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

Mr_Cellophane That's so bizarre that there's really only "two" settings of gain.. 0 and 100... why not just then make it an "on / off" button then?? lol..

There are only two gains that make sense between 0 and 100, i.e., precisely 0 and precisely 100.

However, after gain of 100, you want to be able to adjust the gain so that the sky background raises the dark ADU sufficiently to the point where the shot noise (Poisson distribution) from the sky and from the dark current noise predominates over the uniform distributed quantization noise of the analog-to-digital converter).

There should be enough keywords in that paragraph that you can do a web search if you want to learn more.

Basically, if the noise floor from the ADC input is raised over 10 dB of the quantization noise, then the SNR degradation from the quantization noise becomes only about 0.1 dB, which is pretty negligible for hobby work. But you need to increase the gain for short exposures to get this 10 dB condition. This is why some people need more gain -- they do not use long exposures.

There are generally 4 noise terms that you need to be aware of -- the shot noise from the sky background (depends on your location), the sensor dark current noise (depends on temperature and exposure duration). Read-noise (depends on sensor and camera design and duration to read out data -- that is why a fast RAM buffer is necessary), and finally the ADC quantization noise.

Determining exposure time that is optimal for your location and capture method is therefore different from someone else's. My skies are about SQM 19.3 (gets worse each year when I remeasure :-), which is about Bortle 5. So I pick my exposure times and camera cooling to match that. For example, I do not cool my cameras below -10ºC; there is no need to, given how bright my skies are. You will find some people who try to cool their cameras to unnecessarily low temperatures.

(There is an old maxim in the engineering circles that any fool can build a bridge that does not collapse. But only an engineer can build a bridge that barely does not collapse -- i.e., cheapest and yet meet specifications :-). This applies to cooling, gain, exposure time, and other camera settings.)

With cooling for example, which your camera supports, you just need to cool enough so that the dark current noise is lower than the sky background noise, again by 10 dB. Beyond that point, additional cooling is not material.

ZWO probably just copied how the QHYCCD cameras turn on the HCG. Check out the QHY600, which uses the same Sony IMX455 sensor. You will also find the QHY600 has many more modes than the ASI6200.

Chen