Ultratelephoto photography is easily achieved with your spotting scope and SLR by purchasing this Vanguard T-mount SLR Camera Adapter for Nikon F-Mount Cameras.  You'll need to take precautions to minimize vibrations, such as locking up the mirror if possible as well as using a cable release or remote control to fire the shutter.

First, remove the eyepiece.  Next, attach the SLR camera adapter where the eyepiece would normally be attached to the telescope, and screw in this t-mount adapter to the opposite end of the SLR camera adapter.  Now you can attach your camera to the t-mount and you are ready to begin taking pictures!