Webb Space Telescope

There are four types of mirrors that will fly on NASA's James Webb Space Telescope. They're called the "primary, secondary, tertiary" and fine steering mirrors. Although the 18 primary mirror segments make the biggest splash, the other mirrors are equally as important.

There are 18 hexagonal mirror segments that, when combined, make up the large primary mirror with a collecting area of 25 meters squared (269.1 square feet).The secondary mirror is perfectly rounded and convex, so the reflective surface bulges toward the light source. The tertiary mirror is the third stop for light coming into the telescope and is the only fixed mirror in the system -- all of the other mirrors align to it.

"The tertiary mirror is approximately a meter wide and is designed to accept the light from many field points and relay them through the fine steering mirror to the instruments," said Lee Feinberg, NASA Optical Telescope Element Manager for the Webb telescope at NASA's Goddard Space Flight Center in Greenbelt, Md.

Smith explained, "The light from an object reflects off the primary mirror, the secondary mirror, into the Aft Optics Subsystem's aperture, and off the tertiary and fine steering mirrors, before entering the science instruments in the back of the telescope." The Aft Optics Subsystem sits in the middle of all of those mirrors.

All the mirrors are made of a light metal called beryllium which is very strong for its weight and holds its shape across a range of temperatures.

The James Webb Space Telescope will be the world’s next-generation space observatory and successor to the Hubble Space Telescope. As the most powerful space telescope ever built, Webb will observe the most distant objects in the universe, provide images of the first galaxies ever formed and see unexplored planets around distant stars. The Webb telescope is a joint project of NASA, the European Space Agency and the Canadian Space Agency.