As a 30+ year happy Nikon pro user, I'll weigh in and definitely NOT recommend Nikon for Astro work. I tried for a year with my D700 and it did my head in!
Capture NX is rubbish for astro, one can't run a proper sequence of images beyond 30 secs and just getting it to communicate with the camera on the mac was a real pain in the butt!
I gave up an invested in a CCD and havn't looked back! One loses all the IR and +700 nm info on a standard DSLR. Many DSO's are only really interesting in Ha/red so the images from an unmodded DSLR are always going to be missing critical data.
I'm a proud Ford man so this is like me saying go buy a Holden, but if you're going to do AP on a DSLR, I'm going recommend (gasp) Canon & Backyard EOS
Doesn't stop me wanting a Nikon Df for the studio though!
Cheers
Andy