The SDSS Data Release 2 (DR2)

Contents

• What DR2 contains
• Improvements to image processing
• Caveats for imaging data
• Improvements to spectroscopic processing
• Caveats for spectroscopic data
• Advanced features not yet available in DR2

What DR2 contains

The DR2 imaging data cover 3324 square degrees, and include information on roughly 88 million objects. The DR2 spectroscopic data include data from 574 plates of 640 spectra each, and cover 2627 square degrees.

The DR2 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 2002, and the spectroscopy associated with that area obtained before that date. In fact, 34 square degrees of DR2 imaging data in the Nothern Galactic Cap lie outside this ellipse. While the DR2 scans do not repeat a given area of sky, they do overlap to some extent, and the data in the overlaps are included in DR1 as well. The DR2 includes reprocessing of all data included in DR1, and those data in EDR that pass our data-quality criteria for the official survey. The sky coverage of the imaging and spectroscopic data that make up DR2 are given on the coverage page. The natural unit of imaging data is a run; the DR2 contains data from 105 runs in the best database, and 105 runs in the target database. The DR2 includes all data released as part of the EDR and DR1, reprocessed as described below.

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.

Improvements to image processing

• Model magnitudes

  There was a serious bug in the computation of model magnitudes in the DR1 and EDR processing, having to do with the model of the PSF used. This bug caused systematic errors in the derived scale sizes of galaxies, and caused model magnitudes of bright galaxies to be systematically incorrect. This has now been fixed in DR2. See the description of model magnitudes in the Photometry section of algorithms for a detailed discussion of this problem and its fix. Which magnitudes should I use has further discussion of which magnitudes to use when. The changes in the model magnitudes have also necessitated changes in the target selection for Luminous Red Galaxies.

• The deblender

  The behavior of the deblender of overlapping images has been further improved since the DR1; these changes are most important for bright galaxies of large angular extent (> 1′). In the EDR, and to a lesser extent in the DR1, bright galaxies were occasionally "shredded" by the deblender, i.e., interpreted as two or more objects and taken apart. With improvements in the code that finds the center of large galaxies in the presence of superposed stars, and the deblending of stars superposed on galaxies, this shredding now rarely happens.

• Improvements to PSF modelling

  The PSF is measured from atlas images roughly 7" across for stars; any error in the sky level determined from these images couples to spatial variability of the PSF by the Karhunen-Loève expansion used to model the PSF. This manifested itself in systematic offsets between the PSF and model magnitudes of stars of several hundredths of a magnitude, even with the fixes in the model magnitude code described above. This zero-point term in the PSF is now explicitly suppressed.

  The pixel size is 0.396", giving well-sampled images for the typical seeing of 1" or more. On rare occasions when the seeing became much better than 0.9" (FWHM), the undersampling causes the code that found stars suitable for determining the PSF to miss many objects, yielding an incorrect PSF and therefore poor stellar photometry (the seeing was never good enough in the runs included in DR1, so this error was not triggered). Changes to the thresholds for the selection of PSF stars have solved this problem.

• Photometry of saturated objects

  When an image is saturated in the SDSS imaging data, the wells overflow and a bleed trail results. However, the total number of electrons associated with the object, bleed trail and all, still at least approximately reflects the brightness of the object. For objects for which the flag HAS_SATUR_DN is set in a given band, the imaging pipeline includes the counts associated with the bleed trail of saturated objects in flux measurements. In particular, the fiber, Petrosian, PSF, and model magnitudes include this light, and it is added to the central value of the radial profile (i.e., profMean[0]). As the pipeline works on a single frame at a time, bleed trails that cross frame boundaries will not be properly accounted for. In addition, the fluxes of close pairs of saturated stars whose saturated regions overlap will not be correct.

• Astrometry of objects not detected in r

  Astrometry for each object is referred to the reference frame of the r-band images. DR1 had a bug in the reported right ascension and declination (and all other celestial coordinates, such as l and b) for those rare sources that are not detected in the r band (for example, cool brown dwarfs and z > 5.7 quasars). This bug has been fixed in DR2 and the positions of z-band only detections are now correct.

• Improved proper motions

  The EDR and DR1 match each SDSS object to the nearest object in USNO-A2.0 (Monet et al. 1998), using a 30" matching radius. USNO-A2.0 provides positions at a single epoch (no proper motions are provided), based on POSS-I plates. Proper motions are then calculated based on the SDSS and POSS-I positions, with a typical time baseline of 50 years. For motions greater than ≈40 mas/year, corresponding to separations between the SDSS and USNO-A2.0 positions of greater than 2 arcsec, contamination by false matches becomes significant and rises with increasing motion/separation. The DR2 reductions use USNO-B1.0 (Monet et al. 2003), which provides positions and proper motions based on various Schmidt photographic surveys (primarily POSS-I and POSS-II in the area of sky covered by SDSS). Each SDSS object is matched to the nearest USNO-B1.0 object within 1", after first converting the USNO-B1.0 positions to the epoch of the SDSS observations. This eliminates nearly all of the false matches, yielding much cleaner samples of high proper motion stars. The USNO-B1.0 proper motion is then given for each matching SDSS object.

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 phot ometry, 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 0.3% of the DR2 imaging footprint area (about 300 out of 100,000 fields, or 10/square degree) for DR2 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.

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 DR2: 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.

Improvements to spectroscopic data processing

Improvements in spectrophotometry

There have been three substantial improvements to the algorithms which photometrically calibrate the spectra: (1) improved matching of observed standard stars to models; (2) tying the spectrophotometry directly to the r-band fiber magnitudes measured by the most recent version of the photometric pipeline; and (3) no longer using the "smear" exposures. These are described in detail on the algorithms page for spectrophotometry. These changes result in rms differences between synthesized photometry from the spectra, and the directly measured photometry, of 0.04 mag. The improvements also remove a number of unphysical wiggles which appeared in the blue end of the spectra.

Improvements in radial velocities

An error in the radial velocity templates for some types of stars caused systematic errors of up to 40 km/s in the EDR and DR1; these have now been fixed in DR2. The quality of radial velocities is described in detail under quality of stellar radial velocities.

Spectroscopy caveats

Note about galactic extinction correction

The EDR and DR1 data nominally corrected for galactic extinction. The spectrophotometry in DR2 is vastly improved compared to DR1, but the final calibrated DR2 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 DR2

There are a number of advanced features or data products that are not yet available in DR2, 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 DR2.
• 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.