The Sloan Digital Sky Survey or SDSS is a major redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico.

Named after the Alfred P. Sloan Foundation , it aims to map 25% of the sky and obtain observations on around 100 million objects. It has redshifts for galaxies as far z=0.3 or 0.4, and has been involved in the detection of quasars beyond a redshift 6.

Observations

SDSS uses a dedicated 2.5-m wide-angle optical telescope, and takes images through five filters (named u, g, r, i and z). These images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended (as a galaxy might) and how the brightness on the CCDs relates to various kinds of astronomical magnitude.

Using this data targets are also selected for spectroscopy. The telescope is capable of recording 640 spectra at any one time by feeding an optical fibre for each through holes drilled in an aluminium plate. Each hole is individually positioned for the target in question.

Data releases

To date there have been three data releases:

• The Early Data Release or EDR was released in June 2001 covering 462 square degrees and 53,000 spectra.
• Data Release 1 or DR1 was released in April 2003 covering 2099 square degrees and 186,000 spectra.
• Data Release 2 or DR2 was released in March 2004 covering 3324 square degrees and 367,000 spectra.

Data access

The survey makes the data releases available over the internet. The SkyServer provides a range of interfaces to an underlying Microsoft SQL database. Both spectra and images are available in this way, and interfaces are made very easy to use so that, for example, a full colour image of any region of the sky covered by an SDSS data release can be obtained just by providing the coordinates. The SkyServer also provides a range of tutorials aimed at everyone from schoolchildren up to professional astronomers.

The raw data (from before it was processed into databases of objects) is also available through another internet server.

Results

Along with publications describing the survey itself SDSS data has been used in publications over a huge range of astronomical topics. The SDSS website has a full list of these publications covering distant quasars at the limits of the observable universe, the distribution of galaxies, the properties of stars in our own galaxy and also subjects such as dark matter and dark energy in the universe.

External links

• SDSS Homepage
• The SkyServer