The Ephesian Schism
The Astronomican is a psychic beacon located on Terra, which projects an astropathic beam 70,000 light years across the Milky Way Galaxy which human Navigators utilise to pilot the starships of the Imperium of Man through the otherwise unnavigable chaos of Warpspace.
As the beam generated is psychic it exists within the psychic universe of the Warp. The beam can be psychically sensed from almost anywhere in Warpspace and is vital for faster-than-light travel since the mutant human Navigators use it as the only fixed reference point in the galaxy which is required for a starship to reach its intended destination when it travels through the Warp.
Without it, the Imperium could not survive as an interstellar society as Warp travel would be impossible. It is uncertain how faster-than-light travel was done in the Dark Age of Technology before the Astronomican existed, though it can be safely assumed that a form of faster than light travel that is non-dependent on entering Warpspace and may be similar to Necron phase drive technology was used, or more likely that Warp jumps were simply unable to reach across as many light years as at present because of the lack of a fixed reference point. This is how the Tau’s faster-than-light stardrive technology functions, as their vessels make shallow “Warp dives” across a much smaller length of realspace than current Imperial starships, and is probably how Mankind once operated as well.
In addition to this invisible beacon among the stars, the forging of the Imperium amongst the stars would not have been possible without the creation and continuation of the Astra Telepathica.
This special corps of inter-stellar communicators was created by the Emperor during the final months of the conquest of Terra, as the Emperor had foreseen their absolute future need. For the most part the Emperor did not favour the use of psychic talents in others. As a psyker himself, albeit one of unprecedented power and control, He was well aware of the dangers inherent with human contact with the Warp.
Furthermore the Age of Strife on Old Earth had its own “witch-kings” and possessed psykers to attest to those dangers, some of whom the Emperor had personally slain. However, he was able to identify those strong enough to maintain control of their abilities and so with some prudence certain psykers were employed in a variety of special roles in the Imperium, and the Astropaths were to be such an exception.