What format do you shoot, APS-C or full frame? Lens recommendations might differ.
Third yes vote for SamRokBowYang 14 f/2.8 manual lens, great value for money. On full frame it is really wide. The thing to check for on any astro lens is the degree of coma aberration. Few reviews address this - but see "lenstip review" for a Polish site (English-language) that has a specific coma test for each lens. Samyang has several manual rectilinear ultrawides for APS-C only (10mm f/2.8, 16mm f/2.0) and some for FF (14mm is FF), and a fisheye each for APS-C (8mm) and FF (12mm).
One thing I suggest is to go look at other astro-landscape photographs with EXIFs. What focal length do you like best? If you like 24mm, check out the newly released Sigma 24 f/1.4 Art, not yet reviewed. The power of f/1.4 for astro can hardly be overemphasized. This page by Roger Clark (astronomer and photographer) has some useful information: http://www.clarkvision.com/articles/nightscapes/index.html
go to table 1 to see the effect of f/stop and focal length on photon-gathering capacity - and astrolandscape photography is all about getting as many photons as possible.