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The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P. Sloan Foundation.
Data collection began in 2000, and has mapped over 35% of the sky, with photometric observations of around 500 million objects and spectra for more than 1 million objects. The main galaxy sample has a median redshift of z = 0.1; there are redshifts for luminous red galaxies as far as z = 0.6, for quasars as far as z = 5; and the imaging survey has been involved in the detection of quasars beyond a redshift z = 6.
The most recent data release, DR8[1] includes all photometric observations that will be taken with the SDSS imaging camera, covering 14,555 square degrees on the sky (just over 35% of the full sky).
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SDSS uses a dedicated 2.5-m wide-angle optical telescope, and takes images using photometric system of five filters (named u, g, r, i and z). These images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended (as a galaxy might) and how the brightness on the CCDs relates to various kinds of astronomical magnitude.
The SDSS telescope uses the drift scanning technique,[2] which keeps the telescope fixed and makes use of the Earth's rotation to record small strips of the sky. The image of the stars in the focal plane drifts along the CCD chip, instead of staying fixed as in tracked telescopes. This method allows consistent astrometry over the widest possible field and precision remains unaffected by telescope tracking errors. The disadvantages are minor distortion effects and the CCD has to be written and read in the same time.
The telescope's camera is made up of thirty CCD chips each with a resolution of 2048×2048 pixels, totaling approximately 120 Megapixels.[3] The chips are arranged in five rows of six chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1 and 893.1 nm, with 95% completeness in typical seeing to magnitudes of 22.0, 22.2, 22.2, 21.3, and 20.5, for u, g, r, i, z, respectively.[4] The filters are placed on the camera in the order r,i,u,z,g. To reduce noise the camera is cooled to 190 kelvin (about -80 degrees Celsius) by liquid nitrogen.
Using this photometric data, stars, galaxies, and quasars are also selected for spectroscopy. The spectrograph operates by feeding an individual optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned specifically for a selected target, so every field in which spectra are to be acquired requires a unique plate. The original spectrograph attached to the telescope was capable of recording 640 spectra simultaneously, while the updated spectrograph for SDSS III can record 1000 spectra at once. Over the course of each night, between six and nine plates are typically used for recording spectra.
Every night the telescope produces about 200 GB of data.
During its first phase of operations, 2000–2005, the SDSS imaged more than 8,000 square degrees of the sky in five optical bandpasses, and it obtained spectra of galaxies and quasars selected from 5,700 square degrees of that imaging. It also obtained repeated imaging (roughly 30 scans) of a 300 square degree stripe in the southern Galactic cap.
In 2005 the survey entered a new phase, the SDSS-II, by extending the observations to explore the structure and stellar makeup of the Milky Way, the SEGUE and the Sloan Supernova Survey, which watches after supernova Ia events to measure the distances to far objects.
The survey covers over 7,500 square degrees of the Northern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars. The information on the position and distance of the objects has allowed the large-scale structure of the Universe, with its voids and filaments, to be investigated for the first time.
Almost all of these data were obtained in SDSS-I, but a small part of the footprint was finished in SDSS-II.[5]
The Sloan Extension for Galactic Understanding and Exploration obtained spectra of 240,000 stars (with typical radial velocity of 10 km/s) in order to create a detailed three-dimensional map of the Milky Way.[6] SEGUE data provide evidence for the age, composition and phase space distribution of stars within the various Galactic components, providing crucial clues for understanding the structure, formation and evolution of our Galaxy.
The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of SDSS Data Release 7 (DR7).[7]
Running until the end of the year 2007, the Supernova Survey searched for Type Ia supernovae. The survey rapidly scans a 300 square degree area to detect variable objects and supernovae. It detected 130 confirmed supernovae Ia events in 2005 and a further 197 in 2006.[8]
In mid-2008, SDSS-III was started. It comprises four separate surveys, each conducted on the same 2.5m telescope:[9][10]
The SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS) will map the spatial distribution of luminous red galaxies (LRGs) and quasars to detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Sound waves that propagate in the early universe, like spreading ripples in a pond, imprint a characteristic scale on the positions of galaxies relative to each other.[12]
The original Sloan Extension for Galactic Understanding and Exploration (SEGUE-1) obtained spectra of nearly 240,000 stars of a range of spectral types. Building on this success, SEGUE-2 spectroscopically observed around 120,000 stars, focusing on the in situ stellar halo of the Galaxy, from distances of 10 to 60 kpc.
Combining SEGUE-1 and 2 reveals the complex kinematic and chemical substructure of the Galactic halo and disks, providing essential clues to the assembly and enrichment history of the Galaxy. In particular, the outer halo is expected to be dominated by late-time accretion events. SEGUE can help constrain existing models for the formation of the stellar halo and inform the next generation of high resolution simulations of Galaxy formation. In addition, SEGUE-1 and SEGUE-2 help uncover rare, chemically primitive stars that are fossils of the earliest generations of cosmic star formation.
The APO Galactic Evolution Experiment (APOGEE) will use high-resolution, high signal-to-noise infrared spectroscopy to penetrate the dust that obscures the inner Galaxy. APOGEE will survey 100,000 red giant stars across the full range of the Galactic bulge, bar, disk, and halo.
The Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) will monitor the radial velocities of 11,000 bright stars, with the precision and cadence needed to detect gas giant planets that have orbital periods ranging from several hours to two years.
The survey makes the data releases available over the Internet. The SkyServer provides a range of interfaces to an underlying Microsoft SQL Server. Both spectra and images are available in this way, and interfaces are made very easy to use so that, for example, a full color image of any region of the sky covered by an SDSS data release can be obtained just by providing the coordinates. The data is available for non-commercial use only, without written permission. The SkyServer also provides a range of tutorials aimed at everyone from schoolchildren up to professional astronomers. The DR8, released January 2011,[1] is the eighth major data release and provides images, imaging catalogs, spectra, and redshifts via a variety of search interfaces.
The raw data (from before it was processed into databases of objects) is also available through another Internet server, and through the NASA World Wind program.
Sky in Google Earth includes data from the SDSS, for those regions where such data is available. There are also KML plugins for SDSS photometry and spectroscopy layers,[13] allowing direct access to SkyServer data from within Google Sky.
Following from Technical Fellow Jim Gray's significant contribution on behalf of Microsoft Research with the SkyServer project, Microsoft's WorldWide Telescope makes use of SDSS and other data sources.[14]
Along with publications describing the survey itself, SDSS data has been used in publications over a huge range of astronomical topics. The SDSS website has a full list of these publications covering distant quasars at the limits of the observable universe,[15] the distribution of galaxies, the properties of stars in our own galaxy and also subjects such as dark matter and dark energy in the universe.
|title= specified when using {{Cite web}}". About the SDSS Legacy Survey. http://www.sdss.org/legacy/index.html.
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