I can't comment on Milky Way type shots but when you say 'further into space', what do you mean by that? Sara's the real expert here and as she says there are lots of things to take into consideration. Deep sky astrophotography isn't easy. I use the Sigma 150-600mm on a tracking mount (a proper driven mount you'd use to put a scope on, not a tracking head on a fixed tripod). I also use it for the Sun and Moon, which don't need long shutter speeds and a driven mount. Have a look at my Flickr Astronomy album if you want. The notes for each image tell you which were taken using just the lens. If this is what you had in mind come back and I'll try to help. It may be as simple as a focussing issue, although the answer isn't as simple as it is in daylight.............. But don't rush into changing the lens if you haven't got the basics right.