Mapping the Night Sky The Sloan Digital Sky Survey, with SITe CCDs at its heart, will supply a quantity and quality of data about the universe never before available. A major five-year astronomical undertaking called the Sloan Digital Sky Survey (SDSS) will Figure 1. Apache Point Observatory in the Sacramento Mountains of New Mexico. The SDSS 2.5-m telescope is at left. The small dome right of center houses the monitor telescope, used for calibrations. Optical fibers for spectroscopy are prepositioned each day in the building at right behind the trees. seek to unlock the secrets of the night sky. The SDSS will result in the first five-color charge-coupled-device (CCD) photometric "map" of the North Galactic hemisphere, totaling 10,000 square degrees of coverage. Scientists expect the SDSS to locate some 500 million galaxies and an even greater number of stars, while gathering the spectra of 1 million galaxies and 100,000 quasars. In total, the survey will collect an estimated 10 terabytes of data. Researchers will gain an unprecedented look into the three-dimensional large-scale structure of galaxies (to a median red shift of z=0.1); the evolution, surface density, and morphology of galaxies; and the evolution and large-scale distribution of quasars. Various institutions are jointly sponsoring the SDSS, among them Princeton University, the University of Chicago, Johns Hopkins University, the U.S. Naval Observatory, Fermi National Accelerator Laboratory, the University of Washington, the Institute for Advanced Study, the National Astronomical Observatory of Japan, and the University of Tokyo. Based at New Mexico's Apache Point Observatory (Figure 1),the SDSS employs a dedicated 2.5-meter f/5 telescope built to a distortion-free Ritchey-Chretien optical design. This telescope, with a 3-degree field of view (FOV), achieved first light on May 9 of this year. At the instrument's heart is a large-format CCD camera mosaicked on the telescope's focal plane. Operating in the time-delay-and-integrate (TDI) scanning mode and 10,000 times more   Figure 2. SDSS telescope photometric CCD from SITe. sensitive to light than a consumer digital camera, the SDSS camera is made up of two arrays. A photometric array, consisting of 30 2048-x-2048-pixel CCDs produced by Scientific Imaging Technologies Inc. (SITe), offers an effective imaging area of 72 cm2. These SITe Model SI424A CCDs (Figure 2), with 24-µm pixels, are arranged in six columns of five chips each (Figure 3) Figure 3. SDSS telescope/CCD camera array: 30 photometric CCDs (11-56) and 24 astrometric CCDs (60-76, 81-95). so that two scans cover a filled stripe 2.54 degrees wide. Twenty-four of these CCDs are back-illuminated for improved UV and blue spectral-band sensitivity, and the remaining six are front-illuminated. The second array, an astrometric array, consists of 24 front-illuminated 2048-x-400-pixel custom-built SITe CCDs with 24-µm pixels, allowing SDSS researchers to couple bright astrometric standard stars to celestial objects imaged via the photometric array. An Ideal Match The sheer size of the SI424A CCDs, approximately 4 square inches each, makes them an ideal match for a telescope that captures a relatively large 3-degree FOV. For enhanced sensitivity, each column of five CCDs is encased in a liquid nitrogen-cooled, vacuum-sealed chamber. The six CCDs in each row are fitted with one of five color filters centered at the following wavelengths: 350 nm (blue), 477 nm (green), 623 nm (red), 762 nm (near-IR), and 913 nm (IR). Center-to-center spacing between photometric array columns is 91 mm, slightly less than twice the active width of the CCDs, which allows two scans to cover the 2.54-degree-wide filled stripe (with an 8-percent overlap between scans along each edge). Scanning is at sidereal rate. A star or another celestial body's image crosses the entire photometric array in 5 minutes and 42 seconds, traversing it in the column direction. Effective exposure time is 54.1 seconds for each of the five colors along a column, and spacing in time from one color to the next is 72 seconds. The astrometric array's CCDs are situated in the focal-plane space above and below the photometric array. Twelve CCDs make up the leading astrometric array, above the photometric, and the remaining 12 make up the trailing astrometric array below it. Each astrometric array is arranged in two rows, with one row of six CCDs aligned with the photometric columns, a second row of five CCDs straddling the columns, and the final CCD adjacent to the middle CCD in the row of five. The astrometric CCDs utilize passband filters centered at 623 nm plus ND 3 neutral density filters. Over the Sloan Digital Sky Survey's five-year lifespan, scientists will spend some 200 Figure 4. An aluminum plug plate, showing positions and identification of holes for fiber optic lines that will collect information about galaxies, stars, and quasars. nights observing the heavens through the telescope. Spectral analysis of celestial bodies first involves selecting a group of 640 objects for study. Then the same number of holes are drilled in an aluminum plug plate (Figure 4), with each hole corresponding to the location of a selected star, quasar, or galaxy. These holes are fitted with fiber-optic cables that connect to a high-multiplex-gain, multiobject spectrograph, enabling simultaneous capture of the 640 objects' spectra. The plug plates, interchangeable with the SDSS camera, are placed at the Cassegrain focus position. On a good night for observation, six to nine plug plates are used. New Levels of Data Upon completion, the SDSS will provide a depth of data not available in the sky survey tool now considered the standard: the National Geographic Society-Palomar Observatory Sky Survey. Undertaken between the late 1940s and early 1950s, the Palomar survey utilized more than 900 pairs of 14-inch-square photographic glass plates to record the sky visible from the Northern Hemisphere. The two-color survey centered on the spectrum's visible portion, recording one band in blue and the other in red. While the Palomar survey has served scientists well, it does have significant limitations. Since it is photographic, visually counting and analyzing large numbers of closely clustered stars, galaxies, and other celestial objects in the silver halide emulsion is extremely difficult. In addition, as a two-dimensional study the Palomar survey reveals the positions of celestial objects but does not indicate their distances along the line of sight. As a digital study, however, the Sloan survey is able to concretely distinguish one celestial object from another, no matter how cluttered the sky. The SDSS also is able to detect objects 10 to 20 times fainter than the dimmest objects recorded via the Palomar survey. And when it is complete, the SDSS will provide three-dimensional coordinates for numerous galaxies and quasars. SDSS project scientist Jim Gunn of Princeton University says the Sloan Digital Sky Survey will pointedly demonstrate nature's extreme complexity. Though one cannot predict everything that will come out of the SDSS, the survey should provide solid information about the way matter is distributed in the universe. This data, Gunn explains, should help answer many questions concerning the universe's large-scale structure and its connection with fundamental physics. For more information on Scientific Imaging Technologies' products and applications, contact George Williams, SITe, 14320 S.W. Jenkins Road, Beaverton, OR 97007; (503) 671-0688; fax (503) 671-7110; www.site-inc.com. For more information on the Sloan Digital Sky Survey, see www.sdss.org