@loganlamar, you really want the widest aperture you can get, but I don’t think it will be a huge problem to shoot at f/4.5 as long as you use a long enough shutter speed. 10mm is much better than 18mm for astrophotography because you’re a lot more likely to get a meteor in the shot since you have a wider field of view.
It wouldn’t hurt to try out the 18-200mm, though!