Which wide angle, Canon mount, for star field astro-landscapes?

I've had a Canon EF 20mm f/2.8 and a Sigma EX II 14mm f/2.8 for many years. Lately I've been using them (on a NEX 5N) to learn astro-terrestrial work and I have to say I am pleasantly surprised how well they work for the purpose. I think I'd have to spend a lot of money to get significantly better glass. Put another way, for now, there's a lot more to gain by improving my technique than spending money on lenses. So my advice is, use the lenses you have until you can demonstrate your lenses are holding you back. By then you'll have a better idea what to buy.