The James Webb Space Telescope team is installing telescope mirror segments using a high-precision robotic arm. To precisely install the segments, the robotic arm can move in six directions to maneuver over the telescope structure. While one team of engineers maneuvers the robotic arm, another team of engineers simultaneously takes measurements with lasers to ensure each mirror segment is placed, bolted, and glued perfectly before moving to the next.
References on the structure and the mirrors allow the team to understand where the final mirror surface is located. The team uses reference points on the telescope structure called Spherically Mounted Retroreflectors to accomplish this feat. A laser tracker, similar to the ones used by surveyors, looks at those reference points and can determine where the mirror segments go.