Stopping down isn't going to remove ambient light from a town/ city etc. That light is still going to filter into the shot and essentially reduce the time you can expose your shot for before everything gets washed out by ambient light. At f/20 you're going to find it difficult to pick up the light from the stars anyway unless you have a high ISO - which is the worst thing you cando for long exposures apart fromleaving the lens cap on.


The major downside of taking night shot near city lights is that you will see a lot less of the stars. Getting out of town makes a huge difference. I live in outback Australia and thestars are amazing if you travel into the middle of nowhere.