Mr_Cellophane what do you all recommend you use it at highest gain?
I use higher gains for very short exposures, like with EAA or planetary. Or, for short exposure auto-guiding. So there is defintely a use-case for it. I typically use gains north of 30 dB ("300" in ZWO's lingo) when guiding with 0.5 second exposures.
Whenever you wish to freeze motion by using shorter exposures, is where you need to raise the gain (precisely also the times when you raise the "ISO" of digital cameras). These are also the cases where you don't have to worry about dynamic range, since you won't be capturing enough photons to saturate the sensor anyway.
With very low signal levels from the sensor with short exposures, high gain is needed to overcome the quantization noise of the analog-to-digital converter.
For the long exposures that you use for deep sky astrophotography, the rule of thumb is to pick the lowest gain possible (to get the most dynamic range).
In the last few years, Sony (the 6200 sensor is from Sony) introduced what is called High Conversion Gain (HCG) mode. "Conversion" refers to the analog-to-digital converter (ADC) that converts electrophoton currents into digital numbers (ADU).
When you switch on HCG, you can get a decent reduction in read noise without affecting dynamic range as much as if you did not turn on HCG. Sony has some good white papers about it; try searching the web.
With ZWO cameras, there is no way to explicitly turn on HCG. Instead, with these cameras, ZWO always turn on HCG when the gain is at or over a certain number. For your camera, that gain number is 100 (corresponds to a pre-ADC analog gain of 10 dB).
For non-HCG, the "lowest" gain is 0 (0 dB), for HCG, the "lowest" gain for your camera is 100 (10 dB).
Those are the two discrete gain values that you would typically use with deep sky astrophotography. Any other gain value than those two really don't make much sense for deep sky work (the worst of both worlds for your camera is to pick a gain of 99, by the way -- don't ever choose that gain with the 6200. I read a posting on this forum that they read somewhere that a gain of 99 is optimal for the 2600 and 6200.
If you are interested in ultimate dynamic range, use gain 0. For example, to capture the core of M31 or M42. For dimmer objects, you will find that turning on HCG will give you the best SNR for a given exposure time (but you will get more saturated stars -- caused by the slightly reduced dynamic range).
Different ZWO cameras have different gain when HCG is turned on. I have seen them use 100, and 111, and some other value too, I think. The Sony sensor also has more readout modes than just HCG -- you need to buy other manufacturers' cameras to take advantage of the other modes.
Chen
