Before light from the universe reaches Webb's cameras and science instruments, it will reflect off four different mirrors — the primary, secondary, tertiary, and fine-steering mirrors. The light's third stop along its zigzagging path is the tertiary mirror, housed within the Aft Optics Subsystem at the center of Webb's 21-foot primary mirror. Mary Estacion visits Ball Aerospace in Boulder, Colo., to learn about the tertiary mirror's role and to see how the mirror's optics are being tested.
More information about this video and Webb's mirrors
NASA and Mary Estacion (STScI)
Publication: January 31, 2013