MOSAIC (Phase A completed)

The Multi-Object Spectrograph

Phase A completed, pending start of construction

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).

MOSAIC will provide the ELT with the powerful ability to obtain visible and near-infrared spectroscopy of large samples of astronomical objects, improving upon ESO’s current facilities. It will provide unprecedented observations of objects ranging from stars at the very heart of the Milky Way to the most distant galaxies at the edge of the observable Universe. These future observations will allow astronomers to trace the growth of galaxies and the distribution of matter from shortly after the Big Bang to the present day.

François Hammer, Principal Investigator for MOSAIC, explains: “MOSAIC will be the only ELT instrument able to obtain the sample sizes needed for significant progress in cosmology, extragalactic and galactic science, delivering spectra of a significant number of targets among the thousands included in one exposure with the ELT. In particular, MOSAIC will provide a world-leading and unique capability for spectroscopic follow-up of the most distant galaxies in the Universe, exploiting the legacy of future space missions”.

MOSAIC is being designed as a multi-purpose instrument for the ELT to be used for a broad range of scientific cases. MOSAIC aims to combine the advantages of a highly-multiplexed instrument targeting numerous unresolved sources with an instrument that can resolve sources at high spatial resolution using integral-field units.

The MOSAIC project builds on considerable past work by the consortium on instruments for the Very Large Telescope at the Paranal Observatory, as well as the past conceptual designs of EAGLE and OPTIMOS-EVE for the ELT.


The authoritative technical specifications as offered for astronomical observations are available from the Science Operation page.

Site: Cerro Armazones
Telescope: Extremely Large Telescope
Focus: Five-mirror design — three-mirror on-axis anastigmat + two-fold mirrors used for adaptive optics
Type: Multi-object spectrograph working at the visible and near IR.
Wavelength coverage:

450-1800 nm (proposed)
370-2500 nm (desired)

Spatial resolution: seeing-limited and multi-object adaptive optics
Spectral resolution:

R = 5 000
R = 15 000 (narrow bands)

First light date: N/A
Science goals:
  • Evolution of dark matter
  • ‘Fight light’ galaxies at the highest redshifts
  • Evolution of large-scale structure
  • Mass assembly of galaxies
  • Properties of AGN
  • Resolved stellar populations beyond the Local Group
  • Galaxy archaeology
Images taken with the instrument: Link
Images of the instrument: Link
Press Releases with the instrument: Link


Core partners:

  • Durham University (UK)
  • GEPI, Observatoire de Paris (France)
  • Institut de Recherche en Astrophysique et Planétologie, Toulouse (France)
  • Laboratoire d’Astrophysique de Marseille (France)
  • Laboratório Nacional de Astrofísica (Brazil)
  • Leibniz-Institut für Astrophysik Potsdam (Germany)
  • LESIA, Observatoire de Paris (France)
  • NOVA (Netherlands)
  • Universidade de São Paulo (Brazil)
  • University of Amsterdam (Netherlands)
  • University of Oxford / RAL Space (UK)
  • UK Astronomy Technology Centre (UK)
  • CNRS-INSU (France, leading institution)