Mirror Assembled for Hubble’s Successor

NASA has assembled the primary mirror for the James Webb Space Telescope, a big step on the way to the telescope’s October 2018 launch.

Assembling JWST primary mirror

The final segment of the James Webb Space Telescope is carefully put in place.
NASA / Chris Gunn

NASA's next big space observatory came one step closer to completion last week, as engineers placed the final segment of the James Webb Space Telescope’s primary mirror.

The careful construction of the segmented mirror began in November 2015 at the Goddard Space Flight Center in Greenbelt, Maryland. Each hexagonal segment is 4.2 feet (1.3 meters) across, coated with gold and beryllium, and weighs 88 pounds. There are 18 mirror segments in all. Now fully assembled, the primary mirror is 21.3 feet (6.5 meters) in diameter, making JWST the largest telescope ever fielded in space.

“Scientists and engineers have been working tirelessly to install these incredible, nearly perfect mirrors that will focus light from previously hidden realms of planetary atmospheres, star forming regions, and the very beginnings of the universe,” says associate administrator John Grunsfeld in a NASA press release.

JWST vs. Hubble mirrors

A comparison of the primary mirror on the Hubble Space Telescope versus the James Webb Space Telescope.
NASA

Often touted as the Hubble Space Telescope’s successor, the James Webb Space Telescope (JWST) will actually work at longer wavelengths than its famous cousin: from the long-wavelength end of the visual spectrum into the infrared regime. In that sense, it’s more of a follow-on to the Spitzer Space Telescope. JWST’s primary is also much larger than Hubble’s, which spanned only 2.4 meters, yet JWST’s overall observatory is actually lighter in mass.

The telescope takes its name from NASA's second administrator, James E. Webb, who ensured that NASA’s legacy would feature not just spaceflight but also space science.

On Target for 2018 Launch

James Webb Space Telescope primary mirror

The James Webb Space Telescope in the clean room with all segments in place.
NASA / Chris Gunn

NASA engineers performed the mirror assembly using a robotic arm to position each individual segment. Watch work continue in the Goddard clean room via the James Webb webcam.

“Completing the assembly of the primary mirror is a very significant milestone,” says Lee Feinberg (Goddard optical telescope element manager) in a recent NASA press release. “There is a huge team across the country who contributed to this achievement.” Ball Aerospace & Technologies Corp. built the mirror segments, and Northrop Grumman subcontractor Harris Corporation is carrying out the mirror assembly and installation.

The James Webb Space Telescope uses a three-mirror anistigmat optical configuration, a complex layout that eliminates coma, astigmatism, and spherical aberration. In the coming months, the Harris team will install the secondary mirror and the aft optics assembly. Then engineers at Goddard will conduct acoustic and vibration tests to assure the telescope can withstand a bone-jarring rocket launch. More testing follows: NASA plans to ship the James Webb Space Telescope to the Johnson Space Center in Houston, Texas, for extensive cryogenic optical testing, to ensure the instrument will function in the cold depths of space.

The James Webb Space Telescope is set to launch atop an Ariane 5 rocket from Kourou, French Guiana, in October 2018. Originally slated for a 2011 launch at a budget of $1.6 billion, delays and a ballooning budget led to a threatened cancellation. Yet the mission managed to weather stormy political winds when, instead of cancelling in 2011, Congress capped the budget for development at $8 billion. The assembly of the primary mirror has occurred on schedule for the targeted launch window.

Some Assembly Required

The James Webb Space Telescope has to fold up if it’s going to fit inside the payload faring of the Ariane 5 rocket. Once launched, the telescope will head towards its new home in space: a lissajous (halo) orbit around the L2 Lagrangian point — that is, the telescope will orbit the Sun at a stable position on the opposite side from Earth, 700,000 miles beyond the Moon’s orbit. Then all the parts will have to unfurl and work properly on their own (the video below shows how this will happen). Unlike the Hubble Space Telescope, astronauts won’t be able to visit the JWST for repairs. So why position it so far from Earth? The frigid location gives the telescope a close to noise-free observing environment — especially because with its sunshield, the telescope can block thermal interference from the Sun, Earth, and Moon.

Once open for business, JWST will give astronomers a new window on the universe. It will peer farther back into the early universe to see the first galaxies, penetrate interstellar dust clouds where stars are forming, and give us spectroscopic analysis of exoplanet atmospheres. In fact, James Webb will work in concert with another space-based observatory set to launch the year before it: the Transiting Exoplanet Survey Satellite (TESS), which aims to discover exoplanets around nearby red dwarf stars, ideal targets for JWST’s infrared capabilities.

Exciting times, indeed: the next few years may well mark the dawn of a new golden age of space-based astronomy.

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