Another option that you might not have considered for astro are Nikon manual focus prime lenses with a Nikon to EOS adapter.
For example the 200mm f4 AI lens that I have is a very nice lens that I have used for astro work (see
here and
here). By all accounts the 180mm f2.8 ED AI lens is even better (although more expensive).
I sourced my Nikon to EOS adapter from
here
