 The GTC began its preliminary observations on 13 July 2007, using 12 segments of its primary mirror, made of Zerodur glass-ceramic by the German company Schott AG.Later, the number of segments was increased to a total of 36 hexagonal segments fully controlled by an active optics control system, working together as a reflective unit.Its first instrument was the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS).