What did I learn?
I learned that when it is so dark, you can’t really focus. You have set the lens to manual focus mode, and then learn by heart where the focus ring must be positioned.
About the processing
The photo consists of 9 HDR photos, but the sky comes from only one shot. The reason for that, is that the stars actually move surprisingly fast. An exposure of more than 15 seconds, will show stars moving. It’s really fascinating.