Thanks for the link to Kevn cameras. When I win the lottery, I'll know where to go for cool stuff.
As for the rings of Saturn with a 400mm? Not likely. You would need to use a telescope eyepiece with a barlow and high powered eyepiece to see the rings of Saturn. That adapter was for terrestrial viewing. I don't think that the 400mm would make a bad telescope, just that the device you asked about wouldn't do it.
Well, if you already HAVE the lens, it doesn't cost a whole lot. But at only 4X power, a good pair of binoculars will have more ability to show detail of the moon, planets etc. The advantage of a 400mm f2.8 would be light gathering power. Looking at the Andromeda galaxy or any of hundreds of nebula would be fun.
The converter that KR lists has a 10mm eyepiece built in
Magnification = (Focal length of telescope [in mm]) / (Focal length of eyepiece [in mm]) . So 400mm/10mm=40x
That eyepiece in the old Nikon spotting scope converter looks pretty small. All that big beautiful glass viewed through a tiny objective is pretty much a waste.
If I were you, I would build up a converter with a nice 2" eyepiece. I googled "convert Nikon lens to telescope eyepiece" and found several people have done it.
Here is what a Canon guy did: http://www.iscap.columbia.edu/jank/Photo/Info/Eyepiece_Adapter/index.html
Some people have said they think the Nikon lens might be able to focus to infinity if you use both a barlow (first) and a star diagonal (90 degree) together (but not one at a time).
Edited to add:
I found a premade adapter from this guy: http://moggadapters.com/astro/adapter.asp
He only lists a 1.25" adapter, but that's going to be a whole lot brighter than the one Nikon used to make.