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To exclude one or more of the possible reasons mentioned so far you could do this simple test: Take a dark at 50% and 25% exp time and estimate whether the dark signal is also 50% and 25%. If so, there probably is a constant dark current (at constant temperature).
If not, there may be a temperature rise (dark current grows faster with time) or an electronic defect (dark current is already high in the beginning).
Of course, there may be more than one reason, but thats not very likely.
Erwin
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