APEX at night

The Atacama Pathfinder Experiment (APEX) is a 12-metre antenna to observe the sky in millimetre and submillimetre light. APEX is located at 5100 metres altitude on the Chajnantor Plateau, near San Pedro de Atacama, in the II Region of Chile. In the picture on the right is the 5600-metre-high Cerro Chajnantor. Millimetre and submillimetre radiation is difficult to detect from the ground because it is strongly absorbed by the water vapour in the Earth's atmosphere. For this reason, an extremely dry and high altitude site is needed, such as Chajnantor.