User blog comment:Supahbadmarine/Navigation without the Astronomicon/@comment-24909831-20150829163540

Powerful psykers are capable of acting as minor "Astronomicons" by themselves. That is why they are always aboard the ships, they keep contact with real universe locations. That is how they are explained in a bit older publications at least so to think Astronomicon as the only point of reference within Warp is somewhat incorrect. At least if you ask me.