What lens would you recommend for landscape and astrophotography?
I would go for Sigma Art 20/1.4. Very wide, very fast, and very sharp (
review ). There's some coma near the corners, but it's not as bad as it could be, so can be forgiven. I don't think there's a better lens of this kind. This should be perfect for wide angle nightscapes.
This would be my choice if I had an FF DSLR.