The Sloan Digital Sky Survey (SDSS), the subject of newcomer #5, is the most ambitious astronomical survey project ever undertaken. Its origin goes back to 1988 when a team of observers designed a "next-generation" redshift survey, targeting galaxies and quasars. They planned a 100-fold increase in the number of extragalactic objects with good spectroscopy, a survey of the distances of the nearest million galaxies, plus 100,000 quasars. Adding in the stars of the Milky Way, the sky survey called for the positions and brightnesses of 108 celestial objects. Sunspot, New Mexico is the home base of SDSS: the Apache Point Observatory provides a good site at 2,800 meters, where the atmosphere contains little water vapor. APO is one of darkest sites in the United States, thanks to clean air and distance from large cities. The 2.5-m telescope has the most complex camera ever built, with 30 CCD chips, each of 4 megapixels, cooled by liquid N; the field of view is 3º. A good night of observing produces 200 Gb of data on magnetic tape. Fermilab is the lead center for the data pipelines, automated computer programs that turn the digitized data into useful information on real stars, galaxies, and quasars. The turnaround time needs to be not much more than a week, so that follow-up spectroscopic observations can be planned for the next lunar dark phase. On a good night of spectroscopy anything from 3,000 – 5,500 redshifts is secured, in batches of 640 at a pass using fiber optic feeds. SDSS will survey 104 square degrees, about 1/4 of the whole sky, and yield a 3D picture of the universe through a volume 100 times that explored to date. Hot Paper #5 is termed "Early data release." The spirit of SDSS is that data are made freely available as quickly as possible. The June 2001 release is for 462 deg2, with data on 1.4 x 107 objects, and 54,000 follow-up spectra. The high citation rate is entirely down to the thrilling science in this first release. SDSS is great at finding very high redshift quasars, with record breakers at z=6.0 and 6.2 in this release, and further objects at 6.1, 6.28, and 6.4 announced since publication of #5. These quasars are less than 109 years old, and they host the first generation of supermassive black holes. Their distances are so great that their light has passed through dozens of intervening galaxies, whose properties are imprinted on the quasar absorption-line spectrum. SDSS can probe the distribution of both hot and dark matter. These early results reinforce the current model of a low-density universe with accelerating expansion. SDSS researchers have also looked at the dynamical behavior of thousands of satellite galaxies orbiting around giant parents. They’ve concluded that the gravitational pull of abundant dark matter is speeding up the velocities of the satellites. Although the survey does not tell us what dark matter actually is, it confirms its presence inside galaxies. In a neat twist, SDSS has also unmasked a secret of the solar system. Asteroids show up as tiny streaks in the field of view. There are fewer asteroids with diameters below 4 km than researchers were expecting, which is good news because it suggests future collisions of an asteroid with Earth may be less likely. The colors of asteroids, revealed by SDSS, show that the population between Mars and Jupiter is spatially separated into two types: an inner belt of rocky asteroids, and an outer zone of carbonaceous bodies. 
Dr. Simon Mitton is
Senior Fellow of
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apmaking
and surveying have a long tradition in astronomy, going back to at least
the second century BCE, when Hipparchus compiled a catalog (long since
lost) of 850 stars. Three centuries later, Claudius Ptolemy listed 1,028
star positions, published in his Almagest, the basic text from
antiquity on astronomical theory. Copernicus used Ptolemy’s catalog,
which was superseded by the last of the great pre-telescopic astronomers,
Tycho Brahe. A further 200 years elapsed until William Herschel produced
the first real sky survey: a compilation not just of individual stars, but
also binaries, clusters, gas and dust nebulae, and galaxies. In the course
of this immense undertaking he discovered the planet Uranus, for which
King George III gave him a generous pension for life.
.
