James Webb Space Telescope's groundbreaking optics explained by NASA

The James Webb Space Telescope suffers from astigmatism, but that visual condition is a very good thing for astronomy, NASA assures us.

A new explainer video the agency posted on YouTube talks about the huge telescope's optics and how they will let astronomers peer deep into the universe's history for future science work, ranging across subjects from exoplanets to the formation and evolution of early galaxies.

"This type of telescope enables a wider field of view and minimizes optical distortions," Sophia Roberts, a producer at NASA's Goddard Space Flight Center in Maryland, explains in the video.

Live updates: NASA's James Webb Space Telescope mission

The James Webb Space Telescope's 21.3-foot (6.5 meter) diameter primary mirror, seen deployed during testing here on Earth. (Image credit: NASA/Chris Gunn)

Webb launched on Dec. 25, 2021 and is scheduled to begin science operations this summer. 

The observatory will capture light with its 21.3-foot-wide (6.5 meters), 18-segment primary mirror, reflecting it to a much smaller secondary mirror. The light will then continue to bounce, moving next to a tertiary mirror and at last to a flat, fine steering mirror. 

The steering mirror next pinpoints the light through a hole in the center of the primary mirror, relaying the signal to the science instruments. It's pretty close to a typical Cassegrain telescope design on Earth, Roberts explains in the video, although the telescope has the additional challenge of being large and having to work in extremely cold conditions in space.

Webb gets around this coldness issue in part by having its mirrors made of beryllium, Roberts explains, "because this chemical element is very strong, lightweight and virtually ceases changing shape when it gets really cold." Additionally, the mirror segments include a thin coating of pure gold (less than 100 atoms thick) to reflect the infrared light on which Webb will focus.

Webb will take several months to cool to its operational temperature; it needs to be very cold to observe in infrared light, with a minimum of interference from the environment or from its four science instruments. The mission team also needs to finish aligning the primary mirror segments and check out Webb's instruments before the observatory can begin its science work.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace