Final answer:
The white powder coated aluminum tube of the telescope cools below ambient temperature at night due to its higher emissivity, which allows it to radiate heat more effectively to the cold night sky. This is a phenomenon also considered in infrared astronomy and relates to the principles of radiative cooling and blackbody radiation.
Step-by-step explanation:
The observation that your telescope's aluminum tube coated with white powder cools below ambient temperature at night involves principles of radiative cooling and emissivity. The powder coating on the aluminum tube likely has a higher emissivity than bare aluminum, which means it more efficiently emits infrared radiation. As the telescope and Earth's surface are near 300 K, both are radiating energy in the infrared spectrum with peaks at around 10 micrometers.
At night, the sky is relatively cold and acts as a heat sink. The coated aluminum, with its higher emissivity, will radiate heat more effectively to the cold night sky than the surrounding air, leading to temperatures that can be lower than the ambient air. This is supported by the concept of a blackbody, an idealized physical body that absorbs all incident electromagnetic radiation. The distribution of emitted radiation from a blackbody is solely dependent on its temperature, and as it radiates away energy, its temperature can decrease if not compensated by other heat sources.
A good analogy is provided in the context of astronomy, where telescopes and other equipment must contend with their own thermal emissions when observing faint cosmic sources in the infrared. This is a similar principle to the phenomenon you are experiencing with your telescope.