A Question of Gain

[nickarp2000]

I have an ASi585mc coupled to a skywatcher ED80 Equinox Pro with a ZWO OAG

As its galaxy season I would like to try my hand at taking some extended length subs of lets say for example M51, with sub lengths of around 180 secs plus.

I understand that in order to utilize the well depth of the camera it is recommended to reduce the gain down to "0".

Is this the case, or should I increase the gain to the unity point.

I understand if i set the gain to zero i probably wont see a great deal on the screen after each sub even with a stretch, and presume that in order to know the framing is correct an initial sub with the gain higher is possibly the way to go and then knock it back for the imaging run. (I think I know what I mean but I could be completely off)

Thanks

Nick

[vlaiv]

Gain is just numeric conversion factor - it won't magically increase number of captured photons per pixel.

Same signal will be captured when using gain 0 or unity gain or high gain.

There are however some other differences between these gain settings that will affect your final result:

- full well depth. As long as you don't saturate some parts of your target (unlikely with such short exposures and targets of "normal" brightness - i.e. not M42 or core of M31) - this parameter is irrelevant as stacking increases effective full well depth.

- read noise. As long as you expose long enough that you swamp read noise with some other noise source - usually light pollution noise, but can also be thermal noise if camera is not cooled (both grow with exposure length) - it does not matter how big it is. It will have very minor impact on total result if you keep above rule.

- there is additional error introduced due to quantization - which is masked by read noise.

I would personally use unity gain, but feel free to use any gain setting you wish, and no - single exposure at unity gain and 0 gain will show same level of signal when stretched properly - you won't be left with "blank screen" after exposure at low gain settings.

[nickarp2000]

Thank you very much for yur detailed response.

I presumed unity gain would be better but have noticed many imagers doing DSO with zero gain and I just wondered why

Thanks again

Nick

[vlaiv]

5 minutes ago, nickarp2000 said:

Thank you very much for yur detailed response.

I presumed unity gain would be better but have noticed many imagers doing DSO with zero gain and I just wondered why

Thanks again

Nick

So here are "pros" and "cons" of each.

Zero gain does indeed increase FWC and some might think that is important to capture star color of bright stars. I'd personally use different approach because no FWC is big enough to capture all the bright stars. There are always some targets that will have brighter star still that will clip.

Best way to deal with above is to use few short "filler" exposures - it is much easier to capture very bright parts with shorter exposure then to rely on large full well capacity.

FWC is important in daytime photography - where we take only a single exposure, but in AP - we stack our exposures and we can use all sorts of tricks in stacking to compensate and circumvent limitations of FWC.

On the other hand - when using zero gain - one is likely to destroy faintest signal in the image. Any gain below unity will do this to some extent.

I'll give you an example when using e/ADU value of 2 (so lower gain than unity).

Say you capture 11 electrons and convert that to ADU units. You will (or rather camera will) do that by dividing the two 11e / 2 e/adu = 5.5 ADU - but we always record ADU as whole numbers so we must round it somehow, and it is usually rounded down - so we end up with 5ADU

If we try to restore original signal - we then should multiply with gain or rather e/ADU value so we have 5 ADU * 2 e /ADU = 10e

We were not able to fully restore our original number of captured electrons.

This always happens when we digitize our measurements if we use whole numbers - but there is a difference. For gain values other than 1, but e/ADU lower than 1 - we get this "step" error - that is sort of random. It is not truly random but in presence of read noise it almost becomes random.

Same thing happens with e/ADU larger than 1 - but here error is both more pronounced and also harder to shape - it impacts more. There is reason why most cameras have distinct relation ship between read noise and gain - read noise is always greater at lower gain settings and designers of sensors - don't bother to minimize / correct that - because it suits them.

You need larger read noise to "mask" this quantization error that is larger on low gain settings.

In general use in most cases - you won't see much difference, but if you gather large number of subs and you hunt for faintest signal - my recommendation is to go with unit gain and avoid most of issues with quantization error.

[nickarp2000]

Thank you for the explanation. At least now I won't waste valuable and rare clear nights in experimentation and achieving nothing 

[iantaylor2uk]

I tend to use higher than unity gain on my ZWO 071 MC Pro when I'm using a narrowband L-Enhance filter. Unity gain in that camera is 90, and max gain is 240, and I usually use a gain of 200. 

[vlaiv]

14 minutes ago, iantaylor2uk said:

I tend to use higher than unity gain on my ZWO 071 MC Pro when I'm using a narrowband L-Enhance filter. Unity gain in that camera is 90, and max gain is 240, and I usually use a gain of 200. 

If you want to use high gain to reduce read noise for narrowband and still avoid issues with quantization - here is a neat trick that you can use with ZWO cameras (other might have something similar).

Gain on ZWO cameras is measured in 0.1dB units. This is handy if we know something about dB scale - like the fact that it is calculated like 20 * log_base_10 (ratio).

For ratio of 2 we thus have 20 * log(2) = ~6.02 = 60.2 in units of 0.1dB

Every ~60.2 increase in gain (you can round it to 60) - you have halving of e/ADU value. You can also calculate for other integer values.

These are gain values that one can use to minimize quantization error - as e/ADU value will be based on whole number (reciprocal of whole number) - say if you have e/ADU to be 1/3 or 0.3333 - then

10e / 0.3333 = 30ADU and inverse 30 * 0.3333 = 10e

(you won't get exactly 1/3 e/ADU value - but it will be close that quantization error will show only on very large signal values - and there we simply don't care as SNR is good enough as is).

Using above calculation - gain of 210 is good place to be to minimize quantization errors.