For general pictures of the sky at night a wide-angle is best. I use a 10-20mm Sigma.
The faster the lens the better (large aperture) as you can use shorter exposures to prevent star trails.
A 50mm f1.8 is fine. Small, lightweight and cheap or just try what you've got already.
Expect to stop down a few stops to get acceptable performance
Telephotos are great for large nebulae and the Milky Way star clouds.
Been looking at this myself, main problem I have is light pollution so hoping when the nights draw in to go a little further away from towns/cities.
Got a few interesting time lapse videos using GoPro from my back garden though.
Stick 'em up Fodder lets have a look then.