I am about to get my Sony A7r2 Ha enhanced. It also has a similar MP sensor at 42.4mp. Nikon has the better hot pixel suppression algorithim.
These cameras have very low read noise, much lower than astro CCDs where 5 electrons is considered top class. Sony A7r2 is under 1 electron at some ISOs. D850 would be also.
Thermal noise tends to be very low as well so I don't know how beneficial cooling would be. Its more of an advantage to older noisier Canons.
The main issue I would want to know for Deep Sky imaging with either a Sony or a Nikon is are they able to be flat fielded or not? There is a question mark there.
On another topic I was looking at On Semi's site the other day and see they now have a 43.1mp 35mm sized interline CCD. I wonder when they start making it to astro cams. An A7r2 or D850 may be a cheaper choice. Also if Atik/QHY start getting the A7r2 42.4 backside illuminated CMOS sensor and put it in one of their cooled bodies that would be way better as RAW=RAW not baked like modern camera makers all seem to do. It would also be smaller, lighter and regulated cooling. Flats should pose no problem.
