View source for Astronomy:Sloan Digital Sky Survey

{{short description|Multi-spectral imaging and spectroscopic redshift survey}}
{{Infobox astronomical survey}}
{{cosmology|experiments}}

The '''Sloan Digital Sky Survey''' or '''SDSS''' is a major multi-spectral imaging and spectroscopic [[Astronomy:Redshift survey|redshift survey]] using a dedicated 2.5-m wide-angle [[Astronomy:Optical telescope|optical telescope]] at Apache Point Observatory in New Mexico, United States. The project began in 2000 and was named after the [[Organization:Alfred P. Sloan Foundation|Alfred P. Sloan Foundation]], which contributed significant funding.

A consortium of the [[Organization:University of Washington|University of Washington]] and [[Organization:Princeton University|Princeton University]] was established to conduct a redshift survey. The Astrophysical Research Consortium (ARC) was established in 1984<ref>{{Cite web|last=Peterson|first=Jim|title=A Brief History of the Astrophysical Research Consortium (ARC) and the Apache Point Observatory (APO)|url=https://arc.apo.nmsu.edu/ARC-History.pdf|website=Astrophysical Research Consortium}}</ref> with the additional participation of [[Organization:New Mexico State University|New Mexico State University]] and [[Organization:Washington State University|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.<ref name="leverington1">{{cite book |last1=Leverington |first1=David |title=Encyclopedia of the History of Astronomy and Astrophysics |date=2013 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-89994-9 |pages=299–300}}</ref>

==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 [[Astronomy:Multi-Object Spectrometer|multi-object/multi-fiber spectrograph]] that could take spectra in bulk (several hundred objects at a time) 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 (billions of objects) 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 (flux versus wavelength).  

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), but also Fermilab national laboratory (which supplied compute processing and storage capabilities), as well as colleagues from the computing industry

==Operation==
[[File:SDSS2020.png|thumb|SDSS map shown as a rainbow of colors, located within the observable Universe (the outer sphere, showing fluctuations in the Cosmic Microwave Background). As we look out in distance, we look back in time. So, the location of these signals reveals the expansion rate of the Universe at different times in cosmic history. (2020)]]

Data collection began in 2000;<ref>{{cite journal |last1=Gunn |first1=James E. |last2=Siegmund |first2=Walter A. |last3=Mannery |first3=Edward J. |last4=Owen |first4=Russell E. |last5=Hull |first5=Charles L. |last6=Leger |first6=R. French |display-authors=0 |date=April 2006 |title=The 2.5 m Telescope of the Sloan Digital Sky Survey |journal=The Astronomical Journal |volume=131 |issue=4 |pages=2332–2359 |doi=10.1086/500975 |doi-access=free |arxiv=astro-ph/0602326 |bibcode=2006AJ....131.2332G }}</ref> the final imaging data release (DR9) covers over 35% of the sky, with [[Astronomy:Photometry|photometric]] observations of around nearly 1 billion objects, while the survey continues to acquire [[Physics:Spectrophotometry|spectra]], having so far taken spectra of over 4 million objects. The main galaxy sample has a median [[Astronomy:Redshift|redshift]] of ''z''&nbsp;=&nbsp;0.1; there are redshifts for luminous red galaxies as far as ''z''&nbsp;=&nbsp;0.7, and for [[Astronomy:Quasar|quasar]]s as far as ''z''&nbsp;=&nbsp;5; and the imaging survey has been involved in the detection of quasars beyond a redshift ''z''&nbsp;=&nbsp;6.

Data release 8 (DR8), released in January 2011,<ref name="DataRelease8">{{Cite web |title=SDSS Data Release 8 |url=http://www.sdss3.org/dr8/ |publisher=sdss3.org |access-date=2011-01-10}}</ref> includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky (just over 35% of the full sky). Data release 9 (DR9), released to the public on 31 July 2012,<ref name="DataRelease9">{{Cite web |title=SDSS Data Release 9 |url=http://www.sdss3.org/dr9/ |publisher=sdss3.org |access-date=2012-07-31}}</ref> includes the first results from the Baryon Oscillation Spectroscopic Survey (BOSS), including over 800,000 new spectra. Over 500,000 of the new spectra are of objects in the Universe 7 billion years ago (roughly half the age of the universe).<ref>{{ cite press release
 |title=New 3D Map of Massive Galaxies and Black Holes Offers Clues to Dark Matter, Dark Energy
 |date=8 August 2012 |publisher=New York University
 |url=http://www.nyu.edu/about/news-publications/news/2012/08/08/new-3d-map-of-massive-galaxies-and-black-holes-offers-clues-to-dark-matter-dark-energy.html
}}</ref> Data release 10 (DR10), released to the public on 31 July 2013,<ref name="DataRelease10">{{Cite web |title=SDSS Data Release 10 |url=http://www.sdss3.org/dr10/ |publisher=sdss3.org |access-date=2013-08-04}}</ref> includes all data from previous releases, plus the first results from the APO Galactic Evolution Experiment (APOGEE), 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 [[Astronomy:Chronology of the universe|its expansion history]], and provided data which supports the theory of a flat [[Astronomy:Shape of the universe|geometry of the universe]] and confirms that different regions seem to be [[Astronomy:Expansion of the universe|expanding]] at different speeds.<ref>{{cite news |title=Largest-ever 3D map of the universe released by scientists |url=https://news.sky.com/story/largest-ever-3d-map-of-the-universe-released-by-scientists-12033047 |access-date=18 August 2020 |work=Sky News |language=en}}</ref><ref>{{cite web |title=No need to Mind the Gap: Astrophysicists fill in 11 billion years of our universe's expansion history |url=https://www.sdss.org/press-releases/no-need-to-mind-the-gap/ |publisher=SDSS |access-date=18 August 2020}}</ref>

== Observations ==
SDSS uses a dedicated 2.5&nbsp;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 [[Astronomy:Photometric system|photometric system]] of five filters (named ''u'', ''g'', ''r'', ''i'' and ''z''). These images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended (as a galaxy might) and how the brightness on the [[Engineering:Charge-coupled device|CCDs]] relates to various kinds of [[Astronomy:Apparent magnitude|astronomical magnitude]].

For imaging observations, the SDSS telescope used the drift scanning technique, but with choreographed variation of [[Right ascension|right ascension]], [[Declination|declination]], tracking rate, and image rotation which allows the telescope to track along [[Great circle|great circle]]s and continuously record small strips of the sky.<ref>{{Cite journal | author = David Rabinowitz | title = Drift Scanning (Time-Delay Integration) | date = 2005 | url = http://msc.caltech.edu/workshop/2005/presentations/Rabinowitz.pdf | access-date = 2006-12-27 }}</ref> The image of the stars in the focal plane drifts along the CCD chip, and the charge is electronically shifted along the detectors at exactly the same rate, instead of staying fixed as in tracked telescopes. (Simply parking the telescope as the sky moves is only workable on the [[Celestial equator|celestial equator]], since stars at different [[Declination|declination]] move at different apparent speed). 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 {{resx|2048x2048}} [[Pixel|pixel]]s, totaling approximately 120 megapixels.<ref>{{cite web | title = Key Components of the Survey Telescope | publisher = SDSS | date = 2006-08-29 | url = http://www.sdss.org/background/telescope.html | access-date = 2006-12-27 | url-status = dead | archive-url = https://web.archive.org/web/20070107063936/http://www.sdss.org/background/telescope.html | archive-date = 2007-01-07 }}</ref> The chips are arranged in 5 rows of 6 chips. Each row has a different optical filter with average wavelengths of 355.1 (''u''), 468.6 (''g''), 616.5 (''r''), 748.1 (''i''), and 893.1 (''z''){{nbsp}}[[Nanometre|nm]], with 95% completeness in typical [[Astronomy:Astronomical seeing|seeing]] to [[Astronomy:Apparent magnitude|magnitudes]] of 22.0, 22.2, 22.2, 21.3, and 20.5, for ''u'', ''g'', ''r'', ''i'', ''z'' respectively.<ref>{{cite web | title = SDSS Data Release 7 Summary | publisher = SDSS | date = 2011-03-17 | url = http://www.sdss.org/dr7/}}</ref> The filters are placed on the camera in the order ''r'', ''i'', ''u'', ''z'', ''g''. To reduce noise, the camera is cooled to 190 [[Kelvin|kelvin]]s (about −80{{nbsp}}°C) by [[Chemistry:Liquid nitrogen|liquid nitrogen]].
{| class="wikitable"
|+'''SDSS photometric system''' filters
!
!u
!g
!r
!i
!z
|-
!Mean wavelength (nm)
| align="center"  |355.1 
| align="center" bgcolor="#00a5ff" style="color:#ffffff;" |468.6
| align="center" bgcolor="#ff8200" style="color:#ffffff;" |616.5
| align="center" bgcolor="#a50000" style="color:#ffffff;" |748.1
| align="center"  |893.1
|-
! align="center" |Magnitude limit
| align="center" |22.0
| align="center" |22.2
| align="center" |22.2
| align="center" |21.3
| align="center" |20.5
|}
<small>Note: colors are only approximate and based on wavelength to sRGB representation.</small><ref>{{Cite web |title=Light wavelength to RGB Converter |url=https://www.johndcook.com/wavelength_to_RGB.html |access-date=2023-07-28 |website=www.johndcook.com}}</ref>

Using these photometric data, stars, galaxies, and quasars are also selected for [[Physics:Spectroscopy|spectroscopy]]. The [[Astronomy:Spectrograph|spectrograph]] operates by feeding an individual optical fibre for each target through a hole drilled in an aluminum plate.<ref>{{cite journal |last=Newman |first=Peter R. |display-authors=0 |title=Mass-producing spectra: the SDSS spectrographic system |date=2004 |journal=Proc. SPIE |volume=5492 |pages=533 |doi=10.1117/12.541394 |url=http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1280512 |access-date=3 December 2012 |arxiv = astro-ph/0408167 |series=Ground-based Instrumentation for Astronomy |bibcode=2004SPIE.5492..533N |s2cid=119434705 }}</ref> 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 SDSS{{nbsp}}III can record 1000 spectra at once.  Over the course of 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 fibre tips.

Every night the telescope produces about 200{{nbsp}}[[Gigabyte|GB]] of data.

{{multiple image
 |direction = horizontal
 |align= center
 |width1= 225
 |width2= 225 
 |width3= 225
 |image1=SDSS_Camera.jpg
 |image2=SDSS spectrograph cartridge.JPG
 |image3=SDSS spectrograph plate.JPG
 |caption1=SDSS imaging camera
 |caption2=SDSS spectroscope cartridge
 |caption3=Aluminum plate close up showing optical fibers
}}

== Phases ==
[[File:Quasars Acting as Gravitational Lenses.jpg|thumb|Quasars acting as gravitational lenses. To find these cases of galaxy–quasar combinations acting as lenses, astronomers selected 23,000 quasar spectra from the SDSS.<ref>{{cite news|title=Quasars Acting as Gravitational Lenses|url=http://www.spacetelescope.org/images/potw1212a/|access-date=19 March 2012|newspaper=ESA/Hubble Picture of the Week}}</ref>]]

=== 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 (roughly 30 scans) 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 [[Astronomy:Milky Way|Milky Way]], the SEGUE and the Sloan Supernova Survey, which watches after [[Astronomy:Type Ia supernova|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.<ref>{{cite web|url=http://classic.sdss.org/legacy/index.html|title=About the SDSS Legacy Survey}}</ref>

==== Sloan Extension for Galactic Understanding and Exploration (SEGUE) ====
The Sloan Extension for Galactic Understanding and Exploration obtained spectra of 240,000 stars (with typical radial velocity of 10&nbsp;km/s) in order to create a detailed three-dimensional map of the Milky Way.<ref>{{Cite web|title=Sloan Extension for Galactic Understanding and Exploration|url=http://segue.uchicago.edu|publisher=segue.uchicago.edu|access-date=2008-02-27|url-status=dead|archive-url=https://web.archive.org/web/20080219075929/http://segue.uchicago.edu/|archive-date=2008-02-19}}</ref> 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 [[Astronomy:Galaxy|galaxy]].

The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of SDSS Data Release 7 (DR7).<ref>{{cite journal|last1=Yanny|first1=Brian|last2=Rockosi|first2=Constance|last3=Newberg|first3=Heidi Jo|last4=Knapp|first4=Gillian R.|display-authors=0|title=SEGUE: A Spectroscopic Survey of 240,000 Stars with g = 14-20|journal=The Astronomical Journal|date=1 May 2009|volume=137|issue=5|pages=4377–4399|doi=10.1088/0004-6256/137/5/4377|bibcode = 2009AJ....137.4377Y |arxiv = 0902.1781 |s2cid=39279981}}</ref>

==== Sloan Supernova Survey ====
The SDSS Supernova Survey, which ran from 2005 to 2008, performed repeat imaging of one stripe of sky 2.5° wide centred on the celestial equator, going from 20 hours [[Right ascension|right ascension]] to 4 hours RA so that it was in the southern galactic cap (see Draft:Galactic cap) and did not suffer from galactic extinction.<ref>{{cite journal |journal=The Astronomical Journal |doi=10.1088/0004-6256/135/1/338|display-authors=0 |last1=Joshua Friemann |title=The Sloan Digital Sky Survey-II Supernova Survey: Technical Summary |year=2008 |volume=135|issue=1|pages=338–347|arxiv=0708.2749|bibcode=2008AJ....135..338F|hdl=2152/34451|s2cid=53135988}}</ref> 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.<ref>{{cite journal|journal=Astronomical Journal|title=The Sloan Digital Sky Survey-II Supernova Survey: search algorithm and follow-up observations|first1=Masao|last1=Sako|display-authors=0 |date=2008|volume=135|issue=1|pages=348–373|doi=10.1088/0004-6256/135/1/348|bibcode=2008AJ....135..348S|arxiv = 0708.2750 |s2cid=10089918}}</ref> 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.<ref>{{cite journal |title=The Data Release of the Sloan Digital Sky Survey-II Supernova Survey|first1=Masao|last1=Sako|journal=Publications of the Astronomical Society of the Pacific|year=2018|volume=130|issue=988|page=064002|display-authors=0|doi=10.1088/1538-3873/aab4e0|arxiv=1401.3317|bibcode=2018PASP..130f4002S|s2cid=118342974}}</ref>

=== SDSS III: 2008–2014 ===
In mid-2008, SDSS-III was started. It comprised four separate surveys:<ref>{{Cite web | url=http://www.sdss3.org/surveys/ |title = SDSS-III: Four Surveys Executed Simultaneously - SDSS-III}}</ref>

==== APO Galactic Evolution Experiment (APOGEE) ====
The APO Galactic Evolution Experiment (APOGEE) used high-resolution, high signal-to-noise [[Physics:Infrared spectroscopy|infrared spectroscopy]] to penetrate the [[Astronomy:Cosmic dust|dust]] that obscures the inner Galaxy.<ref>{{cite web|url=http://www.sdss3.org/surveys/apogee.php |title=Sdss-III |publisher=Sdss3.org |access-date=2011-08-14}}</ref> APOGEE surveyed 100,000 red giant stars across the full range of the [[Astronomy:Galactic bulge|galactic bulge]], bar, disk, and [[Astronomy:Galactic halo|halo]]. It increased the number of stars observed at high spectroscopic resolution (R ≈ 20,000 at λ ≈ 1.6{{nbsp}}μm) and high signal-to-noise ratio ({{ratio|100|1}}) by more than a factor of 100.<ref name=sdss3>{{cite web |url=http://www.sdss3.org/collaboration/description.pdf |title=SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems |date=Jan 2008 |pages=29–40}}</ref> The high resolution spectra revealed the abundances of about 15 elements, giving information on the composition of the gas clouds the red giants formed from. APOGEE planned to collect data from 2011 to 2014, with the first data released as part of SDSS DR10 in late 2013.<ref name=DR10Paper>{{cite journal | last=Ahn | first=Christopher P. | title=The Tenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment | display-authors=0 | journal=The Astrophysical Journal Supplement Series | publisher=American Astronomical Society | volume=211 | issue=2 | date=2014-03-18 | issn=0067-0049 | doi=10.1088/0067-0049/211/2/17 | page=17| arxiv=1307.7735 | bibcode=2014ApJS..211...17A | s2cid=7356513 }}</ref>

==== Baryon Oscillation Spectroscopic Survey (BOSS) ====
The SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS) was designed to measure the expansion rate of the [[Astronomy:Universe|Universe]].<ref>{{cite web|title=BOSS: Dark Energy and the Geometry of Space|url=http://www.sdss3.org/surveys/boss.php|work=SDSS III|access-date=26 September 2011}}</ref> It mapped the spatial distribution of luminous red galaxies (LRGs) and quasars to determine their spatial distribution and detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Sound waves that propagate in the early universe, like spreading ripples in a pond, imprint a characteristic scale on the positions of galaxies relative to each other. It was announced that BOSS had measured the scale of the universe to an accuracy of one percent, and was completed in Spring 2014.<ref>{{Cite web|url=http://www.sdss3.org/surveys/boss.php|archive-url = https://web.archive.org/web/20110114204801/http://www.sdss3.org/surveys/boss.php|archive-date = 2011-01-14|title = BOSS: Dark Energy and the Geometry of Space - SDSS-III}}</ref>

==== Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) ====
The Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) monitored the radial velocities of 11,000 bright stars, with the precision and cadence needed to detect gas giant planets that have orbital periods ranging from several hours to two years. This ground-based [[Astronomy:Doppler spectroscopy|Doppler]] survey <ref name="sdss1">{{cite web|url=http://www.sdss3.org/surveys/marvels.php |title=Sdss-III |publisher=Sdss3.org |access-date=2011-08-14}}</ref> used the SDSS telescope and new multi-object Doppler instruments to monitor radial velocities.<ref name="sdss1"/>

The main goal of the project was to generate a large-scale, statistically well-defined sample of [[Astronomy:Giant planet|giant planet]]s. It searched for gaseous planets having orbital periods ranging from hours to 2 years and masses between 0.5 and 10 times that of [[Astronomy:Jupiter|Jupiter]]. A total of 11,000 stars were analyzed with 25–35 observations per star over an 18-month period. It was expected to detect between 150 and 200 new exoplanets,<!-- and what was found? --> and was able to study rare systems, such as planets with extreme eccentricity, and objects in the "brown dwarf desert".<ref name="sdss1"/><ref>{{cite web |author=Publicado por Fran Sevilla |url=http://www.vega00.com/2011/04/carnival-of-space-192-exoplanet.html |title=Carnival of Space #192: Exoplanet discovery and characterization |publisher=Vega 0.0 |access-date=2011-08-14 |url-status=dead |archive-url=https://web.archive.org/web/20110423202723/http://www.vega00.com/2011/04/carnival-of-space-192-exoplanet.html |archive-date=2011-04-23 }}</ref>

The collected data was used as a statistical sample for the theoretical comparison and discovery of rare systems.<ref>{{cite web|url=http://www.aspbooks.org/a/volumes/article_details/?paper_id=29259 |title=The Multi-Object APO Radial-Velocity Exoplanet Large-area Survey (MARVELS) |publisher=aspbooks.org |access-date=2011-08-14}}</ref> The project started in the fall of 2008, and continued until spring 2014.<ref name="sdss1"/><ref>{{cite web|author=Matt Rings |url=http://www.gizmag.com/terapixel-color-digital-image-of-sky-released-by-sdss/17647/ |title=Collaboration results in largest-ever image of the night-time sky |work=Gizmag.com |date=2011-01-23 |access-date=2011-08-14}}</ref>

==== SEGUE-2 ====
The original Sloan Extension for Galactic Understanding and Exploration (SEGUE-1) obtained spectra of nearly 240,000 stars of a range of spectral types. Building on this success, SEGUE-2 spectroscopically observed around 120,000 stars, focusing on the in situ stellar halo of the Milky Way, from distances of 10 to 60{{nbsp}}kpc. SEGUE-2 doubled the sample size of SEGUE-1.<ref>{{cite web|url=http://www.sdss3.org/surveys/segue2.php |title=Sdss-III |publisher=Sdss3.org |access-date=2011-08-14}}</ref>

Combining SEGUE-1 and 2 revealed the complex kinematic and chemical substructure of the galactic halo and disks, providing essential clues to the assembly and enrichment history of the galaxy. In particular, the outer halo was expected to be dominated by late-time accretion events.<!-- and what was found? --> SEGUE data can help constrain existing models for the formation of the stellar halo and inform the next generation of high resolution simulations of galaxy formation. In addition, SEGUE-1 and SEGUE-2 may help uncover rare, chemically primitive stars that are fossils of the earliest generations of cosmic star formation.<!-- so did it? -->

=== SDSS IV: 2014–2020 ===
[[File:Monster in the deep.jpg|thumb|Light from distant galaxies has been smeared and twisted into odd shapes, arcs, and streaks.<ref>{{cite web|title=Monster in the deep|url=https://www.spacetelescope.org/images/potw1818a/|website=www.spacetelescope.org|access-date=30 April 2018}}</ref>]]
The latest generation of the SDSS (SDSS-IV, 2014–2020) is extending precision [[Astronomy:Physical cosmology|cosmological]] measurements to a critical early phase of cosmic history (eBOSS), expanding its infrared spectroscopic survey of the Galaxy in the northern and southern hemispheres (APOGEE-2), and for the first time using the Sloan spectrographs to make spatially resolved maps of individual galaxies (MaNGA).<ref>{{Cite web | url=http://www.sdss.org/surveys/ |title = Sloan Digital Sky Surveys &#124; SDSS}}</ref>

==== APO Galactic Evolution Experiment (APOGEE-2) ====
A stellar survey of the Milky Way, with two major components: a northern survey using the bright time at APO, and a southern survey using the 2.5{{nbsp}}m Du Pont Telescope at Las Campanas.

==== extended Baryon Oscillation Spectroscopic Survey (eBOSS) ====
A cosmological survey of quasars and galaxies, also encompassing subprograms to survey variable objects (TDSS) and X-Ray sources (SPIDERS).

==== Mapping Nearby Galaxies at APO (MaNGA) ====
[[File:Hdp middenvlak.svg|thumb|A simplified graphical representation of a 7-fibre bundle. MaNGA measures 17 galaxies at a time, using bundles of 19, 37, 61, 91, and 127 fibres.<ref name=sdssmanga/>]]
MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), explored the detailed internal structure of nearly 10,000 nearby galaxies 
from 2014 to spring of 2020. Earlier SDSS surveys only allowed spectra to be observed from the center of galaxies. By using two-dimensional arrays of [[Engineering:Optical fiber|optical fiber]]s bundled together into a hexagonal shape, MaNGA was able to use spatially resolved [[Physics:Spectroscopy|spectroscopy]] to construct maps of the areas within galaxies, allowing deeper analysis of their structure, such as [[Astronomy:Radial velocity|radial velocities]] and [[Astronomy:Star formation|star formation]] regions.<ref name=sdssmanga>{{Cite web|url=http://www.sdss.org/surveys/manga/|title=MaNGA {{!}} SDSS|website=www.sdss.org|language=en-US|access-date=2017-04-18}}</ref><ref>{{Cite journal|last1=Bundy|first1=Kevin|last2=Bershady|first2=Matthew A.|last3=Law|first3=David R.|last4=Yan|first4=Renbin|last5=Drory|first5=Niv|last6=MacDonald|first6=Nicholas|last7=Wake|first7=David A.|last8=Cherinka|first8=Brian|last9=Sánchez-Gallego|first9=José R.|date=2015-01-01|title=Overview of the SDSS-IV MaNGA Survey: Mapping nearby Galaxies at Apache Point Observatory|journal=The Astrophysical Journal|language=en|volume=798|issue=1|pages=7|doi=10.1088/0004-637X/798/1/7|issn=0004-637X|bibcode=2015ApJ...798....7B|arxiv = 1412.1482 |s2cid=53707289}}</ref>

=== SDSS-V: 2020–current ===
Apache Point Observatory in New Mexico began to gather data for SDSS-V in October 2020. Apache Point is scheduled to be converted by mid-2021 from plug plates (aluminum plates with manually-placed holes for starlight to shine through) to small automated robot arms, with Las Campanas Observatory in Chile following later in the year. The Milky Way Mapper survey will target the spectra of six million stars. The Black Hole Mapper survey will target galaxies to indirectly analyze their [[Astronomy:Supermassive black hole|supermassive black holes]]. The Local Volume Mapper will target nearby galaxies to analyze their clouds of [[Astronomy:Interstellar medium|interstellar gas]].<ref>{{cite news |last1=Clery |first1=Daniel |title=Astronomy surveys aim to up the pace with army of tiny robots |url=https://www.science.org/content/article/robots-are-speeding-most-boring-job-astronomy |work=Science |date=3 February 2021 |doi=10.1126/science.abg9107}}</ref><ref>{{cite book|first1=T.|last1=Herbst|first2=Pavaman|last2=Bilgi|editor1-first=Heather K|editor1-last=Marshall|editor2-first=Jason|editor2-last=Spyromilio|editor3-first=Tomonori|editor3-last=Usuda|title=Ground-based and Airborne Telescopes VIII|chapter=The SDSS-V local volume mapper telescope system|year=2020|doi=10.1117/12.2561419 |publisher=SPIE|page=114450J|volume=11445|isbn=9781510636774|s2cid=230583048}}</ref>

== Data access ==
[[File:LRG-4-606.jpg|thumb|LRG-4-606 is a Luminous Red Galaxy. LRG is the acronym given to a catalog of bright red galaxies found in the SDSS.]]

The survey makes the data releases available over the Internet. The '''SkyServer''' provides a range of interfaces to an underlying [[Software:Microsoft SQL Server|Microsoft SQL Server]]. Both spectra and images are available in this way, and interfaces are made very easy to use so that, for example, a full color image of any region of the sky covered by an SDSS data release can be obtained just by providing the coordinates. The data are available for non-commercial use only, without written permission. The SkyServer also provides a range of tutorials aimed at everyone from schoolchildren up to professional astronomers. The tenth major data release, DR10, released in July 2013,<ref name="DataRelease10" /> provides images, imaging catalogs, spectra, and redshifts via a variety of search interfaces.

The raw data (from before being processed into databases of objects) are also available through another Internet server, and first experienced as a 'fly-through' via the NASA World Wind program. 

[[Software:Google Sky|Sky in Google Earth]] includes data from the SDSS, for those regions where such data are available. There are also [[Keyhole Markup Language|KML]] plugins for SDSS photometry and spectroscopy layers,<ref>{{Cite web |title=Google Earth KML: SDSS layer |url=http://earth.google.com/gallery/kml_entry.html#tSDSS%20layer |access-date=2008-03-24 |archive-url=https://web.archive.org/web/20080317182401/http://earth.google.com/gallery/kml_entry.html#tSDSS%20layer |archive-date=2008-03-17 |url-status=dead }}</ref> allowing direct access to SkyServer data from within Google Sky.

The data is also available on Hayden Planetarium with a 3D visualizer.

There is also the ever-growing list of data for the [[Astronomy:Stripe 82|Stripe 82]] region of the SDSS.

Following from Technical Fellow [[Biography:Jim Gray (computer scientist)|Jim Gray]]'s contribution on behalf of [[Organization:Microsoft Research|Microsoft Research]] with the SkyServer project, Microsoft's [[Software:WorldWide Telescope|WorldWide Telescope]] makes use of SDSS and other data sources.<ref>{{Cite web|title=When did Microsoft first starting looking at the sky? |url=http://www.worldwidetelescope.org/buzz/FAQ.aspx#mssky |publisher=worldwidetelescope.org |access-date=2008-03-24 |url-status=dead |archive-url=https://web.archive.org/web/20080302205212/http://www.worldwidetelescope.org/buzz/FAQ.aspx |archive-date=2008-03-02 }}</ref>

[[Astronomy:MilkyWay@home|MilkyWay@home]] also used SDSS's data for creating a highly accurate three dimensional model of the Milky Way galaxy.

== Results ==

Along with publications describing the survey itself, SDSS data have 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,<ref>{{Cite web|title=SDSS Scientific and Technical Publications|url=http://www.sdss.org/publications/index.html|publisher=sdss.org|access-date=2008-02-27|url-status=dead|archive-url=https://web.archive.org/web/20080217101608/http://www.sdss.org/publications/index.html|archive-date=2008-02-17}}</ref> the distribution of galaxies, the properties of stars in our own galaxy and also subjects such as [[Astronomy:Dark matter|dark matter]] and [[Astronomy:Dark energy|dark energy]] in the universe.

=== Maps ===
Based on the release of Data Release 9 a new 3D map of massive galaxies and distant black holes was published on August 8, 2012.<ref>{{Cite press release|title=SDSS Science Results|url=http://www.sdss3.org/press/dr9.php|publisher=sdss3.org|access-date=2012-08-08}}</ref>

{{Clear}}

== See also ==
{{colbegin|colwidth=30em}}
* Alex Szalay
* [[Organization:Alfred P. Sloan Foundation|Alfred P. Sloan Foundation]]
* Apache Point Observatory
* [[Astronomy:Dark Energy Spectroscopic Instrument|Dark Energy Spectroscopic Instrument]]
* [[Astronomy:Galaxy color–magnitude diagram|Galaxy color–magnitude diagram]]
* [[Astronomy:Galaxy Zoo|Galaxy Zoo]]
* James E. Gunn
* Sloan Great Wall
{{colend}}

== References ==
{{Reflist|30em}}

== Further reading==
* Ann K. Finkbeiner. ''A Grand and Bold Thing: An Extraordinary New Map of the Universe Ushering In A New Era of Discovery'' (2010), a journalistic history of the project

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* {{APOD |date=7 November 2007 |title=The Sloan Great Wall: Largest Known Structure?}}
* {{APOD |date=13 August 2012 |title=A Flight Through The Universe}}


{{Exoplanet search projects}}
{{Astronomical Surveys}}

[[Category:Alfred P. Sloan Foundation]]
[[Category:Astronomical surveys]]
[[Category:Exoplanet search projects]]
[[Category:Observational astronomy]]
[[Category:Photometric systems]]
[[Category:SDSS objects|*]]

{{Sourceattribution|Sloan Digital Sky Survey}}