Essentially two pieces of wood joined together with a hinge. At the other end they're held apart using a threaded rod, and then I use a motor to drive a nut on the rod so the boards very slowly close together at the same rate as (but opposite direction to) the earth's rotation (roughly 15°/hour). My camera is mounted to a ball head on the top board. This means, relative to the camera, the stars are stationary so I can do long exposures without getting star trails.