Bu I don't think you need an ultradetailed sphere, or am I wrong?
btw for the screenshots it seem that a single 1024x1024 map isn't enough to cover all the background sphere, and probably even 2 maps aren't enough, with images of the complexity of star photos you risk to see odd grain effects, as flipside said.
you should try with 4 or even 8 maps in my opinion, lithunwrap can help a lot to unwrap the sphere, and probably the best solution would be to have one (or two) star photo covering a relatively small portion of the screen, with a tileable starfield background covering all the other areas.
This could require some work, in order to don't see the borders of the star photos