If it's any consolation, a couple of nights back I shot a star field in IR+visible using the Canon 600mm/4L IS at f4. Stars came out tack sharp using autofocus, not bloated as I expecte. Seriously though, most lenses do fine if closed down to f8 and you choose whether you wish to use visible or IR, not both simultaneously. I was surprised that I did not get star bloat when I used the full visible+IR bandwidth with the 600/4. That kind of astrophotography is normally restricted to mirror lenses which are of course free of chromatic aberrations, right across the spectrum.