A new spectroscopic instrument in the high plains of Chile has captured its first light, opening a new era in the study of the southern sky. The telescope, a multi-year international project, achieved this critical milestone on Oct. 18, 2025, signaling the start of a mission that will create a massive public archive of celestial data and could reshape our understanding of the cosmos. The instrument is not a camera that captures images but a spectrograph that analyzes light, providing deep insights into the chemical composition, temperature, and motion of stars and galaxies.
Known as 4MOST, or the 4-metre Multi-Object Spectroscopic Telescope, the instrument is installed on the European Southern Observatory’s VISTA telescope at the Paranal Observatory. Its primary task is to conduct a vast survey of tens of millions of objects over a planned five-year mission, with the potential to operate for as long as 15 years. By studying the detailed spectra of thousands of objects simultaneously, astronomers aim to unravel the formation history of the Milky Way, investigate the evolution of the universe, and probe the enigmatic nature of dark matter and dark energy. The project is led by the Leibniz Institute for Astrophysics Potsdam (AIP) and involves a large consortium of research institutions.
Advanced Spectroscopic Technology
Unlike traditional telescopes that produce images, 4MOST is a wide-field survey facility designed to analyze the constituent colors of light from celestial bodies. At its heart are 2,436 robotic optical fibers, each thinner than a human hair, positioned to capture light from an equal number of objects at the same time. This light is then channeled into three advanced spectrographs that split it into its components, similar to a prism creating a rainbow. The system can resolve the light from each object into 18,000 distinct color components, allowing for incredibly detailed analysis.
This powerful capability allows 4MOST to measure key properties of stars, galaxies, and other phenomena with high efficiency. Observations that would have previously required immense amounts of time on multiple telescopes can now be completed in short exposures of 10 to 20 minutes. The telescope will repeatedly scan large swaths of the southern sky, building its catalog over time. Its unique combination of a large field of view, a high number of simultaneously observed objects, and detailed spectral resolution makes it the most powerful instrument of its kind in the southern hemisphere.
A New Instrument on a Proven Telescope
The 4MOST instrument was installed on the Visible and Infrared Survey Telescope for Astronomy, or VISTA, a 4.1-meter-diameter telescope that has been operating at the Paranal Observatory since 2008. The installation was a significant engineering feat, requiring a complete upgrade of the host telescope. VISTA’s original instrument, a 3-ton infrared camera called VIRCAM, was decommissioned to make way for the new spectrograph.
Engineers replaced many core components of the telescope to accommodate 4MOST. This included installing a new optical corrector featuring four large lenses, the largest of which is nearly a meter in diameter, all aligned with a precision better than 100 micrometers. New technical cameras were also added to properly control the telescope and the instrument. The European Southern Observatory prepared the telescope for the upgrade in advance, returning it to service in the first half of 2025 just in time for 4MOST’s arrival and installation.
Ambitious Scientific Objectives
The data collected by 4MOST will fuel a wide range of astrophysical research for years to come. The project is composed of ten distinct surveys, each targeting different cosmic questions. A primary goal is to perform galactic archaeology, tracing the formation and evolution of our own Milky Way galaxy by studying the chemical compositions and motions of its stars. These observations will help reveal how the galaxy was assembled over billions of years.
Cosmology and Galaxy Evolution
On a grander scale, several surveys will focus on cosmology. By observing distant galaxies and supernova explosions, scientists will map the large-scale structure of the universe and measure its expansion history with greater precision. This research aims to shed light on dark energy, the mysterious force driving the accelerated expansion of the cosmos. Other programs will study how galaxies form and evolve over cosmic time and investigate the distribution of dark matter.
Stellar and Exoplanet Science
Closer to home, 4MOST will investigate the life cycle of stars and the properties of planets outside our solar system. The instrument will also provide crucial follow-up observations for other major telescopes, including the Vera C. Rubin Observatory, which will conduct a vast imaging survey of the sky. By combining imaging from Rubin with spectroscopic data from 4MOST, astronomers will gain a more complete picture of the transient and variable universe.
An International Collaborative Effort
The development of 4MOST was a decade-long project involving the coordinated work of more than 700 scientists and engineers from institutions around the world. While the project was led by the AIP in Germany, significant contributions came from partners in Australia, France, the Netherlands, Sweden, and the United Kingdom. French teams, for instance, played a key role in building two of the spectrographs. Engineers and astronomers at University College London and Durham University in the U.K. were instrumental in designing and building the optical corrector and developing the sophisticated operational software.
The “first light” achievement marks the culmination of this extensive international effort. The initial test observations successfully targeted the Sculptor Galaxy, a spiral galaxy located 11.5 million light-years away, and NGC 288, a dense globular cluster of ancient stars within the Milky Way. These targets demonstrated the instrument’s ability to capture high-quality spectra from a diverse range of celestial objects across its wide field of view. As 4MOST transitions from commissioning to full operational status, it will begin its ambitious 5-year survey, with the resulting data expected to become a major source of exploration for astronomers globally.