Have you tried the "Vertex's Space Flight Workshop" located in the Tutorials section of the Acknex Unlimited site? (http://au.conitec.net/)

It's a little old now, but I have had excellent results with the techniques that are taught in the tutorial.

I usually use one sphere for all the background stars, one or two more with specific celestial phenomena on it, and I think the final results look great.

- WildCat