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The SDSS Data Release 6 (DR6)

Contents

New for DR6

DR6 is the first release since DR2 to have significant changes to the processing software. As it includes the first data from SDSS-II, it also adds substantially different data products - see About SEGUE and the public data release from the SDSS Supernova Survey. The qualitatively new items are:

  • SEGUE imaging catalogs and quality assurance (QA) are available in the SEGUE CAS database. SEGUE targeting flags will eventually be available for all DR6 imaging, including legacy. See the target-selection algorithms page for information on SEGUE targeting.
    For SEGUE imaging runs, note the caveat on low-latitude photometry and reddening corrections!
  • Stellar spectroscopy Stellar Spectral Parameters Pipeline line measurements (sppParams, [Fe/H], log g, Teff) and line index measurements (sppLinesCa, Mg, etc.) are available both in the CAS and the DAS for SEGUE and many SDSS spectra.
    Important! The SEGUE spectra count as special-plate spectra in the CAS and are not considered to be "scienceprimary". Therefore, they are not linked to the photometry in the usual way, e.g., they do not appear in the object browser's default view like galaxy, quasar and stellar spectra from the legacy survey. Moreover, all SEGUE spectra are in the BESTDR6 database, while most of the SEGUE imaging is in the SEGUEDR6 database of the CAS, so that linking imaging and spectroscopy data requires special care. See the SEGUE sample SQL query for instructions on joining SEGUE imaging and spectroscopy.
  • Ubercal improved calibrations using cross-scans to tie the photometry of the entire survey to each other. See the algorithms entry on ubercal
  • Improved spectroscopy reductions The spec2d code that reduces the spectrograph's 2-dimensional CCD images into 1-dimensional spectra has been modified substantially. The most important changes are:
    1. Improved spectrophotometry, now calibrated relative to PSF, not fiber magnitudes. The spectrophotometric flux scale is now brighter by 0.35 mag, or 25%.
    2. Availability of additional data products, including individual 15-minute exposures (DAS only) and sky spectra
  • In the spectro1d pipeline analyzing the reduced 1-dimensional spectra, the algorithm for velocity dispersions has been changed, see Bernardi 2007, The sigma-L correlation in nearby early-type galaxies, AJ, 133, 1954 and there are improved radial-velocity estimates for stars
  • In the CAS database, the following changes have occured:
    • All the columns from the photoAuxAll table (galactic coordinates and astrometric errors in RA and DEC) are now part of photoObjAll and derived views (photoObj, star, photoPrimary etc.). For backwards compatibility, there are still photoAuxAll and photoAux views, but new queries requiring these parameters do not need these auxiliary tables any more.
    • There have been changes in the sector/region code in the CAS
    • A "clean photometry" flag has been added to photoObjAll table to facilitate photometric flag checking.

The quantitatively new items are:

  • Increased sky coverage of the Legacy survey in both imaging and spectroscopy (as with any data release). The contiguous imaging in the north galactic cap is essentially complete now.
  • Additional value-added catalogs (e.g., the DR5 quasar catalog).

Except for the changes in the spectroscopic pipeline spec2d and the CAS changes described above, the pipelines and databases are essentially identical in DR6, DR5, DR4, DR3 and DR2. Thus, DR6 is (very nearly) a proper superset of DR5, which is a superset of DR4, etc. The DR2 included reprocessing of all data included in DR1, and those data in EDR that pass our data-quality criteria for the official survey. For details about what changed in subsequent releases, please refer to About DR4 and About DR5.

What DR6 contains

The DR6 imaging data cover about 8420 square degrees of "legacy" sky, with information on roughly 230 million distinct photometric objects, and about 1200 square degrees of SEGUE sky, with about 57 million objects. The DR6 spectroscopic data include data from 1520 main survey plates of 640 spectra each, and cover 6860 square degrees. In addition, DR6 contains 467 "extra" and "special" plates:

  • 64 "extra" plate/MJD combinations which are repeat observations of 55 distinct main survey plates
  • 383 distinct "special" plates, which includes 162 SEGUE plates (see above), and 226 plates with observations of spectroscopic targets, mostly in the southern galactic cap, which were selected by the collaboration for a series of specialized science programs. Some of these plates are outside of the regular DR6 imaging area; DR6sup provides that missing imaging, among other things.
  • 15 "extraspecial" repeat observations of "special" plates (7 of SEGUE plates, and 8 of other special plates)

There is a separate page describing the special plates in DR6

The DR6 footprint is defined by all non-repeating survey-quality imaging runs within the a priori defined elliptical survey area in the Nothern Galactic Cap, and three stripes in the Southern Galactic Cap obtained prior to 7 July 2006, and the spectroscopy associated with that area as well as the extra and special plates obtained before that date. In fact, 34 square degrees of imaging data in the Nothern Galactic Cap lie outside this ellipse. While the DR6 scans do not repeat a given area of sky, they do overlap to some extent, and the data in the overlaps are included in earlier releases as well. The sky coverage of the imaging and spectroscopic data that make up DR6 are given on the coverage page. The natural unit of imaging data is a run; the DR6 contains data from (about) 244 runs in the best database, and (about) 246 runs in the target database.

A total of 183 square degrees of sky are different runs between target and best, the majority along the Equatorial Stripe in the Fall sky.

We also make available images and associated catalogs from three categories of special runs as a DRsup (supplemental) DAS-only data release. They are:

  • A series of repeat scans of the Equatorial Stripe in the Southern Galactic Cap (Stripe 82);
  • Scans through M31 and the Perseus Cluster;
  • Scans taken at low Galactic latitude as part of the SEGUE project; these runs were used to target stars on special plates.

Imaging caveats

The following caveat is new to DR6.

Systematic relative photometry errors in extreme-coloured stars

Stars with extreme colours can have inconsistent photometry due to slight differences in the photometric response for different camcols. There is a brief description in Ivezic et al. 2007, AJ in press.

The following caveat has been characterised quantitatively now:

Overestimation of sky levels in the vicinity of bright objects