In December 2013 Gaia satellite was launched by ESA (European Space Agency). Since the start of the nominal operations in mid 2014 the instrument is collecting all-sky images of our galaxy. During its five years of operation the satellite will detect a billion of celestial objects with the objective of calculating with accuracies up to tens of microsecond of arc, for each of them, the fundamental astrometric parameters: position, proper motion and parallaxes. The scientific instruments is made of two identical telescopes pointing two fields of view separated by an angle of 106.5° which share a common focal plane. The focal plane is made of 106 CCDs, of which 62 are devoted to astrometry.