Attached Link: http://www.renderosity.com/homepage.ez?Who=Doublecrash&ViewArticle=3919

Another option will be to use ranged spotlights in your dome. If you range the spotlights, the rendertimes will reduce greatly. You just have to find out what range can satisfy your needs. Here's something I wrote long ago about ligthing using a ludicrous number of spots in Bryce (see link). Hope this helps, Stefano