The Atacama Pathfinder Experiment (APEX) is a radio telescope located at 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert in northern Chile, 50 kilometers to the east of San Pedro de Atacama. The main dish has a diameter of 12 meters and consists of 264 aluminium panels with an average surface accuracy of 17 micrometres (r.m.s.).