A 6-m millimetre-wave telescope at an altitude of 5190 m on Cerro Toco in the Atacama desert of northern Chile designed to map the cosmic microwave background at wavelengths of 2, 1.4, and 1.1 mm. It was opened in 2007 and is operated by an international consortium of observatories and institutions led by Princeton University. It was due to become part of the new Simons Observatory in 2019. The ACT is similar to the South Pole Telescope.
http://act.princeton.edu Official telescope website.