It is not hard to focus on infinity (LOL). I know I intentionally swithched to manual focus fo a night launch of the space shuttle as didn't want focus searching at launch when obviously infinity (5 miles away with telephoto). The bigger question is are you still using the 18-55 kit? This lens, while very sharp would duplicate that range, and in addition to the sharpness would gain 2.8 vs 3.5. Me personally, I upgraded the kit lens to the constant 2.8 aperture Tamron 17-50 (non-VC, sharper and cheaper than VC) which would give you a lot more options, including but beyond night sky photography. While it leaves the kit lens in the dust, I suspect the prime may be even sharper but depending on final output (below poster size prints) may be splitting hairs.
Thank you very much, Lawrence. I have upgraded to the D7100 and I'll have a greater flexibility for lenses. I only want something at 2.8 and 18mm or less. There is more choice for me now. Will look into that Tamron. I have heard it's good.