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Sony A7r has great high ISO performance but the noise pattern is a bit wormy compared to a nicer grain pattern with the same sensor in a Nikon D800e.
Sony does a compressed RAW which when in bulb mode has hot pixel suppression which can weaken some faint stars. I have not seen it as an issue and the examples I have seen were all super badly underexposed and you could hardly see the stars anyway. But its worth knowing. If shot not in bulb mode this is not a problem.
A7rii is likely to be the absolute best and unmatchable. Backside illuminated sensors until recently were the realm of $35,000 astro CCD cameras. The on-chip wiring is copper rather than aluminium and has 3.5X faster readout and lower noise.
The camera has a host of other features as well including silent shutter among others. Its the camera to have in 2015.
Greg.
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