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The Scope of DR8

Introduction

SDSS-III has committed to publicly release its raw and reduced data sets. We are doing so using the same highly successful systems used by SDSS-I and -II, including a Catalog Archive Server for retrieval of catalog data from a powerful SQL database and a Science Archive Server for retrieval of calibrated spectra and images.

Data Release 8 (DR8), made publicly available in January 2011, contains the full imaging survey from the SDSS imaging camera (including a large contiguous area in the Southern Galactic Cap, centered on RA ~ 0) and new spectra from the last year of operations of the SDSS spectrograph from the SEGUE-2 project. All of the imaging data have been re-reduced through new pipelines.

A publication accompanies the data release which details the differences between DR8 and its predecessors. Major differences from the DR7 release are:

  1. 2395 deg2 of additional imaging, to give contiguous coverage over 3172 deg2 of the Southern Galactic Cap.
  2. New reductions and calibrations of all imaging, using an improved sky subtraction algorithm.
  3. 211 new plates from the SEGUE-2 survey.
  4. Reanalysis of the stellar parameters of all stars.
DR8 spectroscopic coverage
DR8 imaging and spectroscopic coverage in Equatorial coordinates (plot centered at RA = 6h, or 90 deg). The greyscale shows the coverage of the Legacy spectroscopic survey, while the red and green dots show the SEGUE-1 and SEGUE-2 surveys, respectively.

The figure shows the resulting imaging coverage and spectroscopic coverage for DR8. The spectroscopic surveys were executed through several programs: SDSS Legacy, SDSS special programs, SEGUE-1, and SEGUE-2. In the tables below, we cite a few numbers of interest with regard to the imaging and spectroscopic data.

The data is defined as a set of photometric runs and a set of spectroscopic plates (see the basics on imaging and spectroscopy). We provide links here to ASCII and FITS lists of the runs and the plates. These lists are essentially a summary of all of the data in the data release.

Imaging Data Statistics

Total unique area covered 14,555 square degrees
Total area of imaging (including overlaps) 31,637 square degrees (excluding supernovae runs)
Individual image field size 1361x2048 pixels (0.0337 square degrees)
Number of individual fields 938,046 (excluding supernovae runs)
Number of catalog objects 1,231,051,050
Number of unique detections 932,891,133
Number of unique, primary sources
Total469,053,874
Stars260,562,744
Galaxies208,478,448
Unknown12,682
Effective wavelengths and magnitude limits
(95% completenesss for point sources)
u g r i z
3551 Å 4686 Å 6165 Å 7481 Å 8931 Å
22.0 22.2 22.2 21.3 20.5
Median PSF FWHM, r-band 1.3 arcsec
Pixel scale 0.396 arcsec
Exposure time per band 53.9 sec
Time difference between observations of each band 71.72 sec (in riuzg order)
Relative photometric calibration accuracy
(Padmanabhan et al. 2008)
u g r i z
2% 1% 1% 1% 1%
Global astrometric precision 0.1 arcsec rms (absolute)

Spectroscopic Data Statistics

Area covered
Full unique coverage 9274 square degrees
SEGUE-1 coverage 1438 square degrees
SEGUE-2 coverage 1317 square degrees
Legacy coverage 7966 square degress
Number of plates
Category Total Good/marginal Primary
All programs 2880 2764 2654
Legacy 1926 1869 1794
SEGUE-1 442 427 407
SEGUE-2 211 211 204
Special 301 257 246
Plate area 1.49 deg radius, 6.97 deg2
Fibers per plate 640
Numbers of spectra
Category Total On good or
marginal plates		 Unique
All programs 1,843,200 1,768,960 1,629,129
Main galaxy targets 778,410 755,111 711,726
LRG targets (excluding Main) 106,650 103,662 95,990
SEGUE-1 targets 250,422 242,008 220,851
SEGUE-2 targets 128,112 128,112 118,151
Stars 600,967 577,157 521,990
Galaxies 952,740 921,007 860,836
QSOs 130,300 126,368 116,003
Skies 110,288 103,046 93,187
Unknown 48,905 41,382 37,113
Wavelength coverage 3800 to 9200 Å
Resolution 1800 to 2000
Median S/N at g=20.2 4.2 per pixel (Legacy)
7.0 per pixel (SEGUE-1, -2)
Typical redshift accuracy 30 km/s rms for main galaxy sample (from repeat observations)
4.0 km/s rms for SEGUE near g=18th mag (from repeat observations)
1.8 km/s systematic limit for high signal-to-noise stars
Approximate magnitude limits
(Corrected for Galactic dust extinction)
Main sample galaxies Petrosian r < 17.77 Strauss et al. (2002)
Luminous Red Galaxies Petrosian r < 19.2 Eisenstein et al. (2001)
z < 3 quasars PSF i < 19.1 Richards et al. (2002)
z > 3 quasars PSF i < 20.2 Richards et al. (2002)
SEGUE-1 Faint PSF 17.8 < r < 20.1 Yanny et al. (2009)
SEGUE-1 Bright PSF r < 17.8 Yanny et al. (2009)
SEGUE-2 PSF r < 20.2

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