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Contact: Lynn Chandler
Lynn.chandler-1@nasa.gov
301-286-2806
NASA/Goddard Space Flight Center

Third Light's the Charm

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Caption: 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 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. 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.

Credit: NASA/Rob Gutro and STScI

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