Sloan Digital Sky Survey: Mapping the Universe
Sloan Sweeps the Sky
The astronomers at Apache Point dislike clouds. On their 2,788-meter high, ponderosa pine-studded perch in the southernmost part of the Rockies, New Mexico’s Sacramento Mountains, a perfect night must be perfectly clear. Humidity must be low, with light winds, and neither lightning nor a full moon. John Barentine, one of the eight telescope observers at Apache Point, considers himself an amateur meteorologist as much as an astronomer.
The Sloan telescope points skyward at the Apache Point Observatory in the Sacramento Mountains, New Mexico.
Such ideal conditions happen only one out of every three nights. That’s when the 2.5-meter telescope unfurls from its aluminum-slatted wind baffle to press on with the Sloan Digital Sky Survey. The project is a systematic scan of every object visible—that is, visible to the telescope—in one-half of the northern celestial hemisphere. This has never been done before.
Seven years of observing at Apache Point has tallied over 100 million stars, galaxies, quasars, and other luminous space objects—many unseen until now. “Huge amounts of data have come from this telescope,” says Bruce Gillespie, the observatory’s operations manager. “And when I say huge, I’m talking about more data than that contained by the digitized Library of Congress.”
Other sky surveys have been done—most notably by Caltech’s Palomar Observatory between 1950 and 1957, of about 50,000 space objects—but never with the scope, technology, and usefulness of Sloan’s. The ultimate goal of the project, which is mostly philanthropically supported by the Alfred P. Sloan Foundation, is to create the first-ever map of one-quarter of the heavens in three dimensions. The task is now about 80 percent complete. When finished, the map will be consulted to help answer some of our thorniest questions about the structure and origins of the Universe. “It’s an issue of statistics and volume,” says Barentine. “A lot of the questions we have depend on that statistical information—building up a picture of the Universe as a whole rather than just our particular little corner of it. The Sloan survey gives us access, for the first time, to the information that allows us to address the big-picture questions, which so far have only been the province of theorists.”
Spotlight vs. Floodlight
A portion of the Hubble Deep Field's renowned 1996 image of 1,500 galaxies in a narrow "keyhole" view of the Universe. The image covers an area of sky as wide as a dime viewed from 75 feet away and as deep as the visible horizon of the Universe.
No conventional telescope, or even the Hubble Space Telescope, could do Sloan’s job. “The Hubble Deep Field has produced one of the most famous pictures in astronomy,” says Michael Turner, a theoretician at the University of Chicago and the Fermi National Accelerator Laboratory. “It is a deep and narrow image of the Universe containing about 1,500 galaxies. But the Hubble Deep Field is only one forty-millionth of the total sky. With Sloan, you're seeing one-quarter of the sky.”
With its perpetually cloud-free vantage point in space, Hubble spotlights details about select space objects. Sloan’s much wider field of view casts a floodlight on the celestial landscape. As Earth’s rotation causes the night sky to roll by, Sloan stands at a fixed position on Apache Point, shooting a continuous strip of the heavens with its specially built 142-million-pixel camera. The strips are composited side by side like cosmological wallpaper, creating a seamless map of all the sky available to the telescope.
