Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS ) is a screening of a quarter of the sky by shooting at five wavelengths and subsequent spectroscopy of individual objects.

SDSS is a joint project of institutes in the United States, Japan, Korea and Germany, the funding was initiated by the Alfred P. Sloan Foundation. With a specially designed telescope at Apache Point Observatory, the positions and brightnesses of more than 100 million celestial objects to be measured. With spectra of over a million galaxies and quasars whose distances and properties are to be determined. Unlike previous surveys ( such as the Two Degree Field Galaxy Redshift Survey), SDSS works exclusively with electronic detectors, which, compared to photographic plates linearity and significantly higher sensitivity. The observations are in progress since 1998. Parts of the data base are now astronomers outside the participating institutes available.

Telescope and observational method

Specifically a telescope was built with 2.5 m primary mirror diameter at the Apache Point Observatory for the Sloan Digital Sky Survey. Is equipped the telescope with a camera from a total of thirty large CCD chips, each with 2048 × 2048 pixels. The CCDs are arranged in five rows of six chips. Observe the five rows by various optical filters (U ' g' r ' i' z ') at wavelengths of approximately 354, 476, 628, 769 and 925 nm, the observed sky area is scanned by utilizing the earth's rotation in narrow strips. One night, about 200 GB of image data are obtained. The SDSS is to observe an approximately 10,000 square degrees large area at the north pole of the Milky Way. In the five bands while objects of 22.3, 23.3, 23.1, 22.5 and 20.8 magnitudes with at least five times the noise can be detected. In the autumn of this area is inaccessible. Instead, then a strip of 225 square degrees of the plane of the Milky Way is observed up to about 24.4, 25.3, 25.1, 24.4 and 22.9 mag south.

The brightness and shape of celestial objects is automatically measured and a first classification by type and distance performed.

From selected objects from the survey spectra are obtained. To a plate 640 attached to the corresponding positions holes is brought into the focal plane of the telescope. Glass fibers carry the light of the 640 objects from these holes to a spectrograph. One night, about 6 to 9 such sets of 640- spectra can be obtained.


Although SDSS covers only a part of the sky, but can significantly fainter objects than previous record sky surveys such as the Palomar Observatory Sky Survey. Through precise photometry in five filters the nature and redshift of galaxies, quasars and stars can be estimated immediately. Accurate redshifts and classifications provide the spectra. Further investigations can be carried out by other observatories with even more intense telescopes then if necessary.

An important goal of the SDSS is the mapping of the foam-like large-scale structure of the universe, consisting of clusters of galaxies, filaments with a lower density of galaxies and intervening cavities ( voids) with very few galaxies.

The spectra contain many large sample many different types of galaxies that make up leave their properties determined with good statistical significance. With SDSS but can also be found extremely rare objects. The farthest currently known (2005) quasars at redshift 6 were discovered in the SDSS.

The Distributed - Computing Project MilkyWay @ home also uses the data obtained from the SDSS for research in computer science and computer science Astro.

In addition to results

The work on the SDSS has in addition to the large-scale mapping of a number of other - and by the initiators at the time of unplanned - brought results:


Due to the imaging technique by means of narrow strips in different spectral asteroids can very easily see from the parallax and determine their colors with great accuracy. The number of such measured asteroid has grown by the SDSS project by more than 100 times. Here, the theory seems to confirm that certain asteroid families (eg, Vesta, Eos, Koronis, and Themis asteroid ) each have a common body as the origin. The hypothesis that the surface of an asteroid " weathered " by the solar wind and micrometeorites is supported by the SDSS measurements.

Brown dwarfs

The recordings in two infrared bands with three other colors at the same time allows the SDSS team, very easy and safe to identify brown dwarfs. The previous measurement results led to two more in addition to the previously used M- spectral type - even cooler - spectral types have been defined: L and T. objects with the classification L9 do not have a surface temperature of about 1400 K, T9 dwarfs of only 700 K.

Stellar streams

SDSS data provided brightnesses, colors and highly precise position information for the fixed stars, which are spread over a vast area and common characteristics: they each belong to a stream of stars with common motion and origin. With so many members of the former security globular cluster Palomar 5, and the dwarf galaxy in Sagittarius ( Sagittarius Dwarf Spheroidal Galaxy) could be identified. A stream winds around the entire Milky Way, could come from the Canis Maior dwarf galaxy.

Dwarf galaxies of the Milky Way

Secondary star streams about 10 dwarf galaxies were discovered as companions of the Milky Way using the SDSS data since 2005. These are hardly recognizable due to their very low stellar density with conventional methods.

Gravitational lenses

Among the approximately 50,000 quasars were discovered with SDSS, there is also a number of gravitational lens cases.