That’s a pretty amazing image to catch peering out from your back balcony. The rig used to record such a gem is seen on the right. It’s called a Barn Door tracker and was built by [DCH972]. Details for this build are scattered all over the place, there’s a video (also found below), another album of some of the best images, and plenty of background info in the Reddit thread.
This design is also know as a Haig or Scotch mount. While we’re dropping links all over the place check out the Wikipedia page on the topic. The point of the system is to move the camera in such a way so that the stars appear to hold in the same place even though the earth is moving. There’s an ATmega32u4 breakout board riding on top of the breadboard. It’s doing some pretty heavy math in order to calculate the stepper motor timing. That’s because the mount is like a photo album, hinged at one side and opened on the other by a ball screw. This linear actuation needs to be meshed with the change in angle of the mounting platform, and finally it needs to sync with the movement of the earth. But once a series of images is captured correctly they can be processed into the composite photograph shown above.
If missed that SDR galactic rotation detector from last May you should find it equally compelling.