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\subsection{SDSS - a brief introduction}  The word “survey” in the astronomical context initially referred to what could be called a sky atlas - these were initially hand-drawn sky charts, and later photographic images. Currently the word survey largely denotes catalogues of astronomical sources and their properties (positions, fluxes, morphology, etc.). etc...).  They are used to systematically map the universe and characterize its astrophysical components with the aim of discovering new types of objects or phenomena, and for performing better tests of astrophysical and cosmological models. Surveys are often preceded by the development and introduction of new technology. Where the technological advancement allows us to observe the sky in some new way, for example, viewing images in a previously unexplored wavelength range. The Sloan Digital Sky Survey (SDSS) is one such astronomical sky survey. The SDSS is a five-filter imaging survey consisting mostly of the Northern Galactic Cap combined with a spectroscopic follow-up program. The survey and its extensions (SDSS-II and SDSS-III) cover approximately $14 500 degrees squared$ , which is nearly a third of the entire sky. SDSS’ main telescope is at Apache Point, New Mexico, and it is was specially designed to take wide field $(3^{\circ}\times 3^{\circ})$ images using a mosaic of thirty $2048 \times 2048$ pixel CCDs \cite{Djorgovski_2013}. The survey includes a spectroscopic survey of approximately 1.3 million objects — mostly galaxies, and quasars.