The Nikon D810A is a slight modification to Nikon's existing D810 professional DSLR powerhouse. Its guts have been tweaked to help astrophotographers shoot the beautiful cosmos.
Here's the key change: the D810A's infrared cut filter has been modified to let the hydrogen alpha spectral wavelength pass through. In other words, the camera's optics now don't filter wavelength that's ideal for astrophotography.
At its core, though, the D810A is a camera very similar to the D810: it packs a 36-megapixel full-frame sensor plus Nikon's latest Expeed 4 processing engine. But there are some performance tweaks specific to astrophotography. The D810A's ISO range has been optimised in a narrower range, for a standard settings from 200-12,800. (They're expandable beyond that point, if you're feeling crazy.)
Additionally, there's a new long exposure manual mode that lets you set shutter speeds from 4, all the way to 900 seconds. This gives you plenty of flexibility to be creative with exposures. Also, you know, shooting the dark sky at night takes longer than your every day shutter speeds permit.
For years, Canon sold an astrophotography cameras in the form of the 60Da. The D810A is a much different beast. First of all, it's much more expensive and its high-resolution, full-frame image sensor guts are fancier than what you got from the Canon's APS-C sensor guts. The D810A is a much more substantial camera, though, given the very specialised application, it's hard to say without testing whether it'll be worth the extra cost.
Speaking of which, pricing for the D810A is still TBA, but you can expect something considerably more expensive than the current £2,000 body-only price for the standard D810. If you're gonna shoot for the stars, you're going to have to pay for it.