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Article: 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.

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The Sloan telescope inside its aluminum wind baffle.

Jason Lelchuk for AMNH


“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.

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The interior of the Sloan survey telescope.

Fermilab Visual Media Services


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. 

FIVE-COLOR IMAGES — Light energy travels as a wave of varying lengths. Each wavelength of light has characteristic properties, one being its color. The entire spectrum of visible light wavelengths runs from violet (short wavelengths) to red (long wavelengths). Sloan’s digital camera can detect two different wavelength ranges of visible light (red and green) and three ranges of light not visible to the eye: (ultraviolet, which has shorter wavelengths than violet; infrared, which has longer wavelengths than red; and farther infrared, even longer). Because stars, galaxies, brown dwarfs, and quasars give off light at different wavelengths, Sloan’s breadth of color detection allows it to capture a motley crew of space objects.

SUPERIOR SPECTROGRAPHS — Knowing what general color range of light a galaxy or quasar emits allows astronomers to identify what it is, but not much more. A detailed version of that spectrum can tell astronomers what elements the galaxy is made of, how far away it is, and other properties. “On the Sloan telescope, additional instruments called spectrographs break the light up into not five colors, but several thousand colors,” explains Gunn.

Pluggin_tha_Plugs

Having a human being insert 640 numbered fiber optic cables into the 640 galaxy-specific holes in a plug plate proved more efficient than automating the process. The Sloan survey has used thousands of plates to take spectroscopic measurements of galaxies.

Jason Lelchuk for AMNH


To get precise spectra for objects in an interesting area of sky, a one-meter-diameter aluminum disk called a plug plate is precision-drilled with 640 holes. Each hole corresponds to a point of light from a digital picture. A single optical fiber is inserted by hand into each hole, and then the plug plate is mounted underneath the barrel of the telescope. “The fibers must be positioned exactly because each galaxy’s light falls in a given place, and that fiber has to channel it to the spectrographs,” says Gunn. The telescope’s two spectrographs divide each galaxy’s light into several thousand precise wavelengths. Spectrographs on most other telescopes can measure one object at a time, but Sloan’s do 640 at once.

A plug plate is no longer useful after it the spectrum is taken. The Sloan Digital Sky Survey has stockpiled several thousand plug plates and is tossing around ideas for what do with them. (Coffee tables are a popular suggestion.) Scientists are still unsure of what will become of the Sloan telescope itself after its nine-year photo shoot wraps up in 2008. “The telescope and instruments, especially the spectrographs, are still very much state-of-the-art,” says Gunn. “It is almost certain that some project will go forward with some or all of the equipment.” One possibility is to outfit the telescope with a new type of spectrograph in order to survey arrestingly bright stars for the presence of planets. In the meantime, Sloan’s wide-field pictures of dazzling stars and galaxies continue to turn up other rare cosmological jewels.

Related Links

Sloan Digital Sky Survey Image Gallery
http://www.sdss.org/gallery/index.html

STScI: Telescopes from the Ground Up
http://amazing-space.stsci.edu/resources/explorations/groundup/
Explore the history of telescopes, from Galileo to NASA.

Sloan's Photometric Camera and CCDs
http://www.astro.princeton.edu/PBOOK/camera/camera.htm

The Galaxy Catalog
http://www.astro.princeton.edu/~frei/galaxy_catalog.html
A collection of digital images of 113 nearby galaxies.

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