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Alan when my Peltier fridge is at -10.0C the sensor is at 7.0C. I have measured the temperature rise of the sensor in my Canon 5DH when continuously exposing. This temperature rise above ambient is 17C! I would imagine that a 40D would rise a bit less than this.
Your 40D with the sensor directly cooled will have far less noise even at a sensor temperature of 7.0C. I would imagine the trick would be to drop the sensor temperature to a level where condensation just occurs. Then warm up to clear the sensor and stay above this sensor temperature for these ambient conditions.
The dew point would be the important parameter here. What would help is some mild heating from a dew strap around the lens or telescope barrel to raise the dew point inside the lens or telescope barrel. A filter or clear window with anti reflection coating mounted as close to the sensor as you can manage would stop fresh moist air forming condensation on the sensor.
If I don't continuously expose with dummy exposures when the fridge gets below about 0C to -5C I get condensaton.
Hope my ramblings give you some ideas. Looking forward to what you can achieve with your new cooled 40D. It is well worth it as the dim stuff is at or below the noise levels of uncooled cameras.
I can only imagine what a Canon sensor would do at -40C!
Bert
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