The Chilean site for CMB-S4 telescopes is in the Parque Astronómico Atacama, in the high Atacama desert close to the borders with Argentina and Bolivia. Not only is this one of the driest places on the planet, but at 5200m (17,000ft) it is the highest site on earth for CMB observations, putting the telescopes above the vast majority of the atmosphere. It is also a relatively accessible and developed site, with CMB experiments having been deployed at the site for over 20 years and the ALMA observatory and its associated infrastructure just 10km away.
Located only 23° south of the equator, a very large fraction of the sky can be seen from this site. This allows us to conduct the wide survey needed to detect light relic particles, as well as to catalog huge numbers of galaxy clusters and transient events. The survey footprint, covering about 70% of the sky, will also overlap with several other Southern Hemisphere observatories, including the Dark Energy Survey and the Rubin Observatory, allowing us to study their combined data sets.
Although the South Pole sits at a comparatively modest 2800m (9300ft), its extremely low temperature results in an exceptionally dry, stable atmosphere. The National Science Foundation’s Office of Polar Programs runs the US Antarctic Program (USAP), with the Amundsen-Scott South Pole Station providing a wealth of critical infrastructure for a wide range of experiments. CMB telescopes have been deployed at the South Pole for some 40 years, sited in the so-called “Dark Sector”, an area about 1 km from the station and which is operated as a zone of low radio-frequency emission to reduce the likelihood of interference with the sensitive instruments there. Access to the site is via the USAP’s logistic center in Christchurch, New Zealand, and its McMurdo Station on at Antarctic coast directly south of Christchurch. Fuel and large items of equipment are traversed overland from McMurdo while smaller items of equipment and people are flown in. The site is only accessible during the Austral summer, and experiments at the South Pole leave a skeleton team of scientists to run things during the winter.
Being on the earth’s axis, the same area of sky is always visible from the South Pole. While this limits the total sky area that can be seen, it does enable us to observe the same patch of sky continuously. The South Pole site is therefore ideal for the deep survey required to detect the signals from gravitational waves from inflation, as well as providing a deep complement to the wide survey cluster and transient catalogs.