Simons Observatory, Radio telescope and cosmic microwave background experiment in Atacama Desert, Chile

The Simons Observatory is a radio telescope facility in the Atacama Desert of northern Chile, made up of one large telescope and three smaller ones positioned at around 17,000 ft (5,200 m) above sea level. The telescopes are pointed at the sky to detect cosmic microwave background radiation, a faint radio signal that comes from the early universe.

The Simons Observatory was planned in the 2010s and began operations in the early 2020s, following a series of earlier experiments studying the cosmic microwave background in the same region. It was built on the experience gained from those earlier projects, which had already established the Atacama Desert as a key site for this type of research.

The observatory is named after the Simons Foundation and its founders, hedge-fund billionaire and philanthropist Jim Simons, who died on May 10, 2024, and his wife Marilyn, a trained economist. The collaboration counts over 300 scientists at more than 35 institutions worldwide, and the site's data are shared across the whole team.

The observatory stands at 5,200 m inside the Chajnantor Science Preserve, reached by road from San Pedro de Atacama. Access is controlled by the managing institutions, so arrange authorization in advance. The altitude makes acclimatization advisable, and the site is exposed to intense sun and cold nights.

The three small-aperture telescopes are refractors with three aspheric silicon lenses and a rotating half-wave plate, each with a field of view over 35 degrees. Each sits inside a fixed ground screen that reflects diffraction from co-moving screens back to the sky, a measure against stray ground signals in sidelobes.