Wide field lenses are great tools for night landscape. This Canon lens is very good, with a reasonable maximum aperture, f/3.5, that is the average of similar lenses, but not as fast as a f/2.8 of i.e. Tokina 11-16 f/2.8 (but with other problems, read the review).

For astronomical use the lens has a good potential. At 10 mm the star shape is quite good even at full aperture, where the stars are nearly perfect in the center and with a coma at extreme corners, that is noticeable but not as bad as other lenses. For night sky timelapses I suggest to use fully opened, because also closing some stops the corner quality does not increase as desired. Instead for tracked images, where longer exposures are possible, I suggest to close at f/5.6 to get decent results at the corners, even if chromatic aberration is never fully corrected.
At 22 mm the corner performance is better, but the lens become quite closed, f/4.5, so it is not a good idea to use this focal length for night sky imaging.

Thanks to Marco Tarabini for borrowing this lens.