As soon as I find an adapter for my D40, I will order one of these. I am a little concerned about the portability of a 30 meter rotating pool of mercury, and the payments, but what the heck, it can see back in time.
http://adsabs.harvard.edu/full/1982JRASC..76..245B
http://adsabs.harvard.edu/full/1982JRASC..76..245B