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Tony,
I took the dark frames before/after cooling at ISO800 15 minutes. On the lefthand side are the full frames reduced to 300 x 200 pixels and the right ones 300x200 crops near the frame centre. To make the noise more obvious I've boosted contrast 20x.
The upper frame was made at ambient 27C, sensor temperature 31 C and the lower frame with the cooling employed to lower the sensor to 6C (ambient 24 C).
In fact there is not much difference in noise between a 900 second exposure and a zero second exposure.
Terry
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