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The SDSS Data Release 7 (DR7)

The Sloan Digital Sky Survey (SDSS) consists of a series of three interlocking imaging and spectroscopic surveys, carried out over an eight-year period with a dedicated 2.5m telescope located at Apache Point Observatory in Southern New Mexico. This seventh data release (DR7) from the SDSS represents the completion of this project.

The three surveys that make up the SDSS are:

  • Legacy, an imaging survey in five bands over a contiguous 7646 deg2 high-latitude elliptical region in the Northern Galactic Cap, plus an additional 750 deg2 in the Southern Galactic Cap, together with spectroscopy of complete samples of galaxies and quasars covering about 8200 square degrees. The total imaging area in the Legacy survey is 8423 square degrees.
  • SEGUE (Sloan Extension for Galactic Understanding and Exploration), additional imaging of 3240 deg2 of sky at lower Galactic latitudes, together with spectroscopy of 240,000 stars towards 200 sightlines covering 1400 square degrees (spread throughout the Legacy and SEGUE imaging footprints), to study the structure of the Milky Way.
  • Supernova, The equivalent of about 80 repeated imaging scans of the Southern Equatorial Stripe (ra > 310 or ra < 59; -1.25 > dec < 1.25) obtained in variable weather conditions (some clouds) to search for supernovae in the redshift range 0.1 < z < 0.4.

The catalog derived from the images includes more than 350 million celestial objects, and spectra of 930,000 galaxies, 120,000 quasars, and 460,000 stars. The data are fully calibrated and reduced, carefully checked for quality, and publically accessible through efficient databases. The data have been publicly released in a series of annual data releases, culminating in the final data release, DR7.

Contents

New for DR7

  • The Data Archive Server (DAS) for DR7 has a new interface, offering users the ability to browse all the imaging runs and spectra by run or plate number. Color (g-r-i) pictures of 1.3M imaging fields and wavelength-flux plots of each of 1.6M spectra are available. Bulk download access to over 70TB of raw and processed data are available via wget or rsync interfaces.
  • DR7 contains 577 new plates of spectra beyond DR6 (277 new Legacy, 285 SEGUE, 15 other) for a total of 2564 plates and 1,640,960 spectra (including sky and unidentified objects). All SDSS spectroscopic plates are available from the Spectroscopic Plates page of the DAS.
  • DR7 contains 3240 square degrees of SEGUE lower-latitude and Southern imaging. SEGUE imaging is available as FITS files from the Data Archive Server; you can upload a list of coordiantes and download the appropriate files.
  • DR7 contains the complete Legacy and SEGUE imaging catalogs in the same CAS SQL database as all the Legacy and SEGUE spectra, simplifying cross-id searches. All these catalog data are available from the Catalog Archive Server (CAS).
  • DR7 CAS contains 'propermotions' table with information for over 205M Legacy and SEGUE objects, derived from the UCAC-2 and UCAC-3 USNO astrometric catalogs, with the alignment re-calibrated against the SDSS imaging positions. The parameters stored in this table are available in the CAS Schema Browser entry for the propermotions table.
  • DR7 CAS contains updated (from DR6) astrometry solutions for most objects, using the UCAC-3 catalog when available, and replacing most Tycho-2-based solutions with UCAC-based solutions, which have more matches and include fainter objects. The rms difference in coordinate positions is less than 20 mas for nearly all imaging fields.
  • DR7 CAS contains an all new sppParams table with updated radial velocities of over 460,000 stars and estimates of [Fe/H], log g (surface gravity), effective temperature and stellar type classification information for over 300,000 stellar spectra (sppParams table entry in the CAS Schema Browser).
  • DR7 CAS and DAS contains "ubercalibrated" magnitudes for all objects as the default magnitude selection (psfmag_[ugriz], petromag_[ugriz], etc). The target version of the calibrated magnitudes are still available in a separate table.
  • DR7 CAS contains the imaging RUNS database, separate from the DR7 (Legacy + SEGUE) database. This database contains 530 imaging scans, some of which are not present in the main DR7 database, and thousands of imaging fields from the beginnings and ends of DR7 strip scans, which did not resolve into a complete rectangular stripe on the sky. All photometry from the RUNS database is available in the RUNS Catalog Archive Server (RUNS CAS).
  • DR7 CAS contains the Stripe82 database, distinct from the DR7 and RUNS databases, containing all imaging from SDSS stripe 82 along the Celestial Equator in the Southern Galactic Cap (Stripe 82). Subsets of the 303 individual stripe 82 scans were made public in previous data releases, but with DR7, we now make a coaddition of about 47 South strip and 55 North strip scans into a scan in which objects are detected and cataloged about 2 magnitudes deeper than in any individual stripe82 scan. It includes a total of 303 runs (plus 2 coadd runs), covering any given piece of the approx. 270 deg2 area approximately 80 times. All 303 runs covering Stripe 82 are part of the Stripe 82 database, which can be queried to search for variability (and propermotion, with a bit more effort). Only about one-quarter of the Stripe 82 scans were obtained in photometric conditions, the rest were taken under variable clouds and often poorer than normal seeing. For those runs which are non-photometric, an approximate calibration, using the photometric frames as reference, is derived and available in the CAS stripe82 database, for objects with RA > 310 or RA < 59 degrees.
  • For DR7, as part of the SEGUE effort, the spectroscopic pipeline was enhanced to obtain more accurate radial velocities of stellar objects. The plate-to-plate systematic velocity variations are approximately 2 km/s. Including random scatter and systematics, the overall velocity accuracy of stars has a sigma of 5.5 km/s at g=18.5 mag, degrading to about 10 km/s at g=19.5 and 20 km/s at SEGUE's limit of g=20.3. Over-zealous cosmic ray rejected removed some real emission line features in spectra of narrow emission line galaxies in DR6. This effect has largely been corrected in DR7. Spectrophotometry has been recalculated for all objects in DR7, and is good to levels of 10% or better over the full spectral coverage range of 3900-9100 Ångstroms. The accurate spectrophotometric calibration is a unique feature of the SDSS and SEGUE spectroscopic surveys, present in no other large spectroscopic dataset.
  • With SEGUE's addition of several thousand square degrees of low-latitude (|b| < 30 deg) imaging, the standard high-latitude object detection and measurement code was tested in very crowded fields. In these regions, the standard PHOTO pipeline was augmented by a version of the PanSTARRS object detection pipeline. PanSTARRS-assisted PHOTO also was optimized for stellar PSF measurement and object deblending in low-latitude SEGUE fields. The accuracy of the crowded field photometry approached that of the Legacy survey area, except in regions of extreme crowding (such as the cores of globular clusters). Catalogs of objects processed with this special code are marked in the database with a rerun number of 648 and are available in the DR7 CAS and the DR7 DAS.
  • Updated photo-zs are available in the PhotoZ tables of the CAS. The photoz table is current through DR7, using ubercalibrated magnitudes, and it contains information on nearest neighbor galaxies in a volume to allow analysis of photometric redshift statistics. The PhotoZ2 table, an independent method of deriving photometric redshifts, is up to date through DR6.
  • DR7 CAS contains a number of plates of spectra targeted on known globular and open clusters. The spectra on these plates identified with programnames in the plateX table of the form 'segcluster%' and are used to calibrate the SSPP (SEGUE stellar parameter pipeline) code that derives [Fe/H], log g, and Teff for each star.

Except for the addition of new data and the changes in the spectroscopic, imaging, and sspp pipelines described above, the pipelines and databases are essentially identical in DR7, DR6, DR5, DR4, DR3 and DR2. Thus, DR7 is a proper superset of DR6, which is a superset of DR5, 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.

What DR7 contains

The DR7 imaging data cover about 8423 square degrees of "legacy" sky, with information on roughly 230 million distinct photometric objects, and about 3240 square degrees of SEGUE sky, with about 127 million distinct objects (including many stars at low latitude). The DR7 spectroscopic data include data from 1802 main survey plates of 640 spectra each, and cover 8200 square degrees. In addition, DR7 contains 762 "extra" and "special" plates:

  • 86 "extra" plate/MJD combinations which are repeat observations of 77 distinct main survey plates
  • 676 distinct "special" plates, which includes 410 SEGUE plates, and about 270 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. A few of these plates are outside of the regular DR7 imaging area.
  • 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 DR7

The DR7 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 14 July 2008, and the spectroscopy associated with that area as well as the extra and special plates obtained before that date. In fact, about 34 square degrees of imaging data in the Nothern Galactic Cap lie outside this ellipse. While the DR7 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 DR7 are given on the coverage page. The natural unit of imaging data is a run; the DR7 contains data from 362 runs in the best database, 530 in the RunsDB, and 305 runs in the Stripe82DB, including 2 coadd runs 106 and 206 (the coadd is the co-addition of about 50 repeated North and 50 South scans of stripe 82).

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 in the DAS only. A subset of runs in DAS ALL imaging contains the DAS-only data release, and catalogs are not present in the DR7 CAS, though some (not all) are available in the RUNS CAS database. These runs are indicated by a non-standard stripe number, such as 100, 205, 61, 62, 301, or 293, 90. In DR6, this data was referred to as DR supplemental. They are:

  • Scans through or near M31 (run 3366,3367, stripe 293) and the Perseus Cluster (stripes 61,62);
  • Sagittarius tidal stream scans (stripe 205) and scans near the South Galactic Pole (stripe 90);
  • Scans taken at low Galactic latitude as part of the SEGUE project (plate 797, stripe 301, stripe 100); these runs were used to target stars on special plates.

Imaging caveats

The following caveat is new to DR7

Systematic errors in the photometry of bright galaxies

There is a subtle systematic effect present in the photometry of faint galaxies in the wings of very bright extended galaxies. The effect is due to the way the DR7 version of the PHOTO treats sky subtraction; in particular, the binning of sky over large regions affected by extended wings of low-surface-brightness galaxies. This effect has been carefully documented, and it can be removed in a statistical sense for weak-lensing and other galaxy clustering studies. See the Imaging Caveat on Overestimation of sky levels near bright galaxies for more detail.

The following caveats apply unchanged to DR7

Red leak to the u filter and very red objects

The u filter has a natural red leak around 7100 Å which is supposed to be blocked by an interference coating. However, under the vacuum in the camera, the wavelength cutoff of the interference coating has shifted redward (see the discussion in the EDR paper), allowing some of this red leak through. The extent of this contamination is different for each camera column. It is not completely clear if the effect is deterministic; there is some evidence that it is variable from one run to another with very similar conditions in a given camera column. Roughly speaking, however, this is a 0.02 magnitude effect in the u magnitudes for mid-K stars (and galaxies of similar color), increasing to 0.06 magnitude for M0 stars (r-i ~ 0.5), 0.2 magnitude at r-i ~ 1.2, and 0.3 magnitude at r-i = 1.5. There is a large dispersion in the red leak for the redder stars, caused by three effects:

  • The differences in the detailed red leak response from column to column, beating with the complex red spectra of these objects.
  • The almost certain time variability of the red leak.
  • The red-leak images on the u chips are out of focus and are not centered at the same place as the u image because of lateral color in the optics and differential refraction - this means that the fraction of the red-leak flux recovered by the PSF fitting depends on the amount of centroid displacement.

To make matters even more complicated, this is a detector effect. This means that it is not the real i and z which drive the excess, but the instrumental colors (i.e., including the effects of atmospheric extinction), so the leak is worse at high airmass, when the true ultraviolet flux is heavily absorbed but the infrared flux is relatively unaffected. Given these complications, we cannot recommend a specific correction to the u-band magnitudes of red stars, and warn the user of these data about over-interpreting results on colors involving the u band for stars later than K.

Bias in sky determination

There is a slight and only recently recognized downward bias in the determination of the sky level in the photometry, at the level of roughly 0.1 DN per pixel. This is apparent if one compares large-aperture and PSF photometry of faint stars; the bias is of order 29 mag arcsec-2 in r. This, together with scattered light problems in the u band, can cause of order 10% errors in the u band Petrosian fluxes of large galaxies.

Zeropoint of the photometric system

The SDSS photometry is intended to be on the AB system (Oke & Gunn 1983), by which a magnitude 0 object should have the same counts as a source of Fnu = 3631 Jy. However, this is known not to be exactly true, such that the photometric zeropoints are slightly off the AB standard. We continue to work to pin down these shifts. Our present estimate, based on comparison to the STIS standards of Bohlin, Dickinson, & Calzetti~(2001) and confirmed by SDSS photometry and spectroscopy of fainter hot white dwarfs, is that the u band zeropoint is in error by 0.04 mag, uAB = uSDSS - 0.04 mag, and that g, r, and i are close to AB. These statements are certainly not precise to better than 0.01 mag; in addition, they depend critically on the system response of the SDSS 2.5-meter, which was measured by Doi et al. (2004, in preparation). The z band zeropoint is not as certain at this time, but there is mild evidence that it may be shifted by about 0.02 mag in the sense zAB = zSDSS + 0.02 mag. The large shift in the u band was expected because the adopted magnitude of the SDSS standard BD+17 in Fukugita et al.(1996) was computed at zero airmass, thereby making the assumed u response bluer than that of the USNO system response.

Holes in the imaging data

About 0.3% of the DR7 imaging footprint area (about 25 square degrees) for DR7 are marked as holes. These are indicated in the CAS by setting quality=5 (HOLE) in the tsField file and field table and given in the list of quality holes, which contains further details about the holes and quality flags, including a information about a table in the CAS which allows one to query for quality information about each field of data.

Problems with one u chip

The u chip in the third column of the camera is read out on two amplifiers. On occasion, electronic problems on this chip caused one of the two amplifiers to fail, meaning that half the chip has no detected objects on it. This was a problem for only two of the 105 imaging runs included in DR5: run 2190, which includes a total of 360 frames in two separate contiguous pieces on strip 12N (centered roughly at delta = +5 degrees in the North Galactic Cap; NGC), and run 2189, which includes 76 frames on stripe 36N near the northern boundary of the contiguous area in the NGC. The relevant frames are flagged as bad in the quality flag; in addition, individual objects in this region have the u band flagged as NOTCHECKED_CENTER (or, for objects which straddle the boundary between the two amplifiers, LOCAL_EDGE). Richards et al (2002) describe how the quasar selection algorithm handles such data; the net effect is that no quasars are selected by the ugri branch of the algorithm for these data.

Spectroscopy caveats

The following caveats from DR6 have been corrected in DR7

  • Over-zealous cosmic ray rejected removed some real emission line features in spectra of narrow emission line galaxies in DR6. This effect has largely been corrected in DR7; see the caveat on Clipped Spectral Lines on the Spectra page for more information.
  • In DR6, 28 plates, had a flux scale offset due to replacement of fiber magnitudes by psf magntidues. This problem has been corrected in DR7.
  • In DR6 CAS, the mag_0, mag_1, and mag_2 values in the specobjall table were incorrectly pointing to the u, g, and r respective band synthetic magnitudes for each spectrum. u synthetic magnitude are meaningless since the spectral coverage does not include the u band. In DR7 CAS, these values have been updated to point to the (correct) g, r and i respective synthetic magnitudes.
  • An instability in the spectroscopic flatfields was found in some spectra prior to DR7; this effect has been corrected in DR7. Details on the correction are available in the Spectrophotometry Algorithms page.

The following caveats and notes are new to DR7

  • DAS Only Plates: A few Legacy and SEGUE plates did not have sufficient exposure time to reach their target S/N in order to be declared 'done'. Also, in the case of SEGUE, 3 plates have bright but not faint equivalents (or vice-versa). These plates do not appear in the CAS or in the list of all plates; however, they are available in the DAS. The plate-mjd numbers for these plates are:
     plate-mjd      Why not in CAS platex table:
     -----------------------------------------------------------------------
     0356-51779    S/N too low -- plate not done (not enough exposure)
     1112-53180    S/N too low -- plate not done
     1858-53271    SEGUE test plate, high DEC, never made it to CAS
     2309-54441    S/N too low -- SEGUE plate
     2333-53676    M71 very bright plate, lots of saturation, only 320/640 done
     2535-54632    SEGUE bright, faint matching plate not done 
     2640-54474    BAOTest plate, test plate for SDSS-III 
     2716-54629    SEGUE bright, faint matching plate not done
     2851-54485    SEGUE dup pointing of plate 2045.
     2962-54578    don't have correct tsObj file, Low S/N, plate not done
    
  • Some very strong emission lines appear clipped due to saturated pixels, but were not flagged as such. This occurs very rarely, but the user should be aware of the possibility of objects with extremely strong emission lines and unusual line ratios. Details on this caveat are available under Clipped Spectral Lines on the Spectra page.

The following caveats are unchanged from DR6

A small number of plates suffered from a variety of minor problems affecting the quality of the spectrophotometry (but not of redshifts). See the list under Plates with problematic spectrophotometry on the data products page for spectra.

Zero equivalent width of emission lines, especially H alpha

There is a bug in the line-measurement code that has been in use since DR3 which gives some emission lines an equivalent width of zero, even though there is a significant line detection. The aim of the change introducing the bug had been to determine the equivalent width by integrating the spectrum, instead of using the parameters of a fitted Gaussian. The Gauss-fit equivalent width can be recovered from the fit parameters using the usual expression EW = 2.5066 * sigma * height / continuum.

Note about galactic extinction correction

In the EDR and DR1, the spectroscopic data were nominally corrected for galactic extinction. The spectrophotometry since DR2 is vastly improved compared to DR1, but the final calibrated spectra in DR2 and beyond are not corrected for foreground Galactic reddening (a relatively small effect; the median E(B-V) over the survey is 0.034). Users of spectra should note that the fractional improvement in spectrophotometry from DR1 to DR2 and beyond was much greater than the extinction correction itself. As the SDSS includes a substantial number of spectra of galactic stars, a decision has been taken not to apply any extinction correction to spectra, since it would only be appropriate for extragalactic objects, but to report the observational result of the SDSS, namely, the spectrum including galactic extinction.

Mismatches between the spectroscopic and imaging data

For various reasons, a small fraction of the spectroscopic objects do not have a counterpart in the best object catalogs. In addition, the DR7 does not contain photometric information for some of the special plates, and the retrieval of photometric data from the CAS database requires special care for objects from the special plates, and even more care for SEGUE spectra. See the caveat about mismatches between spectra and images on the data products page for spectra.