Sloan Digital Sky Survey

The Sloan Digital Sky Survey is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 and was named after the Alfred P. Sloan Foundation, which contributed significant funding.
A consortium of the University of Washington and Princeton University was established to conduct a redshift survey. The Astrophysical Research Consortium was established in 1984 with the additional participation of New Mexico State University and Washington State University to manage activities at Apache Point. In 1991, the Sloan Foundation granted the ARC funding for survey efforts and the construction of equipment to carry out the work.

Background

At the time of its design, the SDSS was a pioneering combination of novel instrumentation as well as data reduction and storage techniques that drove major advances in astronomical observations, discoveries, and theory.
The SDSS project was centered around two instruments and data processing pipelines that were groundbreaking for the scale at which they were implemented:

A multi-filter/multi-array scanning CCD camera to take an imaging survey of the sky at high efficiency, followed by
A multi-object/multi-fiber spectrograph that could take spectra in bulk of targets identified from the survey

A major new challenge was how to deal with the exceptional data volume generated by the telescope and instruments. At the time, hundreds of gigabytes of raw data per night was unprecedented, and a collaborating team as complex as the original hardware and engineering team was needed to design a software and storage system for processing the data. From each imaging run, object catalogs, reduced images, and associated files were produced in a highly automated pipeline, yielding the largest astronomical object catalogs available in digital queryable form at the time. For each spectral run, thousands of two-dimensional spectral images had to be processed to automatically extract calibrated spectra.
In the approximate decade it took to achieve these goals, SDSS contributed to notable advances in massive database storage and accessing technology, such as SQL, and was one of the first major astronomical projects to make data available in this form. The model of giving the scientific community and public broad and internet-accessible access to the survey data products was also relatively new at the time.
The collaboration model around the project was also complex but successful, given the large numbers of institutions and individuals needed to bring expertise to the system. Universities and foundations were participants along with the managing partner ARC. Other participants included Fermi National Accelerator Laboratory, which supplied computer processing and storage capabilities, and colleagues from the computing industry.

Operation

Data collection began in 2000; the final imaging data release covers over 35% of the sky, with photometric observations of around nearly 1 billion objects, while the survey continues to acquire spectra, having so far taken spectra of over 4 million objects. The main galaxy sample has a median redshift of z = 0.1; there are redshifts for luminous red galaxies as far as z = 0.7, and for quasars as far as z = 5; and the imaging survey has been involved in the detection of quasars beyond a redshift z = 6.
Data release 8, released in January 2011, includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky. Data release 9, released to the public on 31 July 2012, includes the first results from the Baryon Oscillation Spectroscopic Survey, including over 800,000 new spectra. Over 500,000 of the new spectra are of objects in the Universe 7 billion years ago. Data release 10, released to the public on 31 July 2013, includes all data from previous releases, plus the first results from the APO Galactic Evolution Experiment, including over 57,000 high-resolution infrared spectra of stars in the Milky Way. DR10 also includes over 670,000 new BOSS spectra of galaxies and quasars in the distant universe. The publicly available images from the survey were made between 1998 and 2009.
In July 2020, after a 20-year-long survey, astrophysicists of the Sloan Digital Sky Survey published the largest, most detailed 3D map of the universe so far, filled a gap of 11 billion years in its expansion history, and provided data which supports the theory of a flat geometry of the universe and confirms that different regions seem to be expanding at different speeds.

Observations

SDSS uses a dedicated 2.5 m wide-angle optical telescope; from 1998 to 2009 it observed in both imaging and spectroscopic modes. The imaging camera was retired in late 2009, since then the telescope has observed entirely in spectroscopic mode.
Images were taken using a photometric system of five filters. These images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended and how the brightness on the CCDs relates to various kinds of astronomical magnitude.
For imaging observations, the SDSS telescope used the drift scanning technique, but with a choreographed variation of right ascension, declination, tracking rate, and image rotation which allows the telescope to track along great circles and continuously record small strips of the sky. The image of the stars in the focal plane drifts along the CCD chip, and the charge is electronically shifted along the detectors at the same rate, instead of staying fixed as in tracked telescopes.. This method allows consistent astrometry over the widest possible field and minimises overheads from reading out the detectors. The disadvantage is minor distortion effects.
The telescope's imaging camera is made up of 30 CCD chips, each with a resolution of pixels, totaling approximately 120 megapixels. The chips are arranged in 5 rows of 6 chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1, and 893.1 nm, with 95% completeness in typical seeing to magnitudes of 22.0, 22.2, 22.2, 21.3, and 20.5, for u, g, r, i, z respectively. The filters are placed on the camera in the order r, i, u, z, g. To reduce noise, the camera is cooled to 190 kelvins by liquid nitrogen.

	u	g	r	i	z
Mean wavelength	355.1	468.6	616.5	748.1	893.1
Magnitude limit	22.0	22.2	22.2	21.3	20.5

Note: colors are only approximate and based on wavelength to sRGB representation.
Using these photometric data, stars, galaxies, and quasars are also selected for spectroscopy. The spectrograph operates by feeding an individual optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned specifically for a selected target, so every field in which spectra are to be acquired requires a unique plate. The original spectrograph attached to the telescope was capable of recording 640 spectra simultaneously, while the updated spectrograph for SDSSIII can record 1000 spectra at once. Throughout each night, between six and nine plates are typically used for recording spectra. In spectroscopic mode, the telescope tracks the sky in the standard way, keeping the objects focused on their corresponding fiber tips.
Every night the telescope produces about 200GB of data.

Phases

SDSS-I: 2000–2005

During its first phase of operations, 2000–2005, the SDSS imaged more than 8,000 square degrees of the sky in five optical bandpasses, and it obtained spectra of galaxies and quasars selected from 5,700 square degrees of that imaging. It also obtained repeated imaging of a 300 square-degree stripe in the southern Galactic cap.

SDSS-II: 2005–2008

In 2005 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.

Sloan Legacy Survey

The Sloan Legacy Survey covers over 7,500 square degrees of the Northern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars. The information on the position and distance of the objects has allowed the large-scale structure of the Universe, with its voids and filaments, to be investigated for the first time. Almost all of these data were obtained in SDSS-I, but a small part of the footprint was finished in SDSS-II.

Sloan Extension for Galactic Understanding and Exploration (SEGUE)

The Sloan Extension for Galactic Understanding and Exploration obtained spectra of 240,000 stars to create a detailed three-dimensional map of the Milky Way. SEGUE data provide evidence for the age, composition and phase space distribution of stars within the various Galactic components, providing crucial clues for understanding the structure, formation and evolution of our galaxy.
The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of SDSS Data Release 7.

Sloan Supernova Survey

The SDSS Supernova Survey, which ran from 2005 to 2008, performed repeat imaging of one stripe of sky 2.5° wide centered on the celestial equator, going from 20 hours right ascension to 4 hours RA so that it was in the southern galactic cap and did not suffer from galactic extinction. The project discovered more than 500 type Ia supernovae, Running until the end of the year 2007, the Supernova Survey searched for Type Ia supernovae. The survey rapidly scanned a 300 square degree area to detect variable objects and supernovae. It detected 130 confirmed supernovae Ia events in 2005 and a further 197 in 2006. In 2014 an even larger catalogue was released containing 10,258 variable and transient sources. Of these, 4,607 sources are either confirmed or likely supernovae, which makes this the largest set of supernovae so far compiled.