system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] How did Astropaths navigate the Warp before the Astronomicon?

So it seems to me that the Golden Throne was built during the later stages of the Great Crusade, for the purposes of constructing the Human Webway project and projecting the Astronomicon. I believe that the Emperor could navigate the warp without the Astronomican, but what about his other expeditionary fleets?

Edit: Please replace 'Astropath' with "Navigator" in the title.
gpt
So first off Navigators are the ones who navigate through the warp, not Astropaths. Astropaths send messages through the warp.

To answer your question it is possible to travel through the warp without the Astronomicon, it's just slower (Rogue Traders sometimes have to do this when traveling to areas beyond its reach). Without the Astronomicon's light you basically have to travel in the "best guess" direction and then drop out of the warp every few light years to check your location and reorient. Navigators are still helpful here since they can use their powers to give a better estimate for the ship's travel then a normal human could but they are still a lot less accurate than they would be with the Astronomicom.