Users of these cameras will be fully aware of the characteristics of these cameras and most, like me, will probably have picked on gain 120 as the optimum place to be for achieving unity gain while maximising dynamic range at the minimum cost of read noise.
However from discussions on other fora - one thread on Sharpcap -I have become aware that there may be some peculiarities of IMX294 and related chips that can cause a small proportion of the pixels to fail to reach full saturation with increasing exposure times at lower levels of analogue gain.
ZWO will have implemented their own fixes for this but I thought that I would investigate this phenomenon (as shown here) for my own two cameras that use the 294 and 492 chip.
My conclusions are that both the ASI294 MM and the ASI294MC do not suffer from this problem at any gain settings other than at gains between 120 and 140 - but which unfortunately is also the sweet spot that most of us choose. As you can see from the graphs (Sharpcap histograms) there are maybe 0.1% or less of the pixels that do not behave themselves at gain 120 - in the case of the AS1294 MC these are mainly green pixels (there are twice as many anyway of course) but also red pixels - which are significantly more afflicted than blue. These few pixels fail to saturate irrespective of the exposure time.
The next question of course is does this non linear behaviour in this range of gains actually matter for any practical purpose? I had raised it before in the context of Willem's thread - https://www.astrobin.com/forum/c/astrophotography/deep-sky-processing-techniques/gain-settings-for-flats-vs-lights/ on the wisdom or otherwise of matching gain between flats and lights because it seemed potentially relevant if a number of the pixels in a flat were reporting a lower level of saturation than expected at these particualr gain settings. Also I do not know what relevance this aberrant behaviour of a subpopulation of the pixels up near saturation might have for the their expected behaviour at lower levels of saturation relevant to lights.
So I am just putting this out there to see if any of you folk have more information on this and what it implies in practice. At the moment my instincts are pushing me in the direction of avoiding gain range 120-140 for all future imaging with these cameras -- but I don't know if this is justified.
Tim
However from discussions on other fora - one thread on Sharpcap -I have become aware that there may be some peculiarities of IMX294 and related chips that can cause a small proportion of the pixels to fail to reach full saturation with increasing exposure times at lower levels of analogue gain.
ZWO will have implemented their own fixes for this but I thought that I would investigate this phenomenon (as shown here) for my own two cameras that use the 294 and 492 chip.
My conclusions are that both the ASI294 MM and the ASI294MC do not suffer from this problem at any gain settings other than at gains between 120 and 140 - but which unfortunately is also the sweet spot that most of us choose. As you can see from the graphs (Sharpcap histograms) there are maybe 0.1% or less of the pixels that do not behave themselves at gain 120 - in the case of the AS1294 MC these are mainly green pixels (there are twice as many anyway of course) but also red pixels - which are significantly more afflicted than blue. These few pixels fail to saturate irrespective of the exposure time.
The next question of course is does this non linear behaviour in this range of gains actually matter for any practical purpose? I had raised it before in the context of Willem's thread - https://www.astrobin.com/forum/c/astrophotography/deep-sky-processing-techniques/gain-settings-for-flats-vs-lights/ on the wisdom or otherwise of matching gain between flats and lights because it seemed potentially relevant if a number of the pixels in a flat were reporting a lower level of saturation than expected at these particualr gain settings. Also I do not know what relevance this aberrant behaviour of a subpopulation of the pixels up near saturation might have for the their expected behaviour at lower levels of saturation relevant to lights.
So I am just putting this out there to see if any of you folk have more information on this and what it implies in practice. At the moment my instincts are pushing me in the direction of avoiding gain range 120-140 for all future imaging with these cameras -- but I don't know if this is justified.
Tim
