Located 9,200 feet above sea level, atop the Apache Point Observatory in Sunspot, New Mexico, the Sloan Digital Sky Survey telescope cannot match the incredibly sharp vision of the Hubble Space Telescope, which orbits above Earth’s blurring atmosphere. And, at a modest 2.5 meters (8 feet) across, the Sloan telescope’s main mirror cannot see the incredibly dim objects that the 10-meter (33-foot) Keck telescopes in Hawaii can. What the Sloan telescope does have in spades is a voracious appetite for sky—an appetite that is producing some of the most amazing discoveries in astronomy.
With its giant set of light-sensitive imaging sensors, the Sloan telescope has a field of view so wide it can image 36 full moons’ worth of sky at once (Hubble, in contrast, is limited to a view less than one-tenth of a moon across). Night after night it scans vast swaths of the heavens and downloads its observations into a 73-terabyte (and growing) digital database that covers almost half the night sky as seen from Apache Point. Swept up in the Sloan’s relentless gaze are stars, galaxies, supernovas, nebulas, and more—over 350 million celestial objects in total—adding up to the most complete census of the universe ever conducted.
The result of all this activity is the Sloan Digital Sky Survey (SDSS), originally established “to determine the large-scale structure of the universe,” says Richard Kron, a University of Chicago astrophysicist and a Sloan survey director. “We wanted to map out the galaxies that form clusters and the clusters that form superclusters.” Achieving this goal required a huge step up from the 1950s-era Palomar Sky Survey, whose photographic plates have guided astronomers to celestial curiosities for decades. “We knew that to make real progress, we needed a hundred times more data,” Kron says. The Sloan survey captures the sky in full color rather than just through red and blue filters, produces images twice as sharp as Palomar’s, and detects objects one-tenth the brightness of those detectable by its predecessor. The Sloan also introduced two huge innovations. First, it delivers all the data in digital form, so the images are easy to categorize and study electronically, even from halfway around the world. Second, it does not just capture sky images; it also gauges the distance to many of the objects—a million galaxies and 100,000 quasars so far—that pass through its field of view, providing a unique three-dimensional perspective on deep space.
The Sloan telescope went into operation in 2000 and has since yielded two landmark surveys, known as SDSS-I and SDSS-II. Last August astronomers affiliated with the project gathered in Chicago to review results from SDSS-II and to prepare for a third survey—SDSS-III, of course—which recently began and will continue until 2014.
Taken together, the Sloan results lay out one of the most astonishing stories in science: The visible universe is merely the foam atop a much grander cosmic sea. The vast majority of what is out there is more dynamic and complicated and just plain weirder than the tiny fraction we knew. Only now are we starting to see the universe as it truly is.
