The Zeiss Batis 25m F2 lens made for Sony emount is about the best lens I have ever used. Its awesome.
I have also since gotten the Zeiss Loxia 21mm F2.8 for Sony e-mount and it is rapidly establishing itself as one of the best 21mm lenses ever.
These 2 will be my goto lenses for nightscapes.
Here is one I took over Easter in the 1 hour before the nearly full moon rose.
Zeiss Batis 25mm, Sony a7R2, Polarie, 2 shots one tracked for the stars and one untracked for a sharper landscape.
The a7R2 is a fabulous camera but long exposures at high ISO beyond 30 seconds is not as good as it could be. A7r was better but a7R2 is still very good. Needs a bit more noise reduction as colour noise seems to intrude in the dim areas which was always a Sony strong area. But the a7R2 has very high QE being a backside illuminated CMOS sensor and it seems ISO3200 is as bright as ISO6400 was in the past or at least that is my impression.
http://www.pbase.com/image/162922130/large
Greg.