JJenks

So I have about $400 to spend and I was thinking I would get a new lens for my Nikon D600. I currently have a 28-300mm f3.5-5.6 lens and a star adventurer pro 2i. The only lenses/refactors that I've found are the Samyang/Rokinon 135mm f2 which I think I would get used, and the Askar fma 180 (which I haven't been able to find anywhere actually, lmk where I could get this if you suggest it please). My main targets as of right now are the bigger dso's like Orion, Andromeda, Pleiades, etc. On the one hand, I would think the Askar would be better for the slightly larger focal length which would get better framing on my objects, but on the other, the 135mm is f2 vs f4.5. Which do I choose? or if you know of a better option, let me know! Thanks!