You should not discount the HCG mode. That is a function of the sensor architecture which enables higher sensitivity. Nothing 'fake' about it. It allows the realization of the full intrinsic sensitivity of the low noise sensor.
Note that the frame of reference you are using is very CCD centric which is why you are calling the non-HCG mode 'normal'. In reality there is nothing 'abnormal' about HCG mode in CMOS. CCDs just don't have this capability.
Also where did you see that 2350mv sensitivity is in HCG mode? Is it in the Sony materials?
I think ZWO's website has some analysis on well depth. Also note as I mentioned earlier well depth alone is not meaningful. You need to see it in context of the read noise to understand dynamic range produced.
Ultimately what matters is dynamic range. CCDs require much larger full wells to produce the same dynamic due to the much higher read noise.
From what I have been able to find, only the IMX224 sensor is confirmed to have HCG mode out of all the Sony sensors in my list. That to me means that HCG mode is not "normal", it is a special feature of that sensor. I have found some discussions online about the IMX290 also having HCG mode but have not been able to confirm it. I think it is presumptuous to assume that everyone will use this feature all the time. For solar system imaging for example I would prefer the lower noise and larger SNR of non-HCG mode (note that read noise is not the only source of noise in the image). Do I understand correctly that in HCG mode each pixel is generating twice as many e- for every photon, thus filling up the charge well in half the time? I know read noise is slightly lower in HCG mode, but is not the end result still a reduction in SNR? Agreed, stacking can help to overcome this but in turn you are now committing to using stacking when HCG mode is on.
Quite a few CMOS sensors have HCG mode or something similar, not all are used in Astro applications. Irrespective, HCG mode is an intrinsic feature of the sensor architecture and you cannot exclude it just because other sensors don't have it. Every camera/sensor applies gain... you should think of HCG mode as gain on steroids only possible because of the low read noise. If you tired anything similar with CCDs you would just be amplifying noise.
What people prefer (stacking vs. Single long exposure) is a seperate discussion and should not be mixed with sensor specs. I return to this point below.
Also I think you are missing the point of lower read noise (and any sensor) architectures. The full well size is irrelevant. What matters is dynamic range of the sensor.. Seems counter intuitive especially if you are coming from the CCD world because we have been trained to believe larger the full well the better. The nuance is that this is true only if read noise is the same. The dynamic range of the 829 sensor is exactly the same as the 224 despite the fact that the 829 FW is nearly 50k.
There is no reduction in SNR with shorter exposures using a very low read noise camera. SNR is a ratio. Total exposure time will also be lower but that is a more complex relationship.
Remember if your read noise is <1e you can almost count individual photons but in a camera with 7e read noise you need 7 times the photons for the same SNR (I am dramatically simplyfiing the relationship here but it is directionally correct).
Wrt the preference of stacking vs. single exposure that is a debate as old as stacking : )
You can argue it both ways based on personal preference. My view is that stacking is overall a positive development. Using low read noise cameras and stacking now you can generate much better captures and SNR with simple Alt Az mounts than you could with well polar aligned EQ mounts and long exposures using high read noise CCD cameras a few years ago.
Just look at my 290 thread. Many of those ARPs would have required longer sub exposures at longer focal lengths using traditional CCDs to produce the same detail. I know because I have imaged many of them over the years. I could not be more happier with the technological progress in this field (actually I want a lot more innovation but given the size of this market and the minuscule R&D dollars being invested the progress is quite good).
Edited by Astrojedi, 28 October 2016 - 04:04 PM.