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New Video Brings Webb Telescope's Third Mirror to Light
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. A new video takes viewers behind the scenes for a special look at the tertiary mirror.

The video called, "Third Light's the Charm" is part of an on-going video series about the Webb telescope called "Behind the Webb." The video, produced by Mary Estacion from the Space Telescope Science Institute (STScI) in Baltimore, Md., takes viewers behind the scenes with scientists and engineers who are creating the Webb telescope's components.

Before light from the universe reaches the James Webb Space Telescope'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, Colorado, to learn about the tertiary mirror's role and to see how the mirror's optics are being tested.
Credit: Space Telescope Science Institute
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The 2 minute, 41 second video takes viewers to Ball Aerospace in Boulder, Colo., where Koby Smith, Aft Optics Subsystem Integrated Product Team lead, explains how light is manipulated with the Webb mirrors to get a clearer understanding of the object being focused upon.

Gold-coated Tertiary Mirror

Webb's coated flight tertiary mirror. Dan Patriarca, president of Quantum Coating Incorporated, is in the photo. Credit: Ben Gallagher (Ball Aerospace) and Quantum Coating Incorporated
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depiction of mirror types aboard Webb telescope

This image shows the four types of mirrors on the Webb telescope. From left to right are: a primary mirror segment, the secondary mirror, tertiary mirror and the fine steering mirror. The bottom right shows an artist's conception of the Webb telescope optics with its 18 primary mirror segments. On the bottom row are the three mirror segments shown on the same scale and seen from the rear to illustrate the honeycomb structure that makes this mirrors both very light and mechanically stiff.
Credit: NASA/Ball Aerospace/Tinsley
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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.

Estacion also takes viewers into the optical test tent within a clean room for a look at the tertiary mirror. She and Smith explain the process it still needs to go through to be ready for flight on the Webb telescope.

The optical test tent is used for testing the secondary and tertiary mirrors. Smith shows viewers an interferometer that’s used to measure the surface quality of the optics. It sends out a wavefront of light and compares it to a known reference, and any deviations on that surface will appear as fringes on a camera screen.

Estacion explains that there are more steps in the preparation of the tertiary mirror including vibration and thermal testing before being integrated with the rest of the telescope.

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.

The "Behind the Webb" video series is available in HQ, large and small Quicktime formats, HD, Large and Small WMV formats, and HD, Large and Small Xvid formats.

Related Links

› The "Behind the Webb" series
› About the Webb mirrors' golden coating
› More about Webb's mirrors
› NASA's Webb telescope website
› Webb telescope project website
Rob Gutro
NASA's Goddard Space Flight Center, Greenbelt, Md.