A unique instrument for the biggest telescope



The MOSAIC Consortium wishes a beautiful Christmas and a bewitching new year 2022 to all of You.

MOSAIC http://www.mosaic-elt.eu, entering phase B in 2022.

After Phase A and during the pre-Phase B, the MOSAIC design concept has been consolidated, following collaborative work between the Consortium and ESO. The ESO Council approved on Tuesday Dec. 7th the proposed MOSAIC Construction Agreement which will be signed between the CNRS and ESO in March 2022. Phase B will officially start with the MOSAIC kickoff in June next year. Still, since the project is ready to move forward, an internal kickoff in February will set the real start of the Preliminary Design and Technology Completion phase.
MOSAIC is now a reality and the scientific community will benefit at the horizon 2030 of the only ELT instrument which will open a large discovery space for the science cases where statistics plays a key role.

intraRegistering to the MOSAIC community will keep you updated to with everything related to MOSAIC. Being registered will allow us to take you into account in our science programs and news. You will receive invitations to workshops and symposia. It will take you less than 5 minutes !


We are proud to announce that 250000€ were awarded to MOSAIC by Île-de-France DIMACAV for the prototyping of an integral field unit. The Area of Major Interest ACAV + (Domaine d'Intérêt Majeur ACAV+ - DIMACAV) is labeled by the Île-de-France Region for the period 2017-2020 in order to support Paris region research in the fields of Astrophysics and Conditions of Appearance of Life. Link to the DIMACAV 2020 results

Link to the site of the conference.


                The European Extremely Large Telescope (ELT) will be the world’s largest optical/IR facility for at least a generation. As it is currently the case for the European Very Large Telescope (VLT), the MOSAIC multi-object spectrograph will be the workhorse instrument for the ELT.

The European organisation ESO has embarked on the construction of the Extremely Large Telescope (ELT) in the middle of the Chilean desert. The telescope and its structure reach a volume comparable to eight times that of the Arc de Triomphe in Paris.

A monster of technology

The collecting power of its ultra-giant mirror, 39 metres in diameter, is equivalent to bringing together the 16 largest telescopes in the world. When it is built, probably shortly after 2026, the ELT will be able to observe the weakest sources in the sky. It will be able to study objects so far away that they are inaccessible to other telescopes, unlocking many mysteries in cosmology, about the formation of galaxies, for example the nature of small galaxies or star clusters far beyond our Galaxy or Local Group.

Moreover, since the resolving power of a telescope depends on its size, the ELT will be able to solve the stars with the smallest apparent sizes: the apparent size of the most distant galaxies is much smaller than an arcsecond (for comparison, the Moon’s apparent size is 1800 arcseconds). At these scales, observations are very much affected by the Earth’s atmospheric turbulence, requiring advanced techniques to overcome it, such as adaptive optics.

From the outset of the project, ESO has made a huge commitment to focus all its efforts on ensuring the highest possible spatial resolution. This is equivalent to counting the petals of a daisy 100 kilometres away! The goal is to distinguish an extrasolar planet from its star to distances approximately the thickness of a spiral arm of our Galaxy, in order to study in detail the planetary content of a considerable number of nearby and less nearby stars.


It’s a gamble that’s not insignificant since the main mirror will be made up of 798 segments, each 1.40 metres long, which will have to be aligned with an unequalled precision of only 15 millionths of a millimetre! The other four mirrors of the telescope will have to guarantee the same precision. In particular, two of these mirrors can be mechanically deformed to compensate for tiny variations in the path of light due to atmospheric turbulence. This is made possible by adaptive optics based on complex turbulence analysis systems.

Faced with the challenge of building a mastodon telescope capable of resolving the smallest sources of light in the universe, the European organisation ESO took an even riskier gamble: first install the most sophisticated instruments capable of obtaining the finest spatial resolutions, and only then the two instruments that use the telescope’s maximum collecting power instead (this being guaranteed, whatever the performance of the telescope and its enormous structure).

We are attending SPIE Astronomical Telescopes + Instrumentation in Austin this june 2018. SPIE is an international society advancing an interdisciplinary approach to the science and application of light. You will find below our contribution to the event. Feel free to download our material.


Click here to wiew the article (pdf)



Nearby dwarf galaxies are local analogues of high-redshift and metal-poor stellar populations. Most of these systems ceased star formation long ago, but they retain signatures of their past that can be unraveled by detailed study of their resolved stars. Archaeological examination of dwarf galaxies with resolved stellar spectroscopy provides key insights into the first stars and galaxies, galaxy formation in the smallest dark matter halos, stellar populations in the metal-free and metal-poor universe, the nature of the first stellar explosions, and the origin of the elements. Extremely large telescopes with multi-object R=5,000-30,000 spectroscopy are needed to enable such studies for galaxies of different luminosities throughout the Local Group.


"We recommend the construction of ELTs with multi-object spectrographs to enable archaeological study of dwarf galaxy formation histories across the Local Group."

The Multi-Object Spectrograph, also known as MOSAIC, is a proposed instrument for ESO’s forthcoming Extremely Large Telescope (ELT). MOSAIC is currently in the initial project stage known as Phase A. The study contract was signed at the Paris Observatory on 18 March 2016 by ESO and the CNRS–INSU, the leading technical institute in the MOSAIC consortium. The consortium includes institutions from five countries (France, United Kingdom, The Netherlands, Brazil and Germany) with six associated partners (Austria, Finland, Italy, Portugal, Spain and Sweden).

Read the article on the ESO website


9-13 septembre 2019, Rome

link to the website of the conference

Scientific Rationale

In the next decade, the commissioning of Extremely Large Telescopes (20-40m class) will allow us to see the high redshift universe using new eyes of unprecedented power. By themselves or in combination with other facilities, these new eyes will have the potential to transform our understanding of the formation and early evolution of galaxies and black holes, first light and cosmic reionization, as well as the evolution of the intergalactic and circumgalactic media