NED Frequently Asked Questions -- SDSS Sources and Data

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(Latest revision: 28 March 2012)

SDSS Sources and Data in NED (December 2008)

SDSS Clean Sample

The first extremely large dataset to be integrated into NED involved 154 million objects from the Sloan Digital Sky Survey (SDSS) Data Release 6 (DR6) photometric catalog. These are a subset of the 217 million objects in DR6, which were selected using the following "Clean sample" filters based on recommendations by the SDSS team (see SDSS recommendations for clean samples of objects):

  • All objects with SDSS PRIMARY status bit set,
  • and SDSS flags set as follows
    • BINNED1
    • and not BRIGHT
    • and (not BLENDED or NODEBLEND)
    • and not NOPROFILE
      or (DEBLEND_NOPEAK and r-band PSF mag err > 0.2))

We did not filter out objects with the SATURATED flag set, because although SDSS photometry is unreliable for such sources, this would have omitted useful SDSS positions and size measurements for galaxies with bright nuclei, including many nearby Seyfert galaxies. The requirement that Status=PRIMARY excluded objects with Status=FAMILY that were de-blended by the pipeline into smaller components (these CHILD objects themselves have Status=PRIMARY), and also excluded repeated observations with Status=SECONDARY.

SDSS classified each object as either a "galaxy" or "star", depending on whether the pipeline determined it to be resolved (extended) or unresolved (a point source). Along with objects of type "galaxy", objects of type "star" were also folded into NED, because this SDSS classification is strictly morphological and includes many point sources that are actually QSOs and unresolved, compact galaxies. Whenever possible, both types of SDSS objects were cross-matched to objects in the previous version of NED (which had ~10 million objects total, including ~4 million objects within the SDSS footprint) using our standard position- and data-matching procedures.

SDSS data in NED include:

  • Multiwavelength cross-identifications with observations from tens of thousands of catalogs and journal papers. (Example: multiwavelength cross-IDs with SDSS J074906.51+451033.9)
  • Basic Data, including representative g-band magnitudes, positions, and classifications (Example: SDSS J133840.44+481704.9)
  • Position measurements in the r band (Example: SDSS J133936.13+430238.7)
  • Size measurements in the r band using 4 methods: Isophotal, Petrosian, de Vaucouleurs, and Exponential (Example: SDSS J133934.97+430351.0)
  • Photometric measurements in all 5 bands [ugriz], each with 3 methods (PSF, Model, CModel) for extended objects (Example: SDSS J135554.50+454822.6) and with one method (PSF) for point sources (Example: SDSS J133933.60+430228.5), have been folded into NED's SEDs
  • Links to SDSS SkyServer for detailed pipeline processing flags, spectra and images for each object. (Example: SDSS J074906.50+451033.9)
  • SDSS measurements for large galaxies, including a subset of objects with Status=FAMILY from the standard SDSS pipeline and measurements derived from custom mosaics such as those produced for selected galaxies in the RC3 by David Hogg at NYU.

With over 1.7 billion detailed photometric measurements (PSF, Model and CModel magnitudes for galaxies) in five bands, and hundreds of millions of diameter and position measurements, as well as crucial flags from the pipeline processing, folding in SDSS DR6 (the "Clean Sample" subset described above) expanded NED's tabular database volume by a factor of ~200.


  • While we carefully checked the resulting database of about 163 million objects, there are certain to be anomalies included. Many SDSS objects, for example, are parts of galaxies covering a major portion of a spiral arm. These may be cross-matched to radio sources or H II regions with relatively large position uncertainties that coincidentally overlap the much smaller position error ellipses of the SDSS objects.
  • Some DR6 objects have either no counterparts among objects previously in NED, or generally reliable cross-identifications with objects previously in NED based on observations across the EM spectrum. However, hundreds of thousands of SDSS objects were determined by our cross-comparison software to be possible (candidate) matches with other objects in NED. In addition, there are many cases in which multiple SDSS objects may be associated with single objects observed with lower resolution (e.g., radio, UV, or X-ray surveys).
  • Please see this note about corrections to the u band (CModel only) and z band (PSF and Model only) magnitudes and flux densities.
  • SDSS DR7 and DR8 have been released in the meantime, but have not yet been incorporated into NED.
  • Please let us know about any peculiarities or omissions, so that future releases of NED can more accurately reflect our knowledge of the extragalactic Universe.