Fast wide-angle lens
$$$Astro is an aperture game: going from f/4 to f/1.8 more than doubles the starlight you capture in the same 20 seconds. This is the single biggest image-quality upgrade in night photography.
What to look for: 14–24mm on full frame (10–16mm APS-C), f/2.8 or faster, easy manual focus.
Skip it if: your kit zoom already opens to f/2.8 at 16mm — spend on a tripod first.