Sloan Digital Sky Survey: Mapping the Universe
The 142-Megapixel Digital Camera
Imagine if your digital camera was scaled to the size of a dishwasher. And weighed about 135 kilograms. And cost about $5 million to build. At its barest bones, the Sloan telescope is an outsize digital camera, but one sophisticated enough to capture every luminous object (about 200 million) in large, contiguous swaths of the northern celestial hemisphere. Its goal is to build up an exquisitely detailed picture of the structure of the Universe.
“In 1998, when we were finishing the Sloan telescope, it was the largest digital camera in astronomy,” says Princeton University scientist Jim Gunn. “It takes pictures of objects that are fainter than those you can see with your eye by a factor of about six million.” Gunn is a rare kind of astronomer, proficient in all three of the field’s main areas—observation, theory, and instrumentation. He led the design and building of the Wide Field/Planetary camera on the Hubble Space Telescope and was the mastermind behind the Sloan telescope's complicated technology, having first drawn up “crude optical designs” for the instrument in 1985. Gunn explains why Sloan’s instruments can do what no point-and-shoot can:
WIDE FIELD OF VIEW—In astronomical terms, the Sloan camera’s field of view measures about 5 square degrees. That’s exceptionally wide. For comparison, the Hubble Space Telescope’s field of view is one-160th of Sloan’s. “If you want to map a quarter of the sky, you can't do it one postage stamp at a time,” says Gunn.
GIANT SENSORS—To make an image, a digital camera captures photons of light within its field of view using an electronic version of a photographic plate, called a CCD, or charge-coupled device. The bigger the CCD, the more photons it can capture and the dimmer the objects it can register. A typical digital camera’s single CCD is the size of your pinkie fingernail. “There are 30 of these devices in Sloan’s camera, arranged in six columns of five. Each is almost two inches square,” says Gunn. Their resolving power amounts to 142 megapixels, whereas home-use digital cameras today are 3, 4, or 5 megapixels. “CCDs this big only began to appear about 11 or 12 years ago,” Gunn continues. The CCDs are mounted at the back of the camera lens, paving nearly its entire field of view. They allow for Sloan’s sensitivity to objects many other telescopes would miss.
CONTINUOUS SHOOTING—With an ordinary digital camera, the shutter opens to allow light to hit the CCDs and then closes once its exposure is complete. Instead of peppering the sky with single shots, Sloan stays stationary and images strips of night sky as the stars advance because of Earth’s rotation. “We don’t take a picture, stop to read it, move the telescope, and take another picture,” says Gunn. “The camera is making a tapestry of the sky the whole time. We never stop to close the shutter.” This way, a contiguous area of sky can be imaged very efficiently.
