CTAO's site in the southern hemisphere

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Southern Hemisphere Array

CTAO’s southern hemisphere or CTAO-South site is less than 10 km southeast of the European Southern Observatory’s (ESO’s) existing Paranal Observatory in the Atacama Desert in Chile, which is considered one of the driest and most isolated regions on earth – a paradise for stargazers.

CTAO Southern Hemisphere Site Rendering; credit: Gabriel Pérez Diaz, IAC / Marc-André Besel, CTAO

See more pictures on our Flickr page.

Array Coordinates:

Latitude: 24º 41′ 0.34″ South
Longitude: 70º 18′ 58.84″ West

Proposed Array Layout

While the northern hemisphere array will be more limited in size and will optimize the CTAO’s low- and mid-energy ranges from 20 GeV to 5 TeV, the southern hemisphere array will be centered on the medium and high-energy ranges of CTAO, optimized for gamma-ray energies from 150 GeV to 300 TeV, which implies the specialization of the southern array in Galactic targets. For the first construction phase, the approved Alpha Configuration, the plan is for the site to host a much larger array spread over 3 square kilometers with 14 Medium-Sized Telescopes to cover CTAO’s core energy range and 37 Small-Sized Telescopes to cover CTAO’s highest energy gamma rays.


The Alpha Configuration does not consider LSTs in the CTAO Southern array, but it includes the preparation of the foundation for four of them, as well as the foundation for three more SSTs, to allow for the construction of these telescopes in a future enhancement of the array. The positions of these telescopes are also included in the layout, which defines the geographical positions of the elements in the array (telescopes, calibration systems and atmospheric characterization devices).


Layout of the CTAO Southern array in the Atacama Desert (Chile), according to the Alpha Configuration.

Announcements and Construction Updates

Originally published (in English and Spanish) in March 2020 issue of CTA Newsletter.


Preparations for the south site have continued with layout planning, site surveys and testing, and equipment procurements. The first step for the CTA-South site is to plan the overall layout of the site’s supporting facilities (buildings, power substation, warehouse, etc.) and the required infrastructure, as well as to conduct an analysis of the current telescope positions to re-evaluate their construction feasibility. The telescope locations have been marked in the field and evaluated for potential issues due to loose soil, water accumulation and accessibility. Some difficulties were identified with a few of the telescope locations and could be moved to optimize construction costs.


In order to further test the site, some upgrades have been made to the existing weather tower and new equipment has been installed. The weather tower has a wind turbine now to power operations and is taking data again. Additionally, an existing F/(Ph)otometric Robotic Atmospheric Monitor (FRAM) was relocated to a more optimal position and a second FRAM was installed to characterize and monitor the atmospheric properties of the site before operations. Two additional meteorology towers, a dust analyser and a dust profiler have been procured, along with a diesel generator for the future site works.


As its first major step, the CTAO will begin building the CTA-South access road, data and power connections to the array as soon as the funding becomes available.

On 1 July 2019, Volker Heinz joined the CTAO as the CTA-South Site Manager. As a member of the CTA construction project, and later CTA operations, based in Chile, Volker has a key role in building up the CTA-South site team and leading and coordinating the on-site construction and operation of the observatory. Working closely with the CTAO Project Office, the ESO Project Coordination Office and external contributors involved in CTA, he will be responsible for a broad range of construction, logistics and operations matters. Read the announcement.

ESO’s Director General Xavier Barcons and Federico Ferrini, Managing Director of CTAO, sign the agreement for the construction and operation of CTA’s southern array within ESO’s Paranal site in northern Chile.  The ceremony took place on 19 December 2018 at the ESO offices in Santiago, Chile.

On 19 December 2018, the Cherenkov Telescope Array Observatory (CTAO) and the European Southern Observatory (ESO) signed the final agreements needed for CTA’s southern hemisphere array to be hosted near ESO’s Paranal Observatory in Chile.


A total of three agreements were signed over the course of the week: between the Chilean government and ESO; between ESO and CTAO (read press release in English or Spanish); and between the Chilean National Commission for Science and Technology (CONICYT) and CTAO (read press release in English or Spanish). With these three agreements in place, the CTAO will be able to begin construction on the southern site, which is expected to begin in 2020.


Read the full announcement on our News page.

Signs of progress toward construction are beginning to pop up around CTA’s southern hemisphere site in Chile. Pictured below, shipping containers at Paranal are being refurbished to serve as the CTA site offices and meetings rooms, while piles of stones serve as makeshift markers for the future homes of CTA telescopes.


Site infrastructure design studies are preparing the site for the final design of the facilities and services both above- and below-ground. The geotechnical study, which began in March and is estimated to be completed by the end of April, is being coordinated by the University of Warsaw and conducted by the Chile-based company Sondeal. Below, teams drill 39 boreholes and dig 20 strip excavations to understand the geological conditions of the site, which will be valuable input to the final foundation designs. The topographical study for site characterization will begin soon and be completed in May. The detailed designs of the buildings and technical facilities are in the advanced development stages and are expected to undergo internal reviews before summer 2017.

The number of site characterization instruments on the Armazones 2K site in Chile has grown significantly over the past few months as the start of construction draws closer. A 10-metre tower, the first tower installed on site in early 2014, hosts a weather station that measures temperature, humidity, pressure and wind characteristics. It also includes a Wi-Fi router to transmit data to Paranal via a microwave link. Thirty metres north of the first tower, a 30-metre tower includes three three-dimensional anemometers installed at different heights to measure the wind speed and profile. The “ASC complex” is located 30 metres east of the 10-metre tower and hosts an all-sky camera (ASC), a seismometer and a Sun and Moon photometer. Read the full announcement.

On 15 and 16 July 2015, the Cherenkov Telescope Array (CTA) Resource Board decided to enter into detailed contract negotiations for hosting CTA’s southern hemisphere array on the European Southern Observatory (ESO) Paranal grounds in Chile. The southern site is less than 10 km southeast of ESO’s existing Paranal Observatory in the Atacama Desert, which is considered one of the driest and most isolated regions on earth. In addition to the ideal conditions for year-round observation, collaboration with ESO offers CTA the opportunity to take advantage of existing infrastructure (roads, accommodation, water, electricity, etc.) and access to established facilities and processes for the construction and operation of the observatory. Read the press release.