 Although extremely large telescope designs are large, they can have smaller apertures than the aperture synthesis on many large optical interferometers.However, they may collect much more light, along with other advantages.The Keck Observatory (2 x 10 m) and the Very Large Telescope, of the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile, measure 4 × 8.2 m and 4 × 1.8 m, all on separate mounts but in one building for interferometry.