Behind the Webb: Third Light’s the Charm (Episode 17)

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.