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Sloan Digital Sky Survey
From Wikipedia, the free encyclopedia
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 begun in 2000.
Named after the Alfred P. Sloan Foundation, it aims to map 25% of the sky and obtain observations on around 100 million objects and spectra for 1 million objects. The main galaxy sample has a median redshift of 0.1; there are redshifts for luminous red galaxies as far z=0.4, for quasars as far as z=5; and the imaging survey has been involved in the detection of quasars beyond a redshift 6.
In the year 2006 the survey entered a new phase, the SDSS-II, by extending the obervations to explore the structure and stellar makeup of the Milky Way, the SEGUE and the Sloan Supernove Survey, which watches after supernovae Ia events to measure the distances to far objects.
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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,[1] which lets the telescope fix and makes use of the earth's rotation to record small stripes 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[2]is made up of thirty CCD chips each with a resolution of 2048x2048 pixels, totalling approximately 120 Megapixels. The chips are arranged in five rows of six chips. Each row has a different optical filter with wavelengths of 354, 476, 628, 769 and 925 nm up to a magnitude of respectively 24.4, 25.3, 25.1, 24.4 and 22.9 with a signal to noise ratio of 5. The filters are placed in the order r,i,u,z,g. To enhance the sensitivity the camera is cooled to 190 Kelvin (about -80 degrees Celsius) by liquid nitrogen.
Using this data, targets are also selected for spectroscopy. The telescope is capable of recording 640 spectra at any one time by feeding an optical fibre for each through holes drilled in an aluminium plate. Each hole is individually positioned for the target in question. Per night about six to nine plates are used for recording spectra.
Every night the telescope produces about 200 GBytes of data.
The survey covers over 7,500 square degrees of the Southern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars. This information of the position and distants of the objects allowed to investigate for the first time the large scale structure of the Universe with its voids and filaments.
The Sloan Extension for Galactic Understanding and Exploration searches for the spectra of 240,000 stars with typical radial velocity of 10 km/s for obtaining the structure of the Milky Way and investigating the formation of components.
Running until the end of the year 2007, the Supernova Survey searches for Type Ia supernovae. The survey rapidly scans a 300 square degree area to detect variable objects and supernovae. It detected 129 confirmed supernovae Ia events in 2005, and over 300 total during 2005 and 2006.
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 SkyServer also provides a range of tutorials aimed at everyone from schoolchildren up to professional astronomers. The DR5[1], released June 2006, is the fifth major data release and provides images, imaging catalogs, spectra, and redshifts for download.
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.
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, 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.
- ^ David Rabinowitz (2005). "Drift Scanning (Time-Delay Integration)" (PDF). Retrieved on 2006-12-27.
- ^ Key Components of the Survey Telescope. SDSS (2006-08-29). Retrieved on 2006-12-27.
