Sloan Digital Sky Survey

Sloan Digital Sky Survey

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The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey
Redshift survey
In astronomy, a redshift survey, or galaxy survey, is a survey of a section of the sky to measure the redshift of astronomical objects. Using Hubble's law, the redshift can be used to calculate the distance of an object from Earth. By combining redshift with angular position data, a redshift...

 using a dedicated 2.5-m wide-angle optical telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

 at Apache Point Observatory
Apache Point Observatory
The Apache Point Observatory is located in the Sacramento Mountains in Sunspot, New Mexico 18 miles south of Cloudcroft. The observatory consists of the Astrophysical Research Consortium's 3.5-meter telescope, the Sloan Digital Sky Survey 2.5-m telescope with a 20" photometric telescope,...

 in New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

, United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. The project was named after the Alfred P. Sloan Foundation
Alfred P. Sloan Foundation
The Alfred P. Sloan Foundation is a philanthropic non-profit organization in the United States. It was established in 1934 by Alfred P. Sloan, Jr., then-President and Chief Executive Officer of General Motors.-Overview:...

.

Data collection began in 2000, and has mapped over 35% of the sky, with photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

 observations of around 500 million objects and spectra
Spectrophotometry
In chemistry, spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength...

 for more than 1 million objects. The main galaxy sample has a median redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

 of z = 0.1; there are redshifts for luminous red galaxies as far as z = 0.6, for quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s as far as z = 5; and the imaging survey has been involved in the detection of quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s beyond a redshift z = 6.

The most recent data release, DR8 includes all photometric observations that will be taken with the SDSS imaging camera, covering 14,555 square degrees on the sky (just over 35% of the full sky).

Observations



SDSS uses a dedicated 2.5-m wide-angle optical telescope, and takes images using photometric system
Photometric system
In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...

 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 CCDs
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 relates to various kinds of astronomical magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

.

The SDSS telescope uses the drift scanning technique, which keeps the telescope fixed and makes use of the Earth's rotation to record small strips of the sky. The image of the stars in the focal plane drifts along the CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 chip, instead of staying fixed as in tracked telescopes. This method allows consistent astrometry over the widest possible field and precision remains unaffected by telescope tracking errors. The disadvantages are minor distortion effects and the CCD has to be written and read in the same time.

The telescope's camera is made up of thirty CCD chips each with a resolution of 2048×2048 pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

s, totaling approximately 120 Megapixels. The chips are arranged in five rows of six chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1 and 893.1 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...

, with 95% completeness in typical seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

 to magnitudes
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 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 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 (about -80 degrees Celsius) by liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

.

Using this photometric data, stars, galaxies, and quasars are also selected for spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

. The spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...

 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 SDSS III can record 1000 spectra at once. Over the course of each night, between six and nine plates are typically used for recording spectra.

Every night the telescope produces about 200 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...

 of data.

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 Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, the SEGUE and the Sloan Supernova Survey, which watches after supernova Ia
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...

 events to measure the distances to far objects.

Sloan Legacy Survey


The 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 (with typical radial velocity of 10 km/s) in order 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
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

.

The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of SDSS Data Release 7 (DR7) .

Sloan Supernova Survey


Running until the end of the year 2007, the Supernova Survey searched for Type Ia supernovae. The survey rapidly scans 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.

SDSS III: 2008-2014


In mid-2008, SDSS-III was started. It comprises four separate surveys, each conducted on the same 2.5m telescope:
  • Apache Point Observatory Galactic Evolution Experiment
    Apache Point Observatory Galactic Evolution Experiment
    The Apache Point Observatory Galactic Evolution Experiment is an astronomical survey established to study 100,000 red giant stars encompassing the entire galactic bulge, bar, disk, and halo. It will use high-resolution, high signal-to-noise infrared spectroscopy...

     (APOGEE)
  • Baryon Oscillation Spectroscopic Survey
    Baryon Oscillation Spectroscopic Survey
    The Baryon Oscillation Spectroscopic Survey is an astronomical survey designed to measure the expansion rate of the Universe.It will focus on luminous red galaxies and quasars, mapping the spatial distribution....

     (BOSS)
  • Multi-object APO Radial Velocity Exoplanet Large-area Survey
    Multi-object APO Radial Velocity Exoplanet Large-area Survey
    The Multi-object APO Radial Velocity Exoplanet Large-area Survey ' is a ground-based Doppler survey of 11,000 of the brightest stars. The project will use the SDSS telescope and new multi-object Doppler instruments to monitor radial velocities...

     (MARVELS)
  • Sloan Extension for Galactic Understanding and Exploration 2
    Sloan Extension for Galactic Understanding and Exploration 2
    The Sloan Extension for Galactic Understanding and Exploration 2 is an astronomical survey designed to map the outer reaches of the Milky Way with a spectra of 240,000 stars...

     (SEGUE-2)

Baryon Oscillation Spectroscopic Survey (BOSS)


The SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS) will map the spatial distribution of luminous red galaxies (LRGs) and quasars to 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 .

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 Galaxy, from distances of 10 to 60 kpc.

Combining SEGUE-1 and 2 reveals 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 is expected to be dominated by late-time accretion events. SEGUE 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 help uncover rare, chemically primitive stars that are fossils of the earliest generations of cosmic star formation.

APO Galactic Evolution Experiment (APOGEE)


The APO Galactic Evolution Experiment (APOGEE) will use high-resolution, high signal-to-noise infrared spectroscopy to penetrate the dust that obscures the inner Galaxy. APOGEE will survey 100,000 red giant stars across the full range of the Galactic bulge, bar, disk, and halo.

Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS)


The Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) will monitor 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.

Data access


The survey makes the data releases available over the Internet. The SkyServer provides a range of interfaces to an underlying Microsoft SQL Server
Microsoft SQL Server
Microsoft SQL Server is a relational database server, developed by Microsoft: It is a software product whose primary function is to store and retrieve data as requested by other software applications, be it those on the same computer or those running on another computer across a network...

. 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 is 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 DR8, released January 2011, is the eighth major data release and provides images, imaging catalogs, spectra, and redshifts via a variety of search interfaces.

The raw data (from before it was processed into databases of objects) is also available through another Internet server, and through the NASA World Wind
NASA World Wind
World Wind is an open-source virtual globe developed by NASA and the open source community for use on personal computers. Old versions need Microsoft Windows but the more recent Java version, , is cross platform and provides a suite of . The World Wind Java version was awarded in November 2009...

 program.

Sky in Google Earth
Google Sky
Google Sky is a feature for Google's Google Earth and an online sky/outer space viewer at www.google.com/sky. It was created on August 27, 2007...

 includes data from the SDSS, for those regions where such data is available. There are also KML
Keyhole Markup Language
Keyhole Markup Language is an XML notation for expressing geographic annotation and visualization within Internet-based, two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth, which was originally named Keyhole Earth Viewer. It was created by...

 plugins for SDSS photometry and spectroscopy layers, allowing direct access to SkyServer data from within Google Sky
Google Sky
Google Sky is a feature for Google's Google Earth and an online sky/outer space viewer at www.google.com/sky. It was created on August 27, 2007...

.

Following from Technical Fellow Jim Gray's significant contribution on behalf of Microsoft Research
Microsoft Research
Microsoft Research is the research division of Microsoft created in 1991 for developing various computer science ideas and integrating them into Microsoft products. It currently employs Turing Award winners C.A.R. Hoare, Butler Lampson, and Charles P...

 with the SkyServer project, Microsoft's WorldWide Telescope
WorldWide Telescope
The WorldWide Telescope is a computer program created by Microsoft that displays the astronomical sky as maps, the 3D Universe, visualised data sets and animations. It was announced at the TED Conference in Monterey, California in February 2008. Users are able to pan around outer space and zoom as...

 makes use of SDSS and other data sources.

Results


Along with publications describing the survey itself, SDSS data has 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, the distribution of galaxies, the properties of stars in our own galaxy and also subjects such as dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

 and dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

 in the universe.

See also


  • James E. Gunn
    James E. Gunn
    James Edward Gunn is the Eugene Higgins Professor of Astronomy at Princeton University. Gunn's early theoretical work in astronomy has helped establish the current understanding of how galaxies form, and the properties of the space between galaxies...

  • Alfred P. Sloan
    Alfred P. Sloan
    Alfred Pritchard Sloan, Jr. was an American business executive in the automotive industry. He was a long-time president, chairman, and CEO of General Motors Corporation...

  • Apache Point Observatory
    Apache Point Observatory
    The Apache Point Observatory is located in the Sacramento Mountains in Sunspot, New Mexico 18 miles south of Cloudcroft. The observatory consists of the Astrophysical Research Consortium's 3.5-meter telescope, the Sloan Digital Sky Survey 2.5-m telescope with a 20" photometric telescope,...

  • Galaxy color-magnitude diagram
    Galaxy color-magnitude diagram
    The Galaxy color-magnitude diagram shows the relationship between absolute magnitude, luminosity, and mass of galaxies. A preliminary description of the three areas of this diagram was made in 2003 by Eric F. Bell et al...

  • Galaxy Zoo
    Galaxy Zoo
    Galaxy Zoo is an online astronomy project which invites members of the public to assist in the morphological classification of large numbers of galaxies. It is an example of citizen science as it enlists the help of members of the public to help in scientific research. An improved version—Galaxy...

  • Photometric telescope
  • Sloan Great Wall
    Sloan Great Wall
    The Sloan Great Wall is a cosmic structure formed by a giant wall of galaxies , and to the present day it is the largest known structure in the universe. Its discovery was announced on October 20, 2003 by J. Richard Gott III of Princeton University and Mario Jurić and their colleagues, based on...


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

External links