Illustration par défaut

First light for the VLT’s MOONS spectrograph in Chile

14 years of successful French expertise

3 September 2026

Le spectrographe MOONS vient de franchir avec succès une étape majeure : ses premières observations, réalisées cet été au Very Large Telescope (VLT) de l’ESO au Chili, démontrent déjà les performances exceptionnelles de cet instrument de nouvelle génération. Annoncé par l’ESO le 3 septembre 2026, ce résultat vient couronner plus de 14 années de travail des équipes de l’Observatoire de Paris - PSL et du CNRS au sein d’un ambitieux consortium international.

MOONS (Multi-Object Optical and Near-infrared Spectrograph) is a state-of-the-art multi-object spectrograph installed at the VLT’s Nasmyth focus. Led by the United Kingdom, the project brings together partners from six countries — the United Kingdom, Italy, France, Portugal, Switzerland and Chile — as well as ESO. Since the early stages of the project, the Paris Observatory – PSL and the CNRS have made a major contribution.

The MOONS instrument installed at one of the 8 m telescopes of ESO’s Very Large Telescope (VLT).
Crédit : UK ATC/M. Black

After more than a decade of design, development, assembly and testing in the UK, at the Royal Observatory Edinburgh (Astronomy Technology Centre), MOONS left the UK in December 2025 to travel to the Paranal site in Chile, after a journey of some 11,000 kilometres. Now integrated into the VLT, it has just carried out its first observations, with particularly promising results.

On the left, a view of the MOONS instrument, showing its three cameras. Together, they enable spectra covering a wide range of wavelengths, from 0.65 to 1.8 micrometres, to be obtained in a single observation for 1,000 objects observed simultaneously. On the right, the first observations made by MOONS of the sky. The image shows some of the first fibres pointed towards their targets, as well as examples of the spectra obtained, illustrating the instrument’s performance from its very first nights of observation.
Crédit : ESO/MOONS team/VVV survey

These observations have, in particular, made it possible to target stars located in the plane of the Milky Way, behind thick clouds of cosmic dust. This is because MOONS’ unique feature is its ability to observe in the infrared. It can effectively ‘see’ through this dust and explore regions that were previously difficult to access. Its performance in terms of multiplexing, sensitivity, spectral quality and wavelength coverage has also been brilliantly demonstrated.

A French contribution at the heart of the instrument

A technological marvel weighing 10 tonnes and standing 4.5 metres tall, MOONS is the largest spectrograph ever built in Europe.

With some 1,000 optical fibres, each controlled by a robotic positioner, MOONS can simultaneously observe nearly a thousand targets across a vast field of view. The light is then channelled to two identical spectrographs (each comprising 500 fibres) cooled to very low temperatures (between –143°C and –233°C), in order to minimise spurious infrared emissions. The cryostat, which requires some 5,000 litres of liquid nitrogen, is, in fact, one of the largest ever built for a ground-based astronomical instrument. Once broken down in this way, the light reveals to astronomers numerous properties inherent to cosmic objects, such as their chemical composition, mass and velocity.

The first fibre was connected, marking the start of the installation of the MOONS fibre-optic system. This symbolic milestone was followed by the connection of the remaining 999 fibres, forming a network capable of simultaneously transmitting light from 1,000 celestial objects to the instrument’s spectrographs.
Crédit : I. Guinouard, Observatoire de Paris – PSL
Rear view of the MOONS focal plate, showing the 1,000 optical fibre connections. This image highlights the fibre-optic connection system, the design, integration and testing of which were the responsibility of the Paris Observatory – PSL and the CNRS. This assembly ensures that the light collected at the telescope’s focal point is transmitted to the spectrographs with a very high degree of precision.

In this technological feat, France was responsible for several key components: the fibre-optic link, the slit environment, the shutter and the data reduction pipeline. These developments have, in particular, enabled teams at the Paris Observatory – PSL and the CNRS to acquire cutting-edge expertise in the use of fibres in cryogenic environments.

Ultimately, MOONS will enable us to obtain the spectra of millions of stars and galaxies. Not only will it open a new window onto the formation and evolution of galaxies throughout the history of the Universe – including its expansion, with the most distant galaxies receding from us more rapidly and their light shifted towards longer wavelengths, mainly in the infrared – but it will also provide an unprecedented map of our own Milky Way.

After fourteen years of innovation, international cooperation and engineering of the highest calibre, MOONS’ first light thus marks the culmination of a major scientific and technological endeavour for the Paris Observatory – PSL and the CNRS. The new instrument, now the largest of its kind, joins the family of benchmark spectrographs for ground-based telescopes, alongside MUSE, X-shooter and 4MOST in particular, and ushers in a new era for multi-object spectroscopy in the near-infrared.

Composition of the international consortium

The MOONS instrument was designed and built by a consortium of European and Chilean universities and research institutes, namely:

  • The Science and Technology Facilities Council’s UK Astronomy Technology Centre (UK ATC), United Kingdom (consortium lead)
  • Centro de Astro-ingenieria UC (AIUC), Pontificia Universidad Católica de Chile, Chile
  • ETH Zürich, Switzerland
  • The Paris Observatory – PSL and the CNRS, France
  • INAF, Italy
  • Institute of Astronomy (IoA), University of Cambridge, United Kingdom
  • Instituto de Astrofísica e Ciências do Espaço (IA), Portugal
  • University of Geneva, Switzerland

ESO is an associate member of the consortium and has provided the high-sensitivity scientific detection systems used by MOONS.

ESO press release (3 September 2026):
Revealing hidden stars and galaxies: new MOONS instrument at the Very Large Telescope makes first observations

Scientific contacts

  • Hector FLORES
    French Co-PI for MOONS and LUX, Paris Observatory-PSL, CNRS,
  • Isabelle GUINOUARD
    French Project Manager, UNIDIA, Paris Observatory-PSL, CNRS,

Acknowledgements

We would like to thank all the engineers, technicians and support staff who took part in the design, manufacture, integration and testing of the MOONS instrument, as well as those who managed the administrative and logistical aspects at the Paris Observatory – PSL.
Their expertise and commitment were essential to this success.
We would particularly like to thank:

  • Jean Philippe Amans (Slit and Shutter Manager) UNIDIA, Paris Observatory-PSL CNRS
  • Frédéric Royer (Reduction Pipeline Manager) LIRA, Paris Observatory-PSL CNRS
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