Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) can be considered a continuing series of surveys that began with photometry and continued with spectrography as well. The series is named for the Alfred P. Sloan Foundation. It images all types of astronomical objects, but a major initial goal was redshifts of galaxies, i.e., to produce a vast 3D map. Surveying began in 2000 and has grown to 35% sky coverage, with photometry of over 500 million objects and spectra of over a million. The median redshift of the galaxies is redshift z = 0.1, reaching as far as z = 0.7, with quasars to redshift z = 6 and beyond. The original survey (now called SDSS-I) produced images using the ugriz photometric system of five passbands:

Spectrography was carried out on selected galaxies and quasars. The survey was a milestone, providing an improved set of freely-available all-sky research-quality astronomical data, produced using 2000s technology, carried out with a dedicated telescope (the Sloan 2.5m Telescope) at Apache Point Observatory (APO), New Mexico. Subsequent efforts use additional telescopes, including the du Pont Telescope at Las Campanas Observatory, Chile, adding coverage of more of the southern hemisphere. SDSS efforts:

• SDSS-I (2000-2005, Data Releases 1-5).
• SDSS-II (2005-2008, Data Releases 6-7).
• SDSS-III (2008-2014, BOSS, SEGUE-2, APOGEE, MARVELS, TDSS, Data Releases 8-10).
• SDSS-IV (2014-2020, aiming to finish 2020, APOGEE-2, eBOSS, MaNGA, Data Releases 11-17 as of January 2022).
• SDSS-V (2020-now: optical and infrared spectrography using robotic fiber positioners, so far: Data Release 18).

Among the sub-surveys:

• APOGEE (APO Galactic Evolution Experiment) - infrared survey of the Milky Way interior.
• BOSS (Baryon Oscillation Spectroscopic Survey) - survey to map the spatial resolution of luminous red galaxies and quasars.
• MARVELS (Multi-object APO Radial Velocity Exoplanet Large-area Survey) - survey to find extra-solar planets using radial velocity measurements.
• SEGUE and SEGUE-2 (Sloan Extension for Galactic Understanding and Exploration) - follow-up surveys to SDSS of a group of 119,000 Milky Way stars.
• TDSS (Time Domain Spectroscopic Survey) - spectroscopic investigation of variable sources.
• MaNGA (Mapping Nearby Galaxies at APO) - IFU observations of 10,000 nearby galaxies, yielding a map of information for each galaxy, e.g., showing whether the SF is in the center versus in the arms.

SDSS-V programs:

• MWM (Milky Way Mapper) 4-5 million stars.
• BHM (Black Hole Mapper) 400,000 sources.
• LVM (Local Volume Mapper) stars, star formation and the ISM.

To carry out spectrography on so many objects, for years SDSS used a partially manual process with their multi-object spectrograph: fibers were positioned by hand-plugging them into aluminum plates (plug plates) with holes that were pre-drilled at positions corresponding to locations of objects within the field of view. The holes were drilled and their positions verified using machine-controlled equipment. SDSS has now replaced this procedure with robotic fiber positioners. Spectra have been collected on millions of objects.