From Lexicanum

The Adeptus Astronomica has the duty of maintaining the Astronomican. They recruit members through the Adeptus Astra Telepathica. The Astronomica is largely composed of psykers; its non-psykers are hereditary servants.

The organization is based in Terra's Forbidden Fortress, where even Judges and Inquisitors must receive invitation to enter.

Its head is the Master of the Astronomican, who represents the organization as a High Lord of Terra.

The task of the Adeptus Astronomica is to train the young psykers they receive from the Adeptus Astra Telepathica, so that they can serve in the Chamber of the Astronomican. The whole organisation is basically a training institute run by a class of older members known as Instructors. Some of the Instructors are given the title High Instructor and given control of a certain area of teaching. The day to day runnings such as maintenance are taken care of by a body of administrative functionaries.

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.

The status of Chosen is considered a great honour. To fall short of attaining this goal is not a failure; rather they go on to become Instructors or are absorbed as administrative functionaries in the Forbidden Fortress. Those who do make it to Chosen status are considered above the Instructors and even the Master of the Astronomican. Their heads are shaven and they wear yellow robes and the scarlet badge of the Chosen. The rest of their lives are spent in prayer and contemplation until they are called to serve in the Chamber of the Astronomican.

Sources

• Codex Imperialis (Warhammer 40,000 2nd Edition)
• White Dwarf 140, Jervis Johnson, Andy Jones, Simon Forrest & Rick Priestley