I think your real problem is that you aren't in bulb mode and or have the iso set to low like 100 when it should be at 800/1600. lol i doubt back focus is the issue if its close to the ed100 i own but prove me wrong
Another thing I missed because i'am a canon user not a nikon, just use the moon and center it, lol your got stuff all chance seeing a star on your liveview unless you can zoom in with out knocking the scope haha. I have no idea what software supports nikon's but if you can get something like apt/byeos it will make life way more easy. All my shooting is done through byeos and i use the cannon utils to focus the scope, maybe nikon have some software which lets you do this ask around. I'am sure the nikon boys can help you out anyhow.