Thanks. Although the clouds are not perlin noise - they are the B channel from the cloud/stars texture. I pass the image through some simple calculations to generate different levels of cloud, and various shades of gray. I looked at perlin noise and it seemed a lot of processing for something I could get from a texture. I get join lines where the texture repeats, I can't solve that yet and it is driving me crazy!
The G channel of the texture is the high altitude cloud layer - I still need to add that. Think of it... two layers of cloud with different speeds and directions, densities and cloud types
The other two channels will be stars, but I'm not sure how to get enough resolution from a 1024x1024 texture. I think I might have to make a circular repeating texture for the stars too, rather than just two star domes as I originally planned.