Thousands of eyes on the sky: 4MOST sees first light

Over the weekend, the 4-metre Multi-Object Spectroscopic Telescope (4MOST) successfully completed its first test observations. Installed on the Visible and Infrared Survey Telescope for Astronomy (VISTA), an ESO telescope in Chile, 4MOST is the largest facility of its kind to survey the southern skies. It is expected to capture and analyse the light of more than 25 million different objects during its first five years of operation, to unravel our galactic history, explore the mysteries of dark matter, and investigate the origins of stars, among many other science goals.  

This instrument is designed to capture the light of thousands of cosmic objects simultaneously, using more than 2400 thin optical fibres, each about the width of a human hair. This light is then directed to three separate spectrographs that split it into up to 18 000 colour components (in the visible light range from violet to red), giving us individual spectra. Using these, astronomers can analyse the properties of the observed cosmic sources, including their chemical composition, velocity or distance.  

“Astronomers have been asking for a facility like 4MOST for a long time,” says Joar Brynnel, 4MOST Project Manager at the Leibniz Institute for Astrophysics Potsdam (AIP), Germany, the institute that led the instrument consortium. The number of objects it can observe at the same time, the large field of view (equivalent to the area of 16 full Moons), and the large number of spectral colours it can register simultaneously make the instrument particularly unique.  

“This is a major change in the things we do at ESO, usually when you observe with an instrument, you observe targets for one scientific study at a time.”, says Vincenzo Mainieri, ESO Project Scientist for 4MOST. However, due to the large number of fibres, the instrument can observe sources from many different scientific projects simultaneously. He continues: “4MOST can serve 10 or more scientific studies in parallel in one single observation. This is a way to maximise the instrument’s scientific output.”  

This cutting-edge facility will not only shed light on our own home galaxy, but also peer further out at multiple galaxies to piece together how they form and evolve. By observing distant galaxies, 4MOST will also help us better understand dark matter, an invisible form of matter that permeates galaxies and the space between them. The instrument will also be used to study the evolution of the Universe itself, investigating how it expands and changes over time.  

4MOST has taken the place of the VISTA Infrared Camera (VIRCAM), which had been conducting surveys for the VISTA telescope since 2008. With 4MOST being a spectrograph, an instrument that is fundamentally different from a camera, VISTA needed to be completely upgraded to receive it. “We had to replace many components of the telescope to fit our instrument in, bringing new large optics to the telescope, new technical cameras to control the telescope and then the instrument itself,” says Brynnel. “The upgrade of the VISTA telescope to receive the new components was prepared by ESO in advance of 4MOST’s arrival,” explains Jean-François (Jeff) Pirard, ESO Project Manager for 4MOST “The telescope was returned to service during the first semester of 2025 just in time to receive the new 4MOST instrument.” 

The 4MOST first-light observations, which covered an area of the sky containing the Sculptor Galaxy and the NGC288 star cluster, show off this cutting-edge instrument’s ability to observe multiple targets with an astonishingly wide field of view and numerous optical fibres. In the first run, 4MOST collected spectra for various stars in our Milky Way and for more than a thousand galaxies near and far, demonstrating its impressive capabilities.  

4MOST Principal Investigator Roelof de Jong, Milky Way section head at AIP, remarks: “It is incredible to see the first spectra from our new instrument. The data looks fantastic from the start and bodes well for all the different science projects we want to execute. That we can catch the light that has travelled sometimes for billions of light years into a glass fibre the size of a hair is mindboggling. An incredible feat only made possible by an incredible development team. Can’t wait till having the system operating every night.” 

More Information 

The 4MOST facility is designed, built, and scientifically operated by a consortium of 30 universities and research institutes in Europe and Australia under leadership of the Leibniz Institute for Astrophysics Potsdam (AIP). The main institutes involved in building and operating of the facility are: 

• Leibniz Institute for Astrophysics Potsdam (AIP), Germany
• Macquarie University / Australian Astronomical Optics (AAO), Australia
• Centre de Recherche Astrophysique de Lyon (CRAL), France
• European Southern Observatory (ESO)
• Max Planck Institute for Astronomy (MPIA), Germany
• Max Planck Institute for Extraterrestrial Physics (MPE), Germany
• Nederlandse Onderzoekschool Voor Astronomie (NOVA), The Netherlands
• University of Cambridge, Institute of Astronomy (IoA), United Kingdom
• Universität Hamburg (UHH), Hamburger Sternwarte, Germany
• Universität Heidelberg, Zentrum für Astronomie (ZAH), Germany
• University College London (UCL), United Kingdom

Courtesy of European Southern Observatory