As Sam says, it definitely depends on your target and the amount of sky glow you're encountering, whether due to light pollution or the moon. But personally, I would not use zero gain unless you really need all that dynamic range. I recently did an Orion image to test my new 071, and found that I could use unity gain (90 gain, 20 offset in driver) and get everything from the very dim outer dust to the bright core without blowing out the core. That's some impressive dynamic range out of this camera!
The problem with using zero gain is that the true gain is nearly 3 electrons per ADU count. That means you're losing data between ADU counts and you can get posterization in the dim areas of your image when you stretch it a lot to bring out the faint details. On the other end of the scale, if you go too high in gain, you start losing enough dynamic range so that your stars become saturated if you properly expose to get the faint details.
These are the 3 settings I'd consider using:
Gain 50, Offset 12
- Dynamic range = 13.1 stops
- True gain = 1.5 e-/ADU
Gain 90, Offset 20 (Unity Gain)
- Dynamic range = 12.5 stops
- True gain = 1 e-/ADU
Gain 150, Offset 35
- Dynamic range = 11.8
- True gain = .5e-/ADU
You'll have to do test exposures to see what the proper exposure would be for any of these settings. But I'm in a yellow zone for light pollution (SQM from 20.4 to 20.7), and I find that my exposures range from about 2 to 5 minutes depending on target and sky conditions.
-Dan
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