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Hi Greg.
Well...in short, you were right :-)
Although improvements do diminish as you go lower in temperature, nevertheless my findings illustrate that there are extra benefits of going cooler.
I took samples of 5 bias frames at various temperatures (in 5C increments), stacked them and measured in ImageJ.
As the Mean ADU values for bias frames go down with lower temperatures, so does even more importantly the standard deviation (a measure of noise). It seems that improvements are still significant down to -10 degrees Celsius, and from then on you get very small gains at higher cooling.
Attached are simple graphs that hopefully will help to illustrate that.
The first graph just shows Mean ADU vs Temperature.
The second graph shows ADU percentage difference when going down in 5 C increments.
[For 20C data point the difference is only 3.3% because it is measured between no cooling (ambient) and CCD set cooling to 20 C. ]
The third one shows StDev vs Temperature
One can at least measure his camera and thus correct his thinking when rain has been pouring for the second day in a row...
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