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a peltier is a small thin piece of ceramic that has two wires sticking out of it. If you put 12 volt on these wires, one side of the ceramic goes up in temp and one side goes down in temp.
now, the difference in temp between the hot side and cold side stays constant, so... to get the cold side right down in temp, we need to extract the heat quickly from the hot side, ie by a fan and heat sink.
A simple computer fan and heat sink will mean that the cold side can get to -5 degrees in a few minutes, from my tests anyway.
from there, you attach the cold side to a large metal (aluminium) plate and now it will start to cool down as well and being large, it should retain its coldness. You then blow air over this cold plate to cool the air and then this air in turn cools the mirror evenly.
a peltier is simply a very easy way to turn electricity into hot and cold.
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