The SDSS Data Release 3 (DR3)

Contents

• What DR3 contains
• Caveats for imaging data
• Caveats for spectroscopic data
• Advanced features not yet available in DR3

What DR3 contains

The DR3 imaging data cover 5282 square degrees, and include information on roughly 141 million objects. The DR3 spectroscopic data include data from 826 plates of 640 spectra each, and cover 4188 square degrees.

The DR3 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 1 July 2003, and the spectroscopy associated with that area obtained before that date. In fact, 34 square degrees of imaging data in the Nothern Galactic Cap lie outside this ellipse. While the DR3 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 DR3 are given on the coverage page. The natural unit of imaging data is a run; the DR3 contains data from 162 runs in the best database, and 164 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.

Except for the sky coverage, the pipelines and databases are identical in DR3 and DR2. Thus, DR3 is a proper superset of DR2. 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 from DR1 to DR2, please refer to About DR2 on the DR2 web site.

Imaging caveats

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 F_nu = 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, u_AB = u_SDSS - 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 z_AB = z_SDSS + 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 0.4% of the DR4 imaging footprint area (about 750 out of 200,000 fields, or 10/square degree) for DR3 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 new (for DR3) 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 DR3: 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

Note about galactic extinction correction

The EDR and DR1 data nominally corrected for galactic extinction. The spectrophotometry in DR2/DR3 is vastly improved compared to DR1, but the final calibrated DR2/DR3 spectra are not corrected for foreground Galactic reddening (a relatively small effect; the median E(B-V) over the survey is 0.034). This may be changed in future data releases. Users of spectra should note, though, that the fractional improvement in spectrophotometry is much greater than the extinction correction itself.

Problematic 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.

Mismatches between the spectroscopic and imaging data

For various reasons, a total of 663 spectroscopic objects do not have a counterpart in the best object catalogs, 0.2% of the total. See the caveat about mismatches between spectra and images on the data products page for spectra.

Advanced features not yet available in DR3

There are a number of advanced features or data products that are not yet available in DR3, but will be in the near future.

• Photometric redshifts for galaxies The DR1 Catalog Archive Server had a table with photometric redshifts for galaxies, including galaxies fainter than those in the spectroscopic survey. The redshifts still need to be computed and loaded into the database for DR3.
• Coverage masks Detailed coverage masks which will allow large-scale structure resarchers to easily calculate power spectrum and related quantities are in preparation.
• IQS/SQS The Imaging Query Server (IQS) and Spectro Query Server (SQS) form interfaces have been re-enginered to use a faster database. They currently do not provide a direct link to the DAS data download form; users need to save .csv files with the necessary information and upload these by hand to the DAS form.