Schematic of the stellar beam paths for PRIMA astrometric observations showing the main subsystems and their relative locations (not to scale). The beams propagate through underground light ducts from the telescopes to the delay line tunnel from where they are relayed into the beam combination laboratory. The input channel number of each beam is shown in bold. The PRIMA metrology monitors the path lengths of the four beams between the respective endpoints in the FSU and STS. The two metrology beams that monitor the FSUB feed are shown as dashed lines.