There are a couple of deficiencies with Canon's super telephoto lenses. They can never be as well baffled as an astro APO. They also suffer from vignetting with a full frame sensor camera.
Where they leave astro APO's for dead is that they have a flat field. They also have a single crystal of Magnesium Fluoride as a lens element plus two ED glass elements in the major objective lens group. A Super APO in my opinion.
I have used a 300mm F2.8L Canon lens on a full frame camera and once optimised nothing would come close at this focal length.
At f/2.8 only the very bright stars showed residual aberrations. Limiting the the aperture by an external aperture at f/3.6 even bright stars showed very little secondary aberrations. The other trick is to limit the amount of sky the lens actually 'sees' to not let in any light that does not contribute to the image from being scattered inside the lens and so lower contrast. This can easily be done by extending the lens hood. There are also processing tricks to overcome the lack of perfect focus with the visible spectrum and HA.
See this thread
http://www.iceinspace.com.au/forum/s...ad.php?t=80863
Bert