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Much respect to the cooler boxes / cold fingers people have made, but I found that my DIY Peltier cooled box only managed about -10C on ambient.
However, I can get about the same -10C cooling in Brisbane climate by putting a freezy ice block (the rectangular kind you put in lunchboxes) on the back. Couple of rubber bands to secure, and a shell of encasing polystyrene to insulate. Lasts for about 3 hrs on 15-20C nights. It less wires and a lot quicker to make !
For my purposes, this gets my 450D cool enough so that thermal noise is not such a big problem on the warmer nights, (remember the thermal signal halves every 5-6C). As to how cold is cold enough, you need to consider how much light pollution there is, and what f ratio you are imaging at. The idea is to cool enough so that thermal noise is less than noise from light pollution.
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