Hi guys,
regarding the electronics and stuff that I use for measuring temperatures, it's explained in a seperate article on my site, here:
http://www.acquerra.com.au/astro/equipment/temp-logger/
The temp sensors are quite accurate, reading accuracy of 0.16C, digital output.
As for the reason that the temp rises when the cooling is turned off, there are really two heat sources to blame - one is the core of the mirror as has already been pointed out, and the other one is the hot side of the peltiers (the heatsinks). As soon as you turn off the power the "heat pump" effect stops, and heat can flow back into the system from the hot side.
The fans that are normally used to blow the cold air off the coldplate and onto the mirror then turn into fans that are blowing warm air as the heat from the hot side gets back into the system via the peltiers.
In more recent experiments I've tried turning off the cooling in 2 stages, first turn it down to about 50% for a few minutes and then turn it off. This gives the hot side time to cool down a bit before the heat pump is turned off completely.
Something to remember is that these mirrors are made in a temperature controlled environment, where the mirror as a whole is always at a constant temperature, so the mirror will only be at it's most accurate when it is at the same level of thermal stability.
If you're using (or thinking of using) peltiers then you have to remember that they will introduce a temperature gradient through the glass, from cold on the outside to warm on the inside. You have to turn them off and let the mirror equalise for about 20 mins to 30 mins before it can equalise temperature throughout the glass, and then you'll get the sort of spectacular views that Iceman has found :-)
cheers, Bird