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.
