The Atacama Pathfinder EXperiment (APEX) – a new submillimeter facility for southern skies –

APEX, the Atacama Pathfinder EXperiment, has been successfully commissioned and is operational. This novel submillimeter telescope is located at 5107 m altitude on Llano de Chajnantor in the Chilean High Andes, on what is considered one of the world's outstanding sites for submillimeter astronomy. The primary reflector with 12 m diameter has been carefully adjusted by means of holography. Its surface smoothness of only 17–18 m makes APEX suitable for observations up to 200 m, through all atmospheric submm windows accessible from the ground. First scientific results will be presented in the accompanying papers of this special issue.