Camera Noise
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There are a few things that influence how much camera noise / hot pixels you will get in a night sky image - they are temperature, the warmer the sensor in the camera the more camera noise you get. Time - the longer you exposue for the more camera noise you will get and ISO, the higher that is the more graininess you will get.
The following images were all taken at room temperature 18 degrees and at 3 dregrees (that didn't change much during the corse of the exposures) and had the lens cap on so no light entired the camera, the idea was to get a feeling of what happens as I start with a high ISO and short exposure and end with a low ISO and long exposure - each of the exposures would give a similar amount of light, if pointing at the stars. The 18 degree image is shown and when you move your mouse over the image the 3 degree image appears - this page needs to preload so may take some time on a slow connection
1.5 minutes - ISO 6400
3 minutes - ISO 3200
6 minutes - ISO 1600
11 minutes - ISO 800
22 minutes - ISO 400
45 minutes - ISO 200
90 minutes - ISO 100
In this case Time appears to have a bigger influence over ISO (except on the highest ISO), so if we were only to think about camera noise there appears to be absolutly no advantage in taking a longer exposure and lessening the ISO at this temperature - 18 degrees, you can also see there is a lot less camera noise at lower temperatures, on a frosty morning the images would almost be completely clear. One would say in this case its appears best to use this camera at ISO 3200, but lets see what happens when light enters the camera
(I will add this soon)