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In Atacama, the construction of CTAO-South brings together engineering, astronomy, and data processing to observe atmospheric flashes linked to gamma rays, in an international project focused on the most energetic phenomena in the Universe.
The Atacama Desert in Chile has begun receiving the infrastructure for the CTAO-South, the southern part of the Cherenkov Telescope Array Observatory, an international project dedicated to observing very high-energy gamma rays.
The array will be installed near the Paranal Observatory, operated by the European Southern Observatory, the ESO, and will have more than 50 telescope foundations and about 17 km of internal roads, according to information released by the ESO and the CTAO.
The construction opens a new phase for the observatory, which will be used to study phenomena associated with the highest known energies in the Universe.
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Among the scientific targets mentioned by the CTAO are supernova remnants, regions near black holes, neutron stars, gamma-ray bursts, and possible signals related to dark matter.
At the Chilean site, the arrangement was planned to record gamma rays between 80 GeV and 300 TeV, a range that includes some of the most energetic emissions sought by ground-based astronomy.
The official start of construction of the southern site was celebrated on December 17, 2025, in a ceremony that brought together representatives from the CTAO, the ESO, and Chilean authorities.
The event took place after the signing of contracts for the basic infrastructure of the complex, a necessary step to prepare the bases, access, and support areas where the telescopes will be installed.
Observatory in Atacama will have 51 telescopes
The CTAO-South will be less than 10 km from the Paranal Observatory, in a region used for decades for astronomical projects because of the atmospheric conditions and low level of light interference.
The ESO describes the Atacama as one of the driest and most isolated areas on the planet, a characteristic that favors astronomical observations made from the ground.
The telescopes will not directly record the gamma rays.
When these particles from space hit the Earth’s atmosphere, they produce cascades of secondary particles.
This process generates very quick flashes of bluish light, known as Cherenkov light, which can be captured by specialized instruments.
From these brief signals, scientists can reconstruct the probable direction and energy of the original gamma ray.
The combined analysis of various telescopes allows associating the records with specific regions of the sky and, in some cases, with astrophysical objects or events already identified by other observatories.
The planned configuration for the Chilean site includes 51 telescopes distributed over about 3 km².
There will be 14 medium-sized telescopes, called MSTs, and 37 small-sized telescopes, known as SSTs.
According to the CTAO, the MSTs cover the intermediate energy range, while the SSTs are designed to observe gamma rays at the highest energies.
CTAO will be divided between Chile and Spain
The CTAO will have two sets of telescopes.
In addition to the Chilean facility, the project includes an arrangement in the Northern Hemisphere, in La Palma, in the Canary Islands, Spain.
The division between the two hemispheres allows for expanded sky coverage and the observation of sources not visible from a single latitude.
In Chile, the focus is on medium and high energies, from 80 GeV to 300 TeV, with special attention to galactic targets.
At the northern site, the configuration will be primarily aimed at lower and intermediate energies, from 20 GeV to 50 TeV, according to the CTAO.
This difference does not make the sets competitive.
On the contrary, they compose the same observatory and will have complementary functions.
With both arrangements in operation, the CTAO will be able to observe a broader energy range and monitor phenomena visible in different regions of the sky.
According to the ESO, the observatory is designed to achieve a sensitivity superior to that of current ground-based gamma-ray astronomy instruments.
This estimate is presented by the consortium itself as one of the scientific justifications for the construction of a set with dozens of telescopes, spread over extensive areas and operating in a coordinated manner.
Mirrors and cameras enter the preparation of the telescopes
While civil works advance in Chile, telescope components are developed and tested in European laboratories.
The IRFU, a French institute linked to CEA Paris-Saclay, reported that it participates in the project with the production and validation of mirrors and equipment associated with the medium-sized telescopes.
According to the institute, the contribution includes 700 mirrors destined for seven of the 14 MSTs planned for the southern site.
The development of these pieces began in 2009, in collaboration with the French company Kerdry, based in Brittany.
The final design was completed in 2019, after technical adjustments required by the project.
Before shipping, each mirror undergoes characterization in a dark room at the IRFU.
The first two batches, with 100 mirrors each, were described by the institute as part of the preparation to equip the first MSTs starting in 2026.
The information was kept attributed to the IRFU because the schedule may depend on the progress of the construction and the integration of the telescopes.
Another component in preparation is the NectarCAM, a camera developed for medium-sized telescopes of the CTAO.
According to the IRFU, the instrument measures about 3 m by 3 m by 1.5 m, weighs approximately 2.3 tons, and comprises 1,855 pixels organized in 265 modules.
The reported field of view is about 8 degrees.
There is, however, a discrepancy between the institutional materials consulted.
The CTAO’s technical page states that the MSTs use two camera models and associates the NectarCAM with the northern site, while the FlashCam is linked to the southern site.
Meanwhile, the IRFU material mentions the shipment of the first NectarCAM to the southern site.
For this reason, the information was presented as attributed and the discrepancy was recorded in the final note.
Gamma rays will be converted into sky maps
After capturing the Cherenkov light, the data undergo processing so that the recorded signals are transformed into sky maps, light curves, and energy measurements.
This step is necessary because the telescopes detect indirect effects of gamma rays, not the original particles that arrived from space.
The CTAO adopted Gammapy as an open package for scientific data analysis.
The tool, developed with the participation of researchers from various countries, allows processing of gamma-ray observations and producing results used in studies of cosmic sources.
The adoption of the software was announced by the CTAO in 2021.
The computational dimension is also part of the observatory’s design.
According to the CTAO, the expected data volume before compression could reach hundreds of petabytes per year, with subsequent reduction to about 12 PB annually.
These numbers help explain why the project involves, in addition to the telescopes, processing centers and teams dedicated to data management.
The operational model provides for open scientific access after the proprietary period.
The CTAO also informed that 10% of the observation time of the southern array will be reserved for Chilean researchers, due to the agreements made for the installation of the observatory in the country.
Cosmic phenomena are among the targets of CTAO-South
Gamma-ray astronomy allows the study of environments where particles are accelerated to very high energies.
According to ESO, this observation range can be used to investigate transient events, compact objects, and regions of the Universe that emit radiation under physical conditions different from those observed in visible light.
Among the topics mentioned by teams involved in the project are studies on the central region of the Milky Way, galactic and extragalactic surveys, transient sources, gamma-ray bursts, dark matter, and multi-messenger research.
This last area combines different types of signals, such as gamma photons, neutrinos, and gravitational waves, to analyze the same cosmic event.
The array installed in Chile will have a specific role in this set because of the energy range it will be able to cover and its position in the Southern Hemisphere.
The location favors the observation of regions of the Milky Way that are not seen in the same way by instruments installed in the Northern Hemisphere.
With dozens of telescopes distributed in Atacama, mirrors tested in the laboratory, and camera systems developed to capture very short-duration flashes, the CTAO-South will be used to investigate cosmic sources that still challenge high-energy astronomy.
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