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Originally Posted by Camelopardalis
I think you're right in that there hasn't been a huge leap in sensor performance since the original A7S...although it is rumoured to be followed up later this year.
The problem (for some) with the Sony and Nikon cameras are the spatial filtering they seem to employ in their firmware to smooth out their raw files. Canon doesn't seem to do the same dirty tricks. A 6D is pretty solid, but getting a bit long in the tooth.
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There have been huge leaps in sensor performance since the A7s. The A7s has very large pixels and that is its reason for the high performance. Since then Sony sensors are now mostly backside illuminated which doubles the QE of the sensor and very low noise. The star eater effect has diminished as well but unfortunately not gone. Copper wiring instead of the usual aluminium, dual gain analogue converters which give a sizeable drop in read noise at a certain ISO (640). Improved microlenses and not sure but A7r had different angled microlenses near the edges of the sensor to improve edge performance and reduce colour shifts.Not sure if that continued with other sensors though.
The Sony A7iii is the camera that seems to get the best nightscape shots from Sony. My A7riii is also very good but you do get colour speckle noise in shadows and you do get star eater effect on tracked images that turn the fainter stars green (star eater suppresses red and blue channels on "hot pixels").
EOS R is very good. A tad noisier than the Sonys and not quite as sensitive but the star colours are nicer. No internal intervalometer though.
As to EOS Ra I am I the process of having my EOS R modified by replacing the low pass filter with a Ha friendly one. Essentially the same as the Canon EOS Ra except for the 30X zoom for fine focus. The EOS R does up to 10X zoom, the Sony A7riii up to 12X zoom. This is usually sufficient for focusing though.
An all new Sony A7Siii is set to be released this month. 12 mega pixel, passive cooled sensor with a stacked sensor meaning it has a DDRAM chip set on it for very fast transfer like the A9.
12mp doesn't sound exciting but Sony has some pretty cool sensors. One is a quad bayer that has a Bayer matrix 4X the size of the usual so it goes from 60mp to 15mp with all the advantages of that (lower noise etc). But time will tell what this 12mp sensor has in it.
The Nikon Z6, Son A7iii, Canon 6Dii and EOS R are all good. I imagine the EOS RP would be good but don't know. I think it has the same sensor as the 6D.
Nikon D5300 is a solid performer. Pentax cameras have star tracer where the IBIS shifts the sensor to match the sky rotation.
The new Canon EOS R 5 and 6 may be good. I would like confirmation though that Canon have not introduced their own star eater to reduce noise and gain some extra dynamic range. I hope not, they are the only ones it seems that don't.