stubblejumper can running the cooler at 90-100% for several hours damage the sensor or electronics
There are two good reasons to cool a camera for long exposure.
One of the reasons is to reduce the integrated dark noise. But as long as you don't do 10 minute type exposures, there is no reason to cool the sensor much (notice that dark noise specs is in units per second -- multiply by exposure time to get total integrated dark noise) -- just check the total dark noise with the read noise. If the read noise is 3 to 10 times larger than the integrated dark noise, there is no reason to cool the sensor any further -- the read noise is at that point the main determinant of signal to noise ratio.
With a modern camera like a 2600 or 6200 with 180 second type exposures, even -5ºC is an overkill.
The other reason for cooling a sensor is to get an accurate temperate for the dark frames. If the dark frame temperature is not the same as the light frame temperature, you could be adding noise (instead of subtracting) and other artifacts. To get a stable controlled temperature, you do not want the cooler to get anywhere even close to 100%. I usually try to keep mine below 90%. This is true for both dark frames and light frames. This is also why you also take dark flats at the same temperature as the flats.
Chen
