Yep, as the previous posters said : for wide-field astro photo, you'll need a Samyang lens. They're pretty much the only ones to do wide aperture with almost no coma (maybe Zeiss too, but i've yet to see a test for coma with the 18/21mm Zeiss lenses).
The Canon 16-35 f/4L is mostly comma-free, but F/4 is really not ideal.
Thanks, this is great advice. Your sample pictures were amazing.
Is Samyang just making another version of the Rokinon lens or vice versa?
If I am going to buy another L, I'd rather get something I'll use more than the 24L at that cost.
Made by Samyang, also sold under the Rokinon, Bower and other names.
@ Djaff - wonderful photos!