use merge: overlapping 30 -40% Take picture of Star trails: F2.8, 30S, ISO 3200, or F4, 32 Minutes and ISO 100. To extend photo time, lower ISO and then change to F/11
or use flashlight to add lighting to foreground and use auto focus. If too close to foreground, it's not good to just focus on foreground. when there is a distance from camera to foreground it's ok to focus on foreground.
How to focus in the dark: 1. use infinite (last resort) may need to try several time to get the best focus. 2. Better way: use live view. zoom to one of the star and use manual focus ring to focus. 3. find bright foreground and use auto focus.