The James Webb Space Telescope is scheduled to be deployed in 2013, giving scientists a deeper look into space than the existing Hubble Space Telescope. Its task will be to gather infrared light from objects more than 13 billion years old, using technologies that until recently did not exist.
The new telescope’s primary mirror (below) is more than six meters in diameter, with a surface area seven times that of Hubble’s. The mirror’s size will allow the telescope to collect more light more quickly than previous telescopes and achieve better resolution. “It is extremely lightweight, with very precise optical surfaces,” says John Decker, the deputy associate director of the project at NASA.
To manage such an enormous mirror, engineers have divided it into 18 pieces that will be folded together; they’ll be unfolded while the telescope is traveling to its final destination. Each segment is ground and polished to precise optical specifications (below). Engineers are taking extra precautions to avoid a repeat of the Hubble mishap, in which the mirror was incorrectly ground and polished, causing the telescope to produce blurry images until a service mission adjusted it.
The telescope’s mirror is made out of beryllium, one of the lightest metals known. A close-up view of the material is shown on the this page. Beryllium has exceptional thermal properties that give it stable optical performance at a wide range of temperatures. It is also thermally conductive, which helps keep the mirror’s temperature constant.
The mirror segments will be held in place and supported by a backplane (below) built by Alliant Techsystems. This structure is crucial in that it keeps the mirror steady; any unwanted movement could distort images.
The mirror segments will have seven degrees of freedom: scientists will be able to tip, tilt, and focus them separately without compromising their ability to act as a single optical device. The software that controls the segments was developed by NASA and Ball Aerospace. To validate its performance, Ball engineers have built a one-sixth-scale optical test bed (below). The mirrors in the test bed are a small-scale version of the real thing.
A full-scale model of the telescope (below) was on display in Seattle in January. It is more than 24 meters long and weighs 12,000 pounds.
To help it record faint signals from faraway objects, engineers at Raytheon Vision Systems and Teledyne Technologies have built two sensitive infrared detectors that register mid- and near-infrared wavelengths. The detectors are responsible for turning collected photons into electrons, much as a digital camera does, so that images of stars and galaxies can be recorded electronically. Below, the mid-infrared detector is exhibited by a project scientist at NASA’s Jet Propulsion Laboratory, where it is being tested.
Further enhancing the telescope’s ability to detect faint light is a microshutter built by engineers at NASA’s Goddard Space Flight Center. It serves as a light filter, allowing scientists to select the object they wish to study and block nearer, brighter light sources. With the help of this device, the telescope can efficiently observe more than 100 distant galaxies simultaneously.
Since the telescope will be operating at extremely cold temperatures (30 to 55 K), it must not generate heat that could drown out the radiation scientists are trying to detect. Engineers at Northrop Grumman have designed a large sun shield (below) to block the heat of the Sun and Earth. It consists of five layers of silicon-coated Kapton to reflect the Sun’s heat back into space.
Before the completed telescope is sent into space, a million miles from Earth, it will be tested in a thermal-vacuum chamber (below) at NASA’s Johnson Space Center in Houston. The chamber is 19.8 meters in diameter and 36.6 meters high. Its door alone weighs 40 tons.
Thus far, the chamber has been used mostly to test objects destined for low Earth orbit, so it will need to go through a series of modifications before it can simulate the cold temperatures the James Webb telescope will experience. New helium-cooled panels will be added to existing panels cooled by liquid nitrogen, allowing the chamber to reach a temperature of 30 to 35 K. The helium will also carry heat away from the panels.
The telescope will be put into the chamber by means of a mobile crane. Bringing the chamber and telescope to the desired temperature will take 30 to 40 days.
NASA engineers plan to start testing the telescope in 2010.