13 mirrors polished for Webb telescope

Compiled by Photonics Spectra staff

NASA has completed coating 13 of the James Webb Space Telescope’s 18 primary mirror segments, enabling the observatory to see objects as far away as the first galaxies in the universe.

Mirrors are the most critical part of a telescope and, as such, the quality of their polishing is crucial. The mirrors were polished at L3 Integrated Optical Systems – Tinsley in Richmond, Calif., to accuracies of less than one-millionth of an inch. That accuracy is important for forming the sharpest images when the mirrors cool to 2240 °C in space.

The four types of mirrors on the Webb space telescope are shown along with an artist’s conception of the telescope’s optics with its 18 primary mirror segments. Courtesy of NASA/Ball Aerospace/Tinsley.

The space telescope is composed of four types of mirrors – primary, secondary, tertiary and fine steering. The telescope’s primary mirror has an area of approximately 25 sq m, enabling scientists to capture light from faint, distant objects in the universe faster than previous space observatories have allowed. Made of beryllium, the mirrors will work together to relay images of the sky to the telescope’s science cameras.

Once polished, the mirrors are coated with a microscopically thin layer of gold that enables them to efficiently reflect infrared light. NASA plans to complete the coating of the remaining five mirrors by early next year. The 18 segments will fit together to make one large mirror that measures 6.5 m across.

Ernie Wright, a NASA engineer, looks on as the first six of 18 primary mirror segments are prepped to begin final cryogenic testing at NASA’s Marshall Space Flight Center in Huntsville, Ala. Courtesy of NASA/MSFC/David Higginbotham.

Although many astronomers believed that developing such mirrors could not be done, the telescope’s prime contractor, Northrop Grumman Corp. of Redondo Beach, Calif., invented a mirror technology to make it possible for the Webb telescope to see back in time, said Scott Willoughby, vice president and Webb Telescope Program manager for Northrop Grumman Aerospace Systems.

Although NASA has made great strides in preparing the mirrors for the Webb telescope, the House Subcommittee on Commerce, Justice and Related Agencies voted July 7 to cut its funding. A decision is expected by Sept. 30 to determine whether funds will be restored.