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The Sloan Digital Sky Survey (see www.sdss.org for general information) will map one-quarter of the entire sky and perform a redshift survey of galaxies, quasars and stars. The DR4 is the fourth major data release and provides images, imaging catalogs, spectra, and redshifts for download.

About DR4 explains what is new in DR4, and lists remaining or new caveats and subtleties in the data.

Please refer to the credits page for our sources of funding, participating institutions, how to acknowledge the use of SDSS data in your publications. Please also note how to refer to SDSS sources in your publications using the proper IAU nomenclature for SDSS sources.


Footprint area 6670 sq. deg.
Imaging catalog 180 million unique objects
Data volume
images7.5 TB
catalogs (DAS, fits format)1.5 TB
catalogs (CAS, SQL database)3.0 TB
Average wavelengths and magnitude limits
(95% detection repeatability for point sources)
u g r i z
3551Å 4686Å 6165Å 7481Å 8931Å
22.0 22.2 22.2 21.3 20.5
PSF width 1.4" median in r
Photometric calibration
r u-g g-r r-i i-z
2% 3% 2% 2% 3%
Astrometry < 0.1" rms absolute per coordinate


Spectroscopic area 4783 sq. deg.
Wavelength coverage 3800-9200Å
Resolution 1800
Signal-to-noise >4 per pixel at g=20.2
Redshift accuracy 30 km/sec rms for main galaxy sample (from repeat observations)
Target magnitude limits for main samples Galaxies: Petrosian r <17.77
Quasars: PSF i <19.1
Spectroscopic catalog 849,920 spectra, classified into
565,715 Galaxies
67,382 Quasars (redshift <2.3)
9,101 Quasars (redshift >2.3)
102,714 Stars
50,373 M stars and later
44,363 Sky spectra
10,272 Unknown class
Data volume
calibrated spectra ("2d")51 GB
spectra, redshifts, line measurements ("1d")140 GB

Previous data releases

For reference, we provide links to earlier data releases. The current data release always provides both the largest sky coverage and the highest-quality data and reductions.

This is version v4_20080930_1838 .

If you are using Netscape 4.x and see oversized fonts, please look at the workaround on the Credits page.


DR6 is now available!

Imaging Sky Image
SDSS DR4 Imaging Sky Coverage (Aitoff projection of Equatorial coordinates)

Spectral Sky Image
SDSS DR4 Spectral Sky Coverage (Aitoff projection of Equatorial coordinates)