Thanks for your interest in this and thanks especially for posting those Canon 6D and 7DII images. The images prove something very interesting: in both the JPGs and in the raw files I can see "stand-alone" bright pixels. This is precisely what one would expect to see in a typical raw file with no pre-processing. This is a normal characteristic of any sensor. This absence of pre-processing is why the Canon cameras have always been held in high regard by the astro-imaging community.
The issue I am seeing with the Sony A7S used in Bulb mode (it doesn't appear to affect other modes) is that bright pixels never appear singly but always in pairs with absolutely identical pixel values. When I talk of "pixel doubling" I really mean the absence of single bright pixels - I now regret my inaccuracy of terminology because it might have led to confusion. In any case, the absence of single bright pixels replaced by pairs of identical pixels is a clear indication that some pre-processing algorithm has been applied, because this simply cannot arise by chance across a whole image. To see these identical paired values it is necessary to view the un-debayered raw file using an application such as RawDigger, PixInsight or IRIS where the actual stored pixel values can be viewed directly.
You suggest that the so-called "pixel-doubling" may be "no more than inherent characteristics of a given sensor and its design". However, in the case of the Sony A7S "stand-alone" bright pixels never appear in Bulb mode but they do appear almost everywhere in other modes. This indicates that it is not a characteristic of the sensor itself but of some algorithmic pre-processing performed only in Bulb mode.
My interest in astro-imaging extends beyond simply producing pretty pictures. I am also interested in recording star magnitudes, looking a variable stars etc. Any algorithm that punches a hole in my stars, thinking that the central peak value is a hot pixel, is not much use in that context.
As much as I want to like the Sony A7S (after all, I've paid good money for it and European prices are not cheap), I am seriously beginning to think I should trade it in for something like the Canon 6D or 7DII with its 14 bit unadulterated raw files or for one of the Nikons where the firmware can be hacked to produce an unadulterated long-exposure image.
Edited by sharkmelley, 25 April 2015 - 06:50 PM.