The Astronomican is a psychic beacon located on Terra, which Astrogators use to pilot Imperial spaceships through the otherwise unnavigable chaos of Warpspace. As the beam generated is psychic it exists within the psychic universe of the Warp. It is the duty of the Adeptus Astronomica to maintain all aspects of the Astronomican, including training those who will power it.
The Astronomican was originally constructed in M30 in preparation for the Great Crusade. The Emperor brought in large numbers of Tech-Priests and drafted much of Terra's population for the gargantuan task of constructing the Chamber of the Astronomican at the Forbidden Fortress. At the time of its construction, the Astronomican was the single largest structure on Terra, though this would later be surpassed by the Imperial Palace. In the days of the Great Crusade, the Astronomican was powered by the Emperor's own psychic might directly. Without the beacon of the Astronomican to guide their ships through the Warp, Imperial armies would not have been able to enact the Great Crusade.
Although the Emperor does not provide the energy of the beacon, only He has the psychic power to handle such immense energy and direct it across the Galaxy. The psykers who power the Astronomican are recruited into the organisation through the Adeptus Astra Telepathica. They are initiated into the Adeptus Astronomica as Acolytes, and taught how to use their powers, philosophy, the meaning of their lives, and the Lore of the Astronomican. Some achieve a mystic state, gaining the status of Chosen. It is these Chosen who are eventually called to serve in the Chamber of the Astronomican. Here they fuel the Astronomican beacon with their psychic energies; as their powers are drained the Chosen slowly fade and die. Up to one hundred Psykers die each day powering the Astronomican.
The psychic power generated through their sacrifice of the Psykers is directed by the mind of the Emperor across about fifty thousand light years of the Galaxy. Because Terra is situated in the galactic west, and the galaxy is approximately eighty-five thousand light years across,[1a] the Astronomican does not cover much of the eastern part of the Galaxy on the Eastern Fringe.[10] Warp travel beyond the Astronomican's reach is severely limited - this creates the effective borders of the Imperium.[1a] Because of this, destroying the Astronomican would most likely halt all Imperial Warp travel within the Galaxy, effectively bringing the Imperium to its knees.[1b][4]
Astrogators, as well as other psykers, can sense the Astronomican. It is described as a sensation of silvery light and a chorus of many heavenly voices, beginning first with a solitary voice and progressively growing into an angelic choir.