First up, long camera lenses just won't hack it, as their field of view is just too wide.Originally Posted by Stefen
Based on a 20m field of view at 5,000m you are looking at a field of view of around 5~6 arc mintues. That is not a very large field of view by terrestrial telescope standards.
It means you are going to have to resort to eyepiece projection with a camera to obtain the needed image size on film and that means a slow effective aperture value.
There are other bugbears to worry about apart from the physical size and weight of a telescope, you are going to have to deal with telescope vibration and also ground induced air currents, both of which will conspire to cause serious image degradation. Mountains are particularly bad as they frequently have updrafts and thermal eddies surrounding them.
Given all of the above, I'd strongly recommend you not use a refractor, rather it would make more sense to opt for a SCT type telescope such as a meade ETX 90. It's going to cost you a lot more but it's small physical size and optical qualities are probably the best suited to the job.