The European Southern Observatory (ESO)

by Ludwig Scheibe (TU Berlin), March 2025

The European Southern Observatory is a large intergovernmental collaboration of 16 European countries along with the host country Chile and strategic partners. While ESO’s headquarters are in Garching, Germany, it operates three state-of-the-art astronomical observation facilities in the Atacama desert in Chile, giving astronomers unparalleled access to the southern sky.

The observatory sites

La Silla, ESO’s first observatory, is located about 600km north of Santiago de Chile, at an altitude of 2400 meters above sea level. Among the telescopes there is ESO’s 3.6-meter optical telescope. Installed there is the HARPS spectrograph, which has helped discover over 160 exoplanets using the radial velocity method.

A 360 degree panorama view of the La Silla observatory. The dome in the center is the 3.6-m-telescope, which hosts the planet-finding spectrograph HARPS. Credit: ESO/F. Kamphues under CC BY 4.0.

Paranal, located about 130 km south of Antofagasta at an altitude of 2600 m, is considered ESO’s flagship facility. it features the Very Large Telescope (VLT), a collection of four individual 8-meter optical telescopes that can be interferometrically combined. Here, ESPRESSO is set up, one of the world’s most advanced measurement instruments for radial velocity.

Bird’s eye view of the VLT at ESO’s Paranal observatory. The four large structures are the main telescopes, and the smaller white domes the auxiliary telescopes. Credit: G. Hüdepohl (atacamaphoto.com)/ESO under CC BY 4.0.

Llano de Chajnantor is the third major ESO observing site, located about 50 km east of San Pedro de Atacama at 5000 meters high. It hosts the Atacama Large Millimeter/submillimeter Array (ALMA), an array of 66 antennas whose measurements are combined using interferometry. It studies radiation with wavelengths on the order of milimeters, between infrared and radio waves. Among its many accomplishments is the direct imaging of several discs around very young stars, allowing us observations of exoplanets that are currently forming, such as around the star HL Tauri.

A photo of nineteen of the antennae that comprise the ALMA telescope. Credit: ALMA ESO/NAOJ/NRAO)/W. Garnier (ALMA) under CC BY 4.0.

Why the Atacama?

There are several advantages for setting up observatories in Chile: For one, it offers the possibility to observe interesting targets like the center of the Milky Way or the two galaxies known as the Magellanic clouds. Furthermore, ground-based observatories always have to contend with Earth’s atmosphere disturbing the light. In the Atacama desert, the air is very dry and still, which minimizes this troublesome influences. A third advantage of the location is that it is located far from human cities, meaning it has a very low amount of ambient light pollution.

The future

ESO’s efforts to expand ground-based observation facilities is ongoing. One of the biggest projects currently in the works is the construction of the Extremely Large Telescope (ELT) on the Cerro Armazones mountain about 20 km from Paranal. It will be an optical and infrared telescope with a primary mirror of 39 meter diameter, which will make it the largest optical telescope of the world. Construction started in 2014, and the first test observations are planned for 2029.