Do you have any ideas how I could achieve a more fuzzy look on the outer rim of the atmosphere? I'd prefer a way that doesn't use postprocessing in an external application because I don't have the tools here to combine those passes later on for 1000 single frames.

I already tried to use a SphereFog but Mental Ray doesn't render it.

Here you can see the ramp-shader for my atmosphere, maybe you have any ideas on how to soften it up a bit

EDIT: Hey Dave, hadn't seen your post. I'll take a look at it first. Thx for your help!

EDIT2: Although your link is very interesting, the effect doesn't convince me too much. The point is, that the atmosphere has a height of lets say 100km, by directly mapping the ramp on top the color-texture (like its done in the vid) you completely loose that impression of height. I think I'd prefer mapping the ramp on a sphere with a slightly bigger diameter. That still leaves me with the problem of the sharp edge on the night site. It's probably just a minor tweak in the ramp-shader but I have no idea. I'll play with it some more and also try the shader the video explains, but any ideas are still appreciated.