The Navigator’s Information File has a description of the Astro Compass (or Astrocompass) . This device was used by navigators to determine the true heading of the airplane, to steer a true heading, to identify a star, or as a pelorus, a device without magnetic needles used to read bearings.
It is unclear to me if, or how often, this device was actually used. I ran across an Astro Compass while visiting the United States Air Force Armament Museum at Eglin Air Force Base, near Fort Walton Beach, Florida.
Like pilots, cadets who trained to be bombardiers and navigators were assigned to different air fields for different phases of their training. They had preflight, gunnery and either bombardier or navigator training, or both.