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Originally Posted by netwolf
Question1: What causes the mirror temp to rise when cooling is turned off? Whats heating it?
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Dujon's answer is right, it's heat energy stored in the core of the mirror. Pyrex and most glasses used in mirrors act a bit like a bucket of hot water, that is they loose the heat energy from the periphery first and then the energy gradually flows out from the central core region as the periphery cools.
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Originally Posted by netwolf
Question2: Without cooling, If the mirror is hot why does the tube temp drop bellow ambient would not the heat from the mirror rise up the tube like a chimney? Hence should not the tube temp follow in sync with mirror temp?
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It's a very complex issue involving the concept of black body radiation and the fact that a telescope radiates heat to space (ie: to 0 Kelvin). Also you have to factor in the specific thermal radiation properties of both tube and the material the mirror is made of (glass or ceramic).
It's also an area that gets extremely complex to model mathematically, so rather than bogging this discussion down in formulae and miles of terminology here's the lay version
Each component (mirror, tube, mirror cell) absorbs and radiates heat at different levels. In general a tube will absorb heat and cool faster than a mirror because it's got a greater surface area. If the mirror was in direct contact with the tube there would be heat transferrence, however there's usually about 20-30mm air gap between the mirror and tube and this air isn't sufficiently warmed by the mirror to transfer much heat to the tube. It is enough to produce eddy currents in the air column in the tube though (tube currents).
One thing Bird didn't mention in his article was the accuracy of his measuring gear.