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Okay, I'll bite on this, cause it's something I did a lot of research on a year or so back...
I had heard from a friend that aluminium made a good material for scopes cause it resisted dewing, and I was getting a new tube made from aluminium so I wanted to know what that was all. Google to the rescue...
You can find lots of info on this via google, so here's the short version :-)
Dew forms when a surface is colder than the dewpoint of the surrounding air. Dew forms on horizontal surfaces under a clear sky because that surface has cooled below air temperature by radiative loss up into space. Most materials radiate heat away at wavelengths that are transparent to the air, so they can cool by losing heat without warming up the air next to them. This is called "supercooling" and it's a major cause of tube currents in amateur scopes when the tube is 3 or 4 degrees colder than ambient air and you get a "waterfall" of cold air running down the inside of the tube and washing across the mirror.
If the tubes are painted then the emissivity of the paints will determine what happens. If you have a paint that emits IR at wavelengths not absorbed by air then any surface coated with that paint will supercool under the open night sky.
In the case mentioned here, one paint emits IR at wavelengths absorbed by air, preventing supercooling, and the other one does not.
It so happens that aluminium is a good material for scopes because it emits IR at wavelengths that *are* absorbed by air, so it can't supercool and in fact it will follow the nearby air temp quite closely.
I can't remember who it was, but someone at the starcamp in ballarat commented that my tube was still dry when the table and other stuff around me was heavily dewed, so I went through this explanation.
So the best material for a tube as far as I can see is unpainted aluminium.
regards, Bird
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