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The SDSS Data Release 6 (DR6)


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

Because of scattered light (see the EDR paper [Stoughton et al. 2002]), the background sky in the SDSS images is non-uniform on arc-minute scales. The photometric pipeline determines the median sky value within each 100" square on a grid with 50" spacing, and bilinearly interpolates this sky value to each pixel. This biases the sky bright near large extended galaxies, and as was already reported in the DR4 paper and (Mandelbaum et al. 2005), causes a systematic decrease in the number density of faint objects near bright galaxies. In addition, it also strongly affects the photometry of the bright galaxies themselves, as has been reported by Lauer et al. (2007), Bernardi et al. (2007), and Lisker et al. (2007).

We have quantified this effect by adding simulated galaxies (with exponential or de Vaucouleurs) profiles to SDSS images. The simulated galaxies ranged from apparent magnitude mr=12 to mr=19 in half-magnitude steps, with a one-to-one mapping from mr to Sersic half-light radius determined using the mean observed relation between these quantities for Main sample galaxies with exponential and de Vaucouleurs profiles. Axis ratios of 0.5 and 1 were used, with random position angle for the non-circular simulated galaxies. The results in the r band are shown in the Figure, showing the difference between the input magnitude and the model magnitude returned by the SDSS photometric pipeline, as a function of magnitude.

Also shown is the fractional error in the scale size re. The biases are significant to r=16 for late-type galaxies, and to r=17.5 for early-type galaxies. Also shown is the results of a separate analysis by by Hyde & Bernardi (unpublished) who fit deVaucouleurs models to SDSS images of extended elliptical galaxies, using their own sky subtraction algorithm, which is less likely to overestimate the sky level near extended sources. Their results are quite consistent with the simulations.

Sky misestimation near bright galaxies Sky misestimation near bright galaxies

Upper panel: The error in the r band model magnitude of simulated galaxies with an n=1 (exponential) profile (blue hexagons) and an n=4 (de Vaucouleurs) profile (red crosses) as determined by the photometric pipeline, as a function of magnitude. Fifteen galaxies are simulated at each magnitude for each profile. Also shown are the analogous results from Hyde & Bernardi (unpublished) for three early-type galaxy samples: 54 nearby (z<0.03) early-type galaxies from the ENEAR catalog (da Costa et al. 2000) in black; 280 brightest cluster galaxies from the C4 catalog (Miller et al. 2005) in green; and 9000 early-type galaxies from the Bernardi et al. (2003a) analysis in magenta. Lower panel: The fractional error in the scale size re as a function of magnitude from the simulations and the Hyde & Bernardi analysis.

The following caveats apply unchanged to DR6.

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 DR6 imaging footprint area (about 25 square degrees) for DR6 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

Problematic plates (new: flux scale offset for 28 plates)

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.

A problem new to DR6 is that 28 plate/MJD combinations have an offset in their spectrophotometric flux scale. See spectrophotometry flux scale offset caveat.

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 DR6 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.

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