Hi, Erwin. Maybe this post will help you use a water/glass shader for ring opacity, instead of using your b/w image map in the Opacity Function, use it as a Blending/Mask Shader to a transparent surface layer. Fully white applies the transparency, shades of grey in the blender function controls the transparency all the way from black to white. Works well for rings, propeller blur, etc.
That is, until I get to play around with those planet scales you mentioned, and exclude the image maps, to make it fully procedural, quite probably also with volumetric debris chunks. Tick-Tock...
http://www.planetside.co.uk/forums/index.php/topic,15018.msg146256.html#msg146256
