The Second Data Release of the Sloan Digital Sky Survey

TL;DR

The paper presents the Sloan Digital Sky Survey's Second Data Release (DR2), detailing substantial enhancements in imaging and spectroscopic data products and their processing pipelines. It addresses previously identified photometric and spectroscopic calibration issues by introducing robust flux measurements (notably cmodel magnitudes) and improved spectrophotometric calibration with star-by-star template fitting, yielding higher accuracy in fluxing and colors. DR2 expands public access to 3324 deg$^2$ of imaging (88 million objects) and 367,360 spectra, with dual imaging reductions to support target selection and analysis, thereby strengthening the dataset for cosmology, galaxy evolution, and stellar studies. The release emphasizes data quality, cross-consistency checks between photometry and spectroscopy, and practical guidance for users, while outlining plans for a larger DR3 release to extend coverage and data quality further.

Abstract

The Sloan Digital Sky Survey has validated and made publicly available its Second Data Release. This data release consists of 3324 square degrees of five-band (u g r i z) imaging data with photometry for over 88 million unique objects, 367,360 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 2627 degrees of this area, and tables of measured parameters from these data. The imaging data reach a depth of r ~ 22.2 (95% completeness limit for point sources) and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The imaging data have all been processed through a new version of the SDSS imaging pipeline, in which the most important improvement since the last data release is fixing an error in the model fits to each object. The result is that model magnitudes are now a good proxy for point spread function (PSF) magnitudes for point sources, and Petrosian magnitudes for extended sources. The spectroscopy extends from 3800 A to 9200 A at a resolution of 2000. The spectroscopic software now repairs a systematic error in the radial velocities of certain types of stars, and has substantially improved spectrophotometry. All data included in the SDSS Early Data Release and First Data Release are reprocessed with the improved pipelines, and included in the Second Data Release. The data are publically available as of 2004 March 15 via the web sites http://www.sdss.org/dr2 and http://skyserver.sdss.org .

Figures (4)

• Figure 1: The distribution on the sky of the imaging scans and spectroscopic plates included in DR2. This is an Aitoff projection in equatorial coordinates. The total sky area covered by the imaging is 3324 square degrees, and by the spectroscopy is 2627 square degrees.
• Figure 2: Improvements in model magnitudes for stars and galaxies in the DR2 reductions. The first panel shows the distribution of differences between $r$ band model and Petrosian magnitudes for red ($u-r > 2.22$; Strateva et al. 2001) galaxies brighter than $r_{Petro} = 19$; the three curves are for the old (DR1) reductions (dashed), the current reductions using model magnitudes (dotted; DR2), and the current reductions using cmodel magnitudes (solid; DR2C). The mode and standard deviation (based on the interquartile range) of each distribution are given. The bias in model magnitudes in the DR1 reductions is apparent. The second panel shows the same quantities for blue ($u-r<2.22$) galaxies. The third panel shows the difference between cmodel and PSF magnitudes for $r_{PSF} < 20$ stars, in the DR1 (dotted) and DR2 (solid) reductions; the width of the distribution has decreased by 40% with the new reductions.
• Figure 3: Comparison of synthetic $r$ magnitudes and $g-r, r-i$ colors synthesized from the spectra with photo fiber magnitudes. We have included all objects in DR2 with S/N per pixel $>5$.
• Figure 4: Comparison of white dwarf spectra and models. The grey lines represent 166 individual spectra divided by their best fit model. The heavy line is the median. The equivalent median residuals in DR1 were of order 15% at 4300Å; they are now of order a few percent.