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Mercury, March/April 2003 Table of Contents

The Sloan Digital Sky Survey uses this dedicated 2.5-meter telescope in New Mexico to survey the universe. Courtesy of Fermilab Visual Media Services.

by Joshua A. Frieman and Mark SubbaRao

The Sloan Digital Sky Survey is unlocking some of the universe's deepest secrets, from the solar system and solar neighborhood to the most distant quasars.

From ancient times, humanity has sought to chart the heavens in order to understand our place in the universe. Over the last 200 years, mapmaking has become a major astronomical pursuit. In the 1800s, the Herschel family assembled a Catalogue of Nebulae and Clusters with several thousand objects. In the early 20th century, the advent of large optical telescopes and the recognition that galaxies are external to the Milky Way led to major surveys of the extragalactic universe. The Palomar and UK Schmidt Sky Surveys together produced an all-sky map of several million galaxies. These surveys have been indispensable tools for modern astronomy.

Twentieth-century sky surveys used photographic plates to record the imaging data in at most 2 or 3 optical wavebands. The recent emergence of CCDs, which are 10 to 20 times more sensitive than photographic plates, has permitted deeper imaging in the same exposure time. CCDs also provide more reliable measurement of an object's brightness (photometry) than do plates. The development of cameras employing multiple CCDs has brought the dawn of the era of digital sky surveys.

The Sloan Digital Sky Survey (SDSS), which began taking data in 2000, is the first large-area optical survey designed to exploit this technology. The most ambitious astronomical survey ever undertaken, its primary scientific mission is to study how galaxies cluster on the largest scales. Using a dedicated 2.5-meter telescope at Apache Point Observatory in southern New Mexico, the SDSS will determine positions and brightnesses in five optical wavebands for about 100 million celestial objects (mostly galaxies), covering nearly a quarter of the sky. The Survey will also acquire spectra for over 600,000 galaxies and more than 60,000 quasars, plus tens of thousands of stars, X-ray sources, and radio sources.

We can determine a galaxy's redshift from its spectrum, which tells us how fast it is receding from us because of the expansion of the universe. Through Hubble's law, a galaxy's redshift is proportional to its distance. Thus, a spectroscopic redshift survey provides a 3-dimensional census of the universe. The SDSS redshift survey will be substantially larger than any previous spectroscopic survey.