Giant Mirror for a New Space Telescope

A telescope’s ability to collect light from distant objects is directly related to the size of its mirrors: the larger the mirror, the more light it can collect. So engineers at NASA Goddard Space Flight Center, Northrop Grumman, Ball Aerospace, SSG-L3 Tinsley and AXSYS Technologies have built an extremely large, lightweight cryogenic mirror. Its size will allow it to collect more light faster than previous telescopes and with better resolution. The giant mirror, which will be deployed on the James Webb Space Telescope (JWST), will revolutionize the studies of how stars and planetary systems form and evolve.

The James Webb telescope is scheduled to succeed the Hubble Space Telescope in 2013. Its key component, the new mirror, is composed of 18 segments that make up a total area of almost 25 square meters–seven times the area of the Hubble telescope’s mirror, says John Decker, the deputy associate director of the project at NASA. “The mirror is one of the most revolutionary technologies that will go aboard the JWST,” says Decker. “It is extremely lightweight, with very precise optical surfaces.”

NASA’s new telescope will primarily look at the universe in the infrared spectrum; it will remain stationary with respect to the earth and the sun a million miles away from the earth. It will be operating at tremendously cold temperatures (30 to 55 kelvins) and therefore can’t generate heat that could swamp out what scientists are trying to detect. In contrast, the Hubble telescope orbits the earth detecting optical and ultraviolet wavelengths using a glass mirror that is stable at those temperatures, but it would not be at the temperatures of the James Webb telescope.

The mirror for the new telescope is made out of beryllium, one of the lightest known metals. The material has exceptional thermal properties that allow its optical performance to remain stable at a wide range of temperatures. It is also thermally conductive: that is, it conducts heat across the whole mirror, eliminating temperature gradients.

This is not the first time that this material has been used in space. The primary mirror of the Spitzer Space Telescope, which was launched in August 2003, also uses beryllium, but its mirror is only three-fourths of a meter in diameter. In contrast, the James Webb telescope’s mirror is more than six meters in diameter, making production a far greater challenge, says Lee Feinberg, the telescope’s manager at NASA.

Given the extreme size of the mirror, engineers have divided it into 18 individual pieces, each approximately 1.3 meters in diameter. The segments will be folded together, like the leaves of a drop-leaf table, unfolding while the telescope is traveling to its destination.

Engineers are taking extra precautions to avoid a repeat of the Hubble telescope mishap, in which the mirror was actually ground and polished to an incorrect prescription. As a result, images produced by the Hubble were blurred, and NASA was forced to install corrective optics in a repair mission. Since the James Webb telescope’s mirror is made from beryllium rather than from glass, the specific methods used to produce it differ substantially from those used to produce the Hubble mirror. “The multiple independent-measurement methods that are planned for the JWST mirrors prior to launch will ensure that they have the correct prescriptions,” says Decker.

The mirrors will have seven degrees of freedom and can be individually controlled, allowing scientists to move, tip, tilt, and focus each segment using a new technology developed specifically for the telescope.