The Sony A7r2 series has had some controversy over a hot pixel suppression algorithim Sony uses much the same as Nikon did several years ago.
It can damage stars that are quite tight - I believe its 2x2 pixel sizes it hits.
So tracked images with fine stars may find the dimmer tighter stars attenuated and in some cases (A7s) where the pixels are larger it can punch holes in the star centres etc.
Later models are a tad less bad and the small pixels of the A7r2/3 models seem to escape most of this and its hard to notice.
But a smart poster on DPreview stumbled across a way to turn it off on the earlier models A7r2, A7s2 and probably A7r and A7s. You simply turn the drive mode to continuous Hi or Lo mode and the camera uses 12bit instead of 14bit and saves on processing power. No doubt the engineer thought no one would want long exposures in continuous shooting mode (usually reserved for action).
So I tried it out for the first time under dark skies.
I quite like the result. I am not sure how much was star eater off and how much was the near perfect seeing and very accurate focus. These variables make it hard to judge changes sometimes. But there is no disputing the stars are tight and bright and lots of them so it seems its a win.
Sony A7r2, Sigma Art 14mm F1.8 tracked. 8 x 30 seconds ISO6400 x 3 panels.
http://www.pbase.com/gregbradley/image/167083228/large
Greg.
