Cosmology Large Angular Scale Surveyor, Radio telescope array in Atacama Desert, Chile

The Cosmology Large Angular Scale Surveyor (CLASS) is an array of four microwave telescopes on two altazimuth mounts at the Parque Astronómico de Atacama in Chile. One telescope observes at 40 GHz, one at 90 GHz, and one in bands centred at 150 and 220 GHz, with a resolution of about 1°. Its feedhorn-coupled transition-edge-sensor bolometers are cooled to 0.1 °C above absolute zero.

CLASS has been observing since 2016 from the Parque Astronómico de Atacama. Its 90 GHz detector array was upgraded in 2022 to increase sensitivity, and in 2024 the 90 GHz telescope's variable-delay polarization modulator was replaced by a rotating reflective half-wave plate to improve linear polarization measurements.

CLASS shares the Parque Astronómico de Atacama with ACT, APEX, ALMA, ASTE, CBI, CCAT-prime, NANTEN2, POLARBEAR, Simons Observatory and TAO, all drawn to the same dry, high-altitude conditions. Its name states its method: it surveys large angular scales, about 1°, roughly twice the apparent size of the Sun and Moon. The 90 GHz telescope's variable-delay polarization modulator was swapped in 2024 for a rotating reflective half-wave plate.

CLASS stands at about 5,200 m in the Parque Astronómico de Atacama, on the same high, dry Andean edge of the desert that hosts ACT, ALMA, APEX and Simons Observatory. It is an operating research facility, not a visitor site, so there are no public tours or marked access roads; the array is reached by observatory tracks. The thin air and cold, dry weather suit the telescopes, and the site is chosen for low water vapour.

The array can capture the polarization of cosmic background radiation with exceptional accuracy. This ability allows researchers to detect tiny details from the earliest times of the universe.