The Sloan Digital Sky Survey or SDSS 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 was named after the Alfred P. Sloan Foundation, which contributed significant funding.
Data collection began in 2000, and the final imaging data release covers over 35% of the sky, with photometric observations of around 500 million objects and spectra for more than 3 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 (DR8), released in January 2011, 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, includes the first results from the Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph, 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). Data release 10 (DR10), released to the public on 31 July 2013, includes all data from previous releases, plus the first results from the APO Galactic Evolution Experiment (APOGEE) spectrograph, 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.
Maps, Directions, and Place Reviews
Observations
SDSS uses a dedicated 2.5-m wide-angle optical telescope; from 1998-2009 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 (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 relates to various kinds of astronomical magnitude.
For imaging observations, the SDSS telescope used the drift scanning technique, which tracks the telescope along a great circle on the sky and continuously records 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 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, since stars at different 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 thirty CCD chips each with a resolution of 2048×2048 pixels, 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, 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 kelvin (about -80 °C) by liquid nitrogen.
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 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. 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 GB of data.
Sky Server Video
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 (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, the SEGUE and the Sloan Supernova Survey, which watches after supernova Ia events to measure the distances to far objects.
Sloan Legacy Survey
Sloan Extension for Galactic Understanding and Exploration (SEGUE)
Sloan Supernova Survey
SDSS III: 2008-2014
In mid-2008, SDSS-III was started. It comprises four separate surveys:
APO Galactic Evolution Experiment (APOGEE)
Baryon Oscillation Spectroscopic Survey (BOSS)
Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS)
SEGUE-2
SDSS IV: 2014-2020
The latest generation of the SDSS (SDSS-IV, 2014-2020) is extending precision 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).
APO Galactic Evolution Experiment (APOGEE-2)
extended Baryon Oscillation Spectroscopic Survey (eBOSS)
Mapping Nearby Galaxies at APO (MaNGA)
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. 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, 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.
Sky in Google Earth includes data from the SDSS, for those regions where such data are available. There are also KML plugins for SDSS photometry and spectroscopy layers, 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 Stripe 82 region of the SDSS.
Following from Technical Fellow Jim Gray's contribution on behalf of Microsoft Research with the SkyServer project, Microsoft's WorldWide Telescope makes use of SDSS and other data sources.
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, the distribution of galaxies, the properties of stars in our own galaxy and also subjects such as dark matter and 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.
Source of the article : Wikipedia
EmoticonEmoticon