You may find
this right-angle viewer useful for focusing.. I made it for Canon, but the same principle applies for Nikon cameras.
Another view is
here. Basically, it is a monocular scope (made from old binos), with mirror mounted after objective lens.
Also, have a look at this viewfinder, I made it for my Praktica VLC2, essentially microscope, magnification ~10x. Objective lens is from old 8 mm camera, eyepiece is from old binocular.
The whole thing clicks in the camera instead or standard removable finder.
You will need some sort of tracking for stars.. because you will need exposure times well into minutes.