By Patricia Daukantas
During his plenary speech at last week’s Frontiers in Optics (FiO) meeting, John C. Mather, one of the 2006 Nobel Prize winners in physics, described the optical systems that will go into the James Webb Space Telescope (JWST), which is scheduled for launch in 2013. Mather, a senior astrophysicist at NASA’s Goddard Space Flight Center (USA), spent 15 years taking the Cosmic Background Explorer (COBE) satellite, which measured the faint afterglow from the Big Bang that created the universe.
Mather spent 15 years taking COBE from the proposal stage to its 1989 launch. He was the principal investigator for the COBE instrument called the Far Infrared Absolute Spectrophotometer (FIRAS), which measured the black-body spectrum of the cosmic background radiation from the Big Bang to unprecedented accuracy. (When he first showed his experiment-matches-theory result to an astronomers’ meeting in January 1990, his colleagues gave him a standing ovation.)
Mather’s current project, JWST, involves almost every section of NASA. The planned 6.5-m space telescope looks a little like a solar energy concentrator in the desert, he said, because it needs to have a multilayered sunshade in order to operate at a temperature of 40 K. JWST will be launched on an unmanned Ariane booster rocket and will fly to the L2 Lagrangian point, a stable position about 1 million miles on the opposite side of Earth from the Sun. After showing an animation of the way the telescope is supposed to unfold by itself once it reaches L2, Mather quipped, “If you’re a mechanical engineer, this is either terrifying or thrilling.”
JWST will use a three-mirror anastigmat optical design for a wide field of view. (In addition to the primary and secondary mirrors, it will have a “fine steering mirror” at the Cassegrain focus.) The scientists selected a beryllium mirror because of its superior cryogenic properties—it undergoes much less thermal distortion than the ultra-low-expansion glass used in many ground-based telescopes.
Among JWST’s many cameras will be NIRSpec, a near-infrared imaging spectrograph that can photograph 100 galaxies at once. “If it takes two weeks to get an exposure, we don’t want to be limited to just one galaxy,” Mather said.
Since data from the Hubble Space Telescope showed that galaxies were formed longer ago than scientists had thought, JWST is likely to study the origins of galaxies and the earliest populations of stars. JWST could also study Earth-like transiting planets, or planets that pass between their parent star and the observer. Astrophysicists over the years have proposed a number of schemes for orbiting planet-hunting interferometers, with instruments mounted on widely spaced satellites, but tight budgets at the space agency may mean that not all of those projects will fly.
2008-10 October, Astronomy