I can comfortably say that as of the time of writing this, the Sony 20mm f/1.8 G lens is the best E-mount lens in this focal length for landscape astrophotography. That said, a second-hand Tokina 20mm f/2 MF can be had for an absolute bargain.
It's a big, heavy, expensive lens but it delivers images with phenomenal detail and sharpness and there's very little in the way of distortion or vignetting.