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My only advice is that ANY lens wide open on a full frame will show aberrations in the corners at fast apertures. The L lenses aberrations are just less than others but still noticeable with stars. All have to be stopped down to about f/4 or slower depending on the quality of the lens.
Even my super Tele Canon 300mm F2.8 is finally 'perfect' for stars at the corners at f/3.6 with exterior aperture.
Shorter focal lengths just cannot have both fast speed and be aberration free and usually need to be stopped down.
The good news is that the Canon 24-104mm F4L is very good wide open on a full frame at all FL's. The other good thing, wide open at f/4 there are no iris blade diffraction spikes.
As a dedicated astrophotographer I completely understand the dilemmas of balancing needs and wants.
Hope this helps a bit Mike.
The Canon 5D Mark II is the only real choice of DSLR for low light photography.
Bert
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