Typically to do "digiscoping" you find the scope you are wanting and they will have an adapter to fit a lens, typically either somewhere around 52mm or 58mm. Camera Land has lots of scope options and they even have digiscoping options. That being said though, if $1000 is your budget and if the scope is mainly going to be used as a long lens, you might look at the Tamron 200-500mm lens. With that camera it would be the equivalent of a 750mm f6.3 lens. I don't think autofocus is going to work on the 5100, but it wouldn't digiscoping either. You would have a lens with a tripod collar that would balance it out very well with a body on it and would allow a faster shutter speed with the lens vs. the digiscoping. Just a thought.
If you are going to be using the scope for other stuff mainly then of course that wouldn't work. Either way call Joel up at Camera Land and let him know the details and I am sure he can help. Make sure to mention to him you are a Campfire member.
