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Originally Posted by bird
The explanation is that aluminium radiates heat at many wavelengths, some of which are absorbed by air, so it doesn't "see" the open night sky, whereas steel radiates at wavelengths that are mostly transparent to the air.
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I have also seen mention that the colour of paint used on a telescope tube makes a difference. IIRC, a white OTA will radiate more and get colder than a black one.
I wonder if there have been any comprehensive studies done on materials used commonly in telescopes.