivity in the IR and it still has OK blue response for optical instruments. Aluminum is also pretty good, but not quite as good as silver. More important than the metal is how clean the mirror is kept. Black objects are good thermal radiators. A telescope tube at night sees 2pi steradians of the sky, which is very cold, so the tube radiates heat like there's no tomorrow.
I don't think that white paint generally gaurantees low emissivity since paint is a rather complex chemical. Some of the telescopes on Mauna Kea are painted in a low emissivity paint, and can be white, blue, or other colors. White/unpainted tubes are useful in reflecting sunlight and keeping your scope cool if you set it up before sunset. A black scope will get hotter in the sunlight, making the cool down problem worse. My scope has an aluminum tube that I left unpainted, mostly because I'm lazy but also for the thermal issues mentioned above. By far the worst thermal problem I have isn't the tube, but the 19 lbs hunk of glass at the bottom of the tube with no ventilation.

Cheers
Mike Connelley
Posted 02/21/2007 12:16PM #4
Some discussion of emissivity with links:

http://edhiker.home.comcast.net/telescope12inch.html