Thanks for the feedback guys:)
I see what you mean about "incomplete", an earlier version had a lake/river that followed about the long inner curve (a slightly altered version of Gom's file). It was a render hog and I had trouble getting the "shore" texture to flow along with the curve.
Images like this throw up all kinds of issues, the primary ones being that its trying to break almost all the rules upon which landscape rendering software work by.
For example atmospheres don't work this way! All the software I've ever come across assumes that the atmosphere thickness decays as you get further from the centre of the planet. Well in this image the planet is hidden and is above the bent planes that form the tunnel. So the atmosphere is decaying in the wrong direction! It's not that noticeable in this image. Another issue is that the light sources on an internally lit structure like this (its the same for a cylinder (Eon) or sphere (Dyson))create a very even shadowless light, quite flat and uninteresting with rays coming in from all directions but mostly above.
Populations will of course only work in the mid to near distance (close enough to not notice the curve)
I'm in London for a few days so I've set it to render a larger version, higher detail and AA and better quality, the vp has also been moved.
When I get back I'll post the result.

Richard
ps: Oh the light placement. I made a model of the Generator unit (
in its final position, y=-137 km ish) and another model of where the light would be. The generator was imported and copied and pasted with a rotation of 0.3deg in the z? axis (the last one). This gave a curve of generator units. The same was done with the light model so I had a small bounding box where each light was going to be. I then simply moved individual lights sources too cover the modelled lights and then deleted the light models. Simples