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All of what I am talking about does rely on one thing that I have not seen written explicitly but that is implied, that the sensor is 65Ke full well regardless of gain, that is that the gain is applied to the read output not to the capture. Given the gain is expressed in terms of electrons per ADU I think it reasonable.
Assuming that, at the gain that gives the best noise profile (120, just over unity for the ASI294) you have 0-65Ke range on the sensor, which needs 16 bits to quantise, but a 14 bit ADC. Examination of subs suggests that at 120 gain, data is truncated at 16Ke, the limit of 14 bits. I found some features in M42 that could be readily identified on both 30 and 300 second unstretched subs. At 30 seconds the .fit file it showed a brightness level of around 12K, divide that by four to get back to the 14 bit level and it is around 3K off the sensor. Multiply that by 10 to get a reasonable guestimate of what the sensor would read for the same feature at 300 seconds and it would be about 30Ke, about half of the sensors range. In the .fit file for 300 seconds it is topped out at max value. Rough working would suggest that particular feature should saturate the sensor after about 600 seconds (a bit less due to dark current) but with 16K steps in the ADC output, it would saturate in the output file somewhere around 150 seconds.
Rather than being a loss of dynamic range, the 14 bit conversion to me is a loss of potential dynamic range that would be very useful for astro imaging, you should be able to capture more, and stretch less.
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