The Sloan Digital Sky Survey or SDSS is a photometric system of five filters (named u, g, r, i and z) imaging and spectroscopic redshift survey using a dedicated 2.5 meter wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. Data collection began in 2000, and the final imaging data release covers over 35% of the sky, with photometric observations of around 500 million objects and spectra for more than 3 million objects. The SDSS telescope uses the drift scanning technique, which keeps the telescope fixed and makes use of the Earth's rotation to record small strips of the sky. The image of the stars in the focal plane drifts along the CCD chip, instead of staying fixed as in tracked telescopes. This method allows consistent astrometry over the widest possible field and precision remains unaffected by telescope tracking errors. The disadvantages are minor distortion effects and the CCD has to be written and read in the same time.