How do you build a mirror thirty-nine meters across? The European Southern Observatory designed its Extremely Large Telescope around hundreds of polished hexagons. Seven hundred and ninety-eight segments will form its primary mirror. Each one has to work with its neighbors.

If neighboring segments shift, the optical surface falls out of alignment. Sensors along their edges measure tiny changes in relative position. Those measurements tell the control system where corrections are needed.

Three positioning actuators on each segment assembly adjust its height and tilt. Small corrections help the segments behave like a single giant mirror. The challenge is keeping hundreds of separate pieces aligned while the telescope observes.