The Astronomican is a psychic beacon which the Navigators utilize to pilot the spaceships of the Imperium through the otherwise unnavigable chaos of warp space. 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 Adeptus Astronomica is based on Terra within the Forbidden Fortress, built within the Himalayas. The leader of the organization is known as the Master of the Astronomican, representing the organization as a High Lord of Terra.
Young psykers are initiated into the Astronomica as Acolytes. They are taught not just how to use and control their psychic powers but are introduced to the Lore of the Astronomican. They learn the value of their lives, study philosophy and are gradually brought to an inner understanding of the universe and the nature of the warp. Those who achieve this mystic state become the Chosen and 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. This psychic power is directed by the mind of the Emperor across about 50 thousand light years of the galaxy. Because Terra is situated in the galactic west, the Astronomican does not cover the extreme eastern part of the galaxy. Warp travel beyond the Astronomican’s reach is severely limited – this creates the effective borders of the Imperium.
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.
Destroying the Astronomican would most likely halt all Imperial warp travel within the galaxy, effectively bringing the Imperium to its knees.
The Emperor of Mankind used thousands of psykers to man the Astronomican when he was participating in the Great Crusade. He, however, did not sacrifice them; rather they were trained to maintain it themselves, but with immense physical stresses, the Emperor merely created the process for this. However, when the Librariums and the Astronimican was attacked in the Horus Heresy climax, the secrets of this were lost, and the Imperium was forced to sacrifice thousands of psykers as sustenance for the now Ascended God-Emperor