Telescope complete; Project to map universe can begin

The Sloan Digital Sky Survey Telescope at Apache Point is now complete.

The final piece of an advanced telescope that will give astronomers their clearest view yet of the large-scale structure of the universe has arrived at its permanent home. The delivery Feb. 18 of the spectrograph for the Sloan Digital Sky Survey's telescope at Apache Point Observatory in New Mexico marked a milestone in the most ambitious sky-mapping project ever undertaken.

An international team of astronomers from eight research institutions, including the UW, will collaborate in the project, which will use a specially constructed telescope located in New Mexico's Sacramento Mountains to produce an unprecedented three-dimensional picture of the universe.

UW engineers installed the Sky Survey telescope's primary mirror in January. And the final piece of the telescope, the second of two spectrographs built at The Johns Hopkins University, will arrive this month at Apache Point.

Spectrographs separate radiation from celestial objects into individual bands, or colors. The spectra reveal important information about the objects emitting light, including their distance and chemical composition. Within the Sky Survey's planned five-year lifetime, astronomers will catalog more galaxy spectra than have been examined in the entire history of astronomy.

Besides its spectrographic capability, the Sky Survey's 2.5-meter diameter telescope will incorporate another technological advance: at its focus will be an array of charge-coupled devices, or CCDs, small silicon chips that convert incoming light to electrical signals. While the typical large telescope has one, or at most a few, CCDs in its focal plane, the Sky Survey telescope will contain 54 of the chips.

The camera for the Sky Survey telescope is the most complex imaging instrument ever developed for astronomy. The increase in sensitivity provided by the camera will yield a dramatic increase in the quality of data about a very large section of the sky.

The Sky Survey will produce a catalog of the positions and brightnesses of more than 100 million celestial objects, as well as the spectra for about a million galaxies and 100,000 quasars. Ultimately, the catalog produced by the Sky Survey will be available to the public and to astronomers around the world for a wide range of future studies.

"The Sky Survey catalog will be the astronomer's field guide to the heavens for the next fifty years," said Bruce Margon, UW professor of astronomy, who recently was named the Survey's scientific director. "Our only current comprehensive guide comes from the Palomar Sky Survey of 40 years ago. That survey used photographic plates, making it 50 times less sensitive than the Sloan Digital Sky Survey. It was in only two colors, compared to our five, and of course it was not in digital form."

The collaboration expects the telescope to receive first light in May, and astronomers hope to be collecting useful data in about a year.

Astronomers will use the images and spectra from the telescope to learn the distances and locations of galaxies in order to build the most complete three-dimensional map of the cosmos ever attempted. The map will provide clues to the overall structure and evolution of the universe.

The Sky Survey will systematically map one quarter of the sky, providing a three-dimensional picture of the universe through a volume 100 times larger than that explored to date.

The galaxies that the team will map are arranged in patterns of groups, clusters, filaments, walls and voids of various sizes. The volume of the universe that will be surveyed is so big that the largest structures scientists might see are in a state little changed from the primordial pattern laid down in the earliest phases of the Big Bang.

Apache Point Observatory, site of the Sloan Digital Sky Survey telescope, is owned by the Astrophysical Research Consortium (ARC), a collaboration of seven research institutions, and operated by New Mexico State University. The Sky Survey collaboration includes scientists from the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, Princeton University, the United States Naval Observatory and the University of Washington. Funding for the project is provided by the Alfred P. Sloan Foundation, the National Science Foundation, the U.S. Department of Energy, Sky Survey member institutions, and others.

"The Sloan Digital Sky Survey represents a new way of doing astronomy," said Professor of Astrophysics Tim Heckman of Johns Hopkins University, the project's CEO. "Traditionally, we have built general-purpose telescopes, to be used by small groups in separate studies. But it is a big problem to map the universe. It requires a new approach, with specialized technology and instruments, and the collaboration of large numbers of scientists." ¶