User blog comment:Supahbadmarine/Navigation without the Astronomicon/@comment-8395098-20150830074204

It's also possible to navigate the warp without a psyker. However this requires a knowledge of forces and a calculative power beyond most human ability. Using special supercomputers the Cult Mechanicus and golden age humans were able to navigate the warp. Similarly the Tau (while to a lesser extent) do the same with their micro-jumps. And from the understanding developed by the Rogue Trader game line it's that the navigators use the astronomicon as a point of reference when calculating their warp jump path and use it less for actual guidance while their ship travels through the warp.