High Quality Content by WIKIPEDIA articles! The ESO 3.6 m Telescope is an optical reflecting telescope run by the European Southern Observatory at La Silla Observatory, Chile since 1977, with a clear aperture of about 3.6 meters (140 in.) and 8.6 m2 area. It received an overhaul in 1999 and a new secondary in 2004. It was one of the largest optical telescopes in the world when it was completed in the late 1970s, and has supported many advanced optical and scientific achievements. It presented one of the first Adaptive Optics system available to the astronomical community, ADONIS: ADaptive Optics Near Infrared System in the 1980s.