Jack is right -- try mksource to see if that will work. Otherwise, you can try creating several smaller illum sources and scatter them around your sky, with the glow behind.
-Greg
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From: Jack de Valpine <[email protected]>
Date: September 14, 2009 1:51:14 PM PDTHi Mark,
Is your high-res image an hdr? If so would it be worth looking into mksource to recover broad areas of the sky (clusters of star) as illum sources? Then your high res image can be mapped onto a glow source with the lighting provided by a set of illum sources (multiple ones with varying solid angles), derived from the hdr.
-Jack de Valpine