If the budget can be stretched to almost $1500 Paul there's a beautiful Prime that's usually overlooked because it's an "awkward" FL for normal photography.
It's the very very sharp
Canon 135mm f/2 and I believe it's a killer lens.
Yes it's not considered "wide field" compared to the ones mentioned and the budget is blowing out but I suppose it depends on how you define widefield for astro work compared to say a telescope.
I'm very keen to give this one a try for astro and terrestrial work !
I also like the ones mentioned so far (10-22mm, 17-40mm) but as you know primes are sharper, lower f-ratios allow you to stop down the lens to minimise CA while still giving you brighter pics at same exposures but of course there's the cost difference.