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. The project was named after the Alfred P. Sloan Foundation.
The survey was begun in 2000, and 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 as 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 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 SDSS telescope uses the drift scanning technique, 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 is made up of thirty CCD chips each with a resolution of 2048x2048 pixels, totaling 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 aluminum 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 GB of data.
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, 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.
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.