Sloan Digital Sky Survey

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The Sloan Digital Sky Survey

Introduction

The Sloan Digital Sky Survey (SDSS) is one of the most ambitious and influential surveys in the history of astronomy. Over eight years of operations (SDSS-I, 2000-2005; SDSS-II, 2005-2008), it obtained deep, multi-color images covering more than a quarter of the sky and created 3-dimensional maps containing more than 930,000 galaxies and more than 120,000 quasars. SDSS data have been released to the scientific community and the general public in annual increments, with the final public data release from SDSS-II scheduled for October 31, 2008. SDSS-III, a program of four new surveys using SDSS facilities, began observations in July 2008, and will continue through 2014.

Instrumentation

The SDSS used a dedicated 2.5-meter telescope at Apache Point Observatory, New Mexico, equipped with two powerful special-purpose instruments. The 120-megapixel camera imaged 1.5 square degrees of sky at a time, about eight times the area of the full moon. A pair of spectrographs fed by optical fibers measured spectra of (and hence distances to) more than 600 galaxies and quasars in a single observation. A custom-designed set of software pipelines kept pace with the enormous data flow from the telescope.

Scope

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.

With new financial support and an expanded collaboration including 25 institutions around the globe, SDSS-II carried out three distinct surveys:

  • The Sloan Legacy Survey completed the original SDSS imaging and spectroscopic goals. The final dataset includes 230 million celestial objects detected in 8,400 square degrees of imaging and spectra of 930,000 galaxies, 120,000 quasars, and 225,000 stars.
  • SEGUE (the Sloan Extension for Galactic Understanding and Exploration) probed the structure and history of the Milky Way galaxy, with new imaging of 3500 square degrees and spectra of 240,000 stars in a variety of categories in selected fields.
  • The Sloan Supernova Survey carried out repeat imaging of the 300 square degree southern equatorial stripe to discover and measure supernovae and other variable objects. In the course of three 3-month campaigns, the supernova survey discovered nearly 500 spectroscopically confirmed Type Ia supernovae, which are being used to determine the history of the accelerating cosmic expansion over the last 4 billion years.

SDSS data have supported fundamental work across an extraordinary range of astronomical disciplines, including the properties of galaxies, the evolution of quasars, the structure and stellar populations of the Milky Way, the dwarf galaxy companions of the Milky Way and M31, asteroids and other small bodies in the solar system, and the large scale structure and matter and energy contents of the universe; this site includes For a brief overview of SDSS science contributions. A more in-depth view can be found in the online materials from the 2008 Symposium The Sloan Digital Sky Survey: Asteroids to Cosmology.

Datasets

The [www.sdss.org Sloan Digital Sky Survey] will map one-quarter of the entire sky and perform a redshift survey of galaxies, quasars and stars. The DR7 is the seventh major data release and provides images, imaging catalogs, spectra, and redshifts for download. It is the third and final data release of SDSS-II, an extension of the original SDSS consisting of three subprojects: The Legacy Survey, SEGUE and a Supernova survey.

Data products overview

Images - using SDSS as your telescope

SDSS provides FITS image files of the corrected frames in five bands, a mask that records how each pixel was used in the imaging pipelines, 4 x 4 binned images of the corrected frames after detected objects have been removed, and "atlas" images, which include all significant pixels around each object. This page describes imaging frames, how to get them and related files from the survey archive, and how to work with them. Object lists - using SDSS as your photometric catalog

The calibrated object lists are FITS tables containing positions, fluxes, and shapes of all objects detected at >5 sigma on the images. This page describes the lists, how to get them and related files from the archive, and how to work with them.

Spectra and spectroscopic parameters - using SDSS as your spectrograph

  • "2d" spectra: FITS files of the flux- and wavelength-calibrated, sky-subtracted spectra, with error and mask arrays and resolution at each pixel. 640 spectra per file.
  • "1d" spectra: FITS files with the calibrated spectra and error and mask arrays, redshift, spectral classification, and detected lines of each spectrum.

The page describes calibrated spectra, how to get them and related files from the archive, and how to work with them.

Tiling - using SDSS as your complete survey

"Tiling" means optimizing the placement of spectroscopic tiles on the sky and assignment of spectroscopic fibers to targets in the spectroscopic survey. This section describes the tiling files, how to get them and related files from the archive, and how to characterise the survey's non-uniformity using them.

Other data products

These include astrometric calibrations, photometric calibrations, gif and postscript plots of spectra, and finding charts in postscript/jpeg/fits formats.

Value Added Data Products

The SDSS collaboration has created catalogs and other data products based on public survey data that are of general use. IAU designations for SDSS objects

Use IAU designations in publications.

Data model

The flat-file data model contains detailed descriptions of all data generated by the data processing. The Archive Intro page and the SQL Schema Browser describe how data are stored in the Catalog Archive Server (CAS).

Data quality overviews

Imaging

There are various quality assurance algorithms for photometry, some of which consider each run in isolation, and some of which consider overlapping parts of different runs. Since DR4, we make available the summary and single-run QA data and plots, which analyze the PSF, the difference between aperture and PSF magnitudes of stars, and the location of the stellar locus in color-color space. Details are available on the QA pages.

The following overviews cover the quality of other aspects of imaging.

Spectroscopy

Download the Datasets

Proper Nomenclature/IAU designations

The official SDSS designation for an object is

SDSS JHHMMSS.ss+DDMMSS.s

where the coordinates are truncated, not rounded. This format must be used at least once for every object listed in a paper using SDSS data.

When abbreviating the object name in the text please use the "J" to indicate the equinox of the coordinates. For example

SDSS J123456.89-012345.6

could be abbreviated as

SDSSJ1234 SDSSJ1234-0123

Please refer to the CDS dictionary on SDSS for further information.

Why bother?

The following is an excerpt from a message by Helene Dickel, the Chair of the IAU Working Group Designations.

To illustrate the kind of confusion that shortened names can lead to, consider the source in the Fan et al. paper in ApJ 526, L57, 1999 properly designated SDSSp J153259.96-003944.1 which is then referred to as SDSS 1533-00 for brevity. Not only is there no J but the RA has been rounded instead of truncated. Without the explicit J, one may assume these are B1950 coordinates. If a subsequent author only gives the shortened name, then a reader searching in NED with IAU format on 1533-00, assuming B1950 coordinates, specifying acronym SDSS but nothing for the object type, would be given 243 objects that might be the one in question. Most begin with J2000 RA of 1533, 1534, or 1535. You can narrow the choices if you realize that SDSS source names are based on J2000 coordinates and that this source is a QSO. Then there are 9 choices which include SDSS J153306.42+000635.1 and SDSSp J153259.96-003944.1 Searching for SDSS 1532-0039 assuming J2000 coordinates and QSO yields two choices SDSS J153243.67-004342.5 and SDSSp J153259.96-003944.1 However, if you think SDSS 1532-0039 uses B1950 coordinates and don't specify a QSO, you are given 11 possibilities which start with RA 1534 or 1535 and Dec mostly start with 004, NONE of which include the relevant source SDSSp J153259.96-003944.1!

It is not just the earlier papers that give shortened names without the J. A recent paper in AJ gives SDSSHHMM+DDMM with no full coordinates and a website as the reference. It took over 2 hours with the help of NED to track down the original SDSSp designations and references for those three SDSS sources plus several other sources whose designations were equally corrupted. Following discussions with the Editor and Author, an Addendum is being published which includes a table giving the short name, the official SDSS or SDSSp full designation (and the full designation of the other sources), precise J2000 coordinates, and the published reference for each source. Editors and journal readers would appreciate having this information already available when you publish your papers.

Data usage policy

Acknowledging SDSS: Non-Commercial Use

Non-commercial scientific and technical publications using SDSS data should include the following acknowledgment:

Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/.

The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

Please also refer to the appropriate SDSS technical publications.

Acknowledging SDSS: Commercial Use

Data from the SDSS public archive may not be used for any commercial publication or other commercial purpose except with explicit approval by the Astrophysical Research Consortium (ARC). Requests for such use should be directed to the ARC Corporate Office via ARC's Business Manager as follows:

     Michael L. Evans
     ARC Business Manager
     c/o University of Washington
     Office of Research, Box 351202
     Seattle, WA 98195
     Phone: 206-685-7857

Email: evans@astro.washington.edu

Further Information

http://www.sdss.org/dr7/access/index.html