@tharso: thanks. Not there yet I think.
So for those who are interested: I took a completely different tack. I decided to use a flat cylinder with limited depth and then use a photoshop gradient image in the opacity slot. It worked but it wasn't uniform in a circular way. So then I was browsing the forum and someone else was doing something with the falloff node in a diffuse texture. So I tried that and I got this.... I think it looks a lot better (though still not perfect.. atmospheric edge needs to shine just a little bit. Not sure how to make it shine...) Next up: greebles on the spaceship so it looks a little less like lego...
