APEX, the Atacama Pathfinder Experiment, is a 12 m diameter submillimetre telescope at 5100 m altitude in the Atacama Desert in northern Chile.

APEX was inaugurated in September 2005, and provides new opportunities for astronomers to use submillimetre observations to study the first galaxies and how stars and planets are formed.

The APEX project (Güsten et al. 2006, Astronomy & Astrophysics, vol. 454, L3) is a collaboration between Onsala Space Observatory, Max Planck Institut für Radioastronomie (Germany), and the European Southern Observatory to construct and operate a modified ALMA prototype antenna as a single dish on the high altitude site of Llano Chajnantor. The telescope was supplied by VERTEX Antennentechnik in Germany. Onsala Space Observatory, through the Group for Advanced Receiver Development (GARD), provides facility heterodyne receivers (SHFI) for several frequency bands up to 1.3 THz (Belitsky et al. 2006, Proceedings of SPIE Vol. 6275, 62750G; for astronomy related information see Vassilev et al. 2008, Astronomy & Astrophysics, vol. 490, 1157). The entire Astronomy & Astrophysics issue 454 No 2 (August 2006) was an APEX special booklet.

In scientific publications containing APEX data please acknowledge the project with the following footnote: "Based on observations with the Atacama Pathfinder EXperiment (APEX) telescope. APEX is a collaboration between the Max Planck Institute for Radio Astronomy, the European Southern Observatory, and the Onsala Space Observatory."