If you are really interested in deep sky photography, you should go in to a shop that deals with that. Yes, Canon is the preferred choice until you get into dedicated cameras for astro photography... and that may be the way you would want to go? For night sky photography, your nikon will be fine.
"No matter how slow the film, Spirit always stands still long enough for the photographer it has chosen" Minor White
Thom Hogan makes a short comment on this in his D700 Guide - page 82/83. basically, the difference between Nikon and Canon's way of processing noise, makes the Canon better suited for AstroPhotography.
now don't ask me to explain - this is extremely technical stuff that is far beyond my interest in photography.