Nikon Japan announced the shipping date for the D810a DSLR designed for astrophotography: May 28th,2015.
The camera was announced on February 6th and is currently available for pre-order at Amazon, B&H, Adorama, Jessops and WEX.
Additional information on the D810a can be found here.
You can also read Nikon’s article “Using the D810A DSLR for Deep Space and Nebulae Astrophotography”.