Astronomican

The Astronomican is a psychic beacon on earth, projecting a beam 50,000 light years across space. The beam can be "seen" from almost anywhere in warp space and is vital for faster-than-light travel since the mutant human "Navigators" utilize it as a reference point to reach their intended destination. Without it, the Imperium could not survive as warp travel would be impossible.

History
The Hall of the Astronomican is based in the Himalayan mountains of Terra. It was built by the Emperor as a focus for his fathomless psychic power. Originally, the Emperor fueled and directed the beacon himself, projecting a beam of unimaginable power and range through the warp.

During the Horus Heresy, the Emperor's Imperial Webway project drew more and more of the Emperor's might away from the Astronomican, weakening its signal. Malcador the Sigilite suggested a compromise: the beacon would be fueled instead by sacrificial psykers, freeing the Emperor to finish his work. The Emperor approved the notion, but also made it clear that it could be only a temporary solution.

Tragically the Emperor would never resume his former role, though he would continue to calibrate the beam's frequency. Because of its vital role in the running of the Imperium, it has great religious significance and is also called the Light of the Emperor, the Ray of Hope or the Golden Path.

In the present, the Astronomican is a complex piece of machinery powered by ten thousand specially trained psykers, whose life-forces are exhausted in a matter of months. Their ranks must therefore be replenished constantly with fresh recruits. .

Limitations of the Astronomican
Warp storm activity can affect the overall range of the Astronomican but it generally has a range of approximately fifty thousand light years. As the galaxy is about eighty-five thousand light years in diameter and Earth is approximately thirty thousand light years towards the galactic west, the Astronomican barely penetrates the eastern fringe of the galaxy; even less so in the wake of the warp-smothering Tyranid Hive Fleets.