A project to investigate the large-scale structure of the Universe by mapping the three-dimensional distribution of galaxies and quasars over a quarter of the sky, mostly around the north galactic pole but with three strips near the south galactic pole. The survey started in 2000 and used a wide-field 2.5-m telescope at Apache Point Observatory, New Mexico. The first phase of observations, SDSS-I, was completed in 2005–08. It was followed by SDSS-II, in three parts: the Sloan Legacy Survey, to fill remaining gaps in the main survey; SEGUE (Sloan Extension for Galactic Understanding and Exploration), to map the structure and chemical makeup of our own Galaxy from the spectra and radial velocities of 230 000 stars; and the Sloan Supernova Survey, searching for distant Type Ia supernovae to measure changes in the rate of expansion of the Universe. Its successor, SDSS-III (2008-14) consisted of four surveys: the Baryon Oscillation Spectroscopic Survey (BOSS) which mapped the clustering of galaxies resulting from density fluctuations in the early Universe; SEGUE-2, continuing the original SEGUE study with the spectra of an additional 119 000 stars; the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a spectroscopic survey of over 100 000 red giant stars at infrared wavelengths; and the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS), which monitored the radial velocities of 11 000 bright stars for signs of orbiting planets. The latest instalment of the survey, SDSS-IV, began in 2014 and consists of three main parts: APOGEE-2, observing 300 000 stars with the Apache Point Observatory and the 2.5-meter du Pont Telescope at Las Campanas Observatory, Chile; the Extended Baryon Oscillation Spectroscopic Survey, eBOSS, to measure the expansion history of the Universe over the past 10 billion years; and Mapping Nearby Galaxies at APO (MaNGA) to explore the internal structure of some 10 000 nearby galaxies. In addition, the eBOSS survey has two subdivisions: the Time-Domain Spectroscopic Survey (TDSS) to study variable stars and quasars, and the SPectroscopic IDentification of ERosita Sources (SPIDERS) to provide optical spectroscopy of X-ray sources. SDSS-IV will continue until 2020.
http://www.sdss.org/ Official project website with many images.