Samyang has bargain lenses that have been coma-minimal to date. Look at the lenstip reviews of 14mm 24mm 35mm - a true bargain set for astrophotographers. I gather that the Nifty Fifty Canon 50mm f/1.8 II is fine and coma free when stopped down to f/4 - you can't get much cheaper than that. On my 6D I use the 14mm f/2.8 at f/4 for maximal astrophotographic sharpness, but it will be ok at f/2.8 in my experience. I like manual focus lenses. The Zeiss 21mm f/2.8 and Canon TS-E 24mm f/3.5 are superb, and if you have unlimited money, go for the TS-E to get the tilt for your daylight landscapes. I happened across a used Zeiss 21 at my local store, bought it, use it at f/2.8, coma free. Coma is reasonable on the Sigma 35mm f/1.4, best results at f/2.8, fine results f/2 in my experience. I am experimenting with inherited and my own old legacy manual-everything lenses for 50ish and 100ish focal lengths. These 40 year old fast normal lenses have coma wide open - aspherical elements were rare back then. I get great sharpness and ok coma at f/2.8, no coma at f/4 with the old Nikkor 50mm f/1.2 AIS. It is also a fun special effects lens for the marked aberrations at f/1.2.