Rokinon/Samyang/Bower (same company) have some great lenses for astrophotography like:
They are affordable and have wide apertures to get as much light in as possible. They also stay sharp with the aperture as wide as it gets both have very low levels of coma (comatic aberration). They are available in 8-10 different mounts for numerous cameras so it is very likely that they will be compatibility probably won't be a problem.
The 24mm f/1.4 is considered one of the best night photography and astrophotography lens you can buy. "Excellently sharp, especially when stopped to f/2. Manual focus...fast, wide and shows very little aberration problems." - lonelyspeck.com/24mm
The 14mm f/2.8 is cheaper and one of the most affordable full frame nightscape lenses. It's manual focus too, pretty big and heavy but the ultra-wide angle is perfect for sweeping landscapes. "It’s slow focus ring and inaccurate distance scale are minor quirks but a little bit of manual focusing practice with the lens will help most photographers adjust to its use. Given that most full-frame alternatives are at least $1000 more expensive, the Rokinon 14mm f/2.8 is one of the best super-wide astrophotography and landscape lenses available for any interchangeable lens camera system. If you’re trying to decide on what lens you should buy for landscape astrophotography or Milky Way photos, stop right there and buy this lens first." - lonelyspeck.com/14mm
All in all, two great lenses, if you can afford it, the Rokinon/Samyang/Bower 24mm f/1.4 is the best choice for all-around usability as long as you get a good one because a few have decentering issues. Buy it from Amazon and test it to make sure you have a good one using this quick decentering test.
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