A unique instrument for the biggest telescope


MOSAIC in a nutshell

MOSAIC is a multi-object and multi-integral field spectrograph that will use the widest possible field of view provided by the ELT.  The MOSAIC top-level requirements have been based on a comprehensive White Paper  summarizing the very numerous scientific cases for a multi-object spectrograph on the ELT. MOSAIC will have three operating modes: a high multiplex mode (HMM) covering the visible and near-infrared domain; a high definition mode (HDM) that will provide spatially resolved observations in the near-infrared; and a multi light bucket integral field mode for the Inter-Galactic Medium mode (IGM).

Proposed MOS optical path appears in this artist view, as explained in the ESO video (see SCIENCE)

High multiplex mode (HMM)

This mode allows us to observe simultaneously 200 astronomical objects in the visible domain, or 100 in near infrared, in a single exposure.

Science drivers: Resolved stellar population of local group galaxies and astro-archeology (SC #4 and #5), the origin of dwarf galaxies (SC #3)

Why multiplex ? Even with the unbeatable collecting power of the ELT, the observed astronomical sources  are so faint, that it will require several hours to obtain spectra. The high multiplex mode enables us to observe hundreds of targets simultaneoulsy in a single exposure of a few hours (1h at ELT). Without multiplex capability, i.e. possibility to observe several objects at the same time, the same observations would require several hundreds of hours (100h).

High multiplex instruments build by the consortium: FORS@VLT, FLAMES@VLT, MOONS@VLT, WEAVE@WHT


HMM Visible
Operating bandwidth
Number of objects observed simultaneously
Diameter of the aperture on sky
Spectral Resolution (λ/Δλ)
Limiting magnitude
0.45 - 0.8 μm
5000 & 15000
RAB = 26


HMM Near-infrared
Operating bandwidth
Number of object observed simultaneously
Diameter of the aperture on sky
Spectral Resolution (λ/Δλ)
Limiting magnitude
0.8 - 1.8 μm
5000 & 15000
HAB = 28


High definition mode (HDM)

Integral field units (IFUs) are used in this mode, combined with the multi-object adaptive optics system (MOAO). It allows to obtain spatially resolved observations at high definition for 10 astronomical objects in the ELT field of view.


Operating bandwidth
IFU field of view
 Number of object observed simultaneously
 Spectral Resolution (λ/Δλ)
 Spatial pixel size
 Ensquared Energy  ( adaptive optics performance)
 Limiting magnitude
0.8 - 1.8 μm
2.0 x 2.0”
 75 mas
 > 25% EE
 JAB = 28 ( 40h)


Science drivers: Detecting & studying first galaxies (SC #1), how galaxies grow in mass (SC #3)

What is integrated field spectroscopy ?

This technology combines spectrographic and imaging capabilities. The integral field unit slices the astronomical object into several regions, each of them being spectrographied. This leads to spatially resolved observations of the object, by providing a spectrum for each region;


 Instrument build by the consortium: GIRAFFE@VLT, KMOS@VLT, MUSE@VLT




MOSAIC Conceptual design

The design is based on two principles: first, MOSAIC will have a shared focal-plate with multi-function tiles which can serve as pick-offs for any of the modes and AO functions, and second, it will include shared-slit spectrographs for which the spectrograph optics and detectors can be used either for the high-multiplex mode or for the IFU mode.

Shared focal-plate

MOSAIC focal plane is filled by hexagonal tiles. On each tile there will be a locally controled positioner that allocates for the HMM apertures and for the HDM pick-off mirror.  Figure below shows a conceptual design for the MOSAIC focal plane and the implementation of the 4 observational modes.

The two high multiplex modes (HMM) will operate in seeing limited or ground layer adaptive optics (GLAO) conditions with the following specifications:
  • HMM-VIS: 200 sub-fields of 0.80” in diameter to be allocated within a 32 arcmin^2 field. Each sub-field consists in bundles of several microlens + fibres.
  • HMM-NIR: 100 sub-fields of 0.60” in diameter consisting in dual apertures for optimal sky-subtraction.
The two integrated field spectroscopy modes HMM and IGM will operate with the following specifications:
  • HDM: High definition mode, operating with multi object adaptive optics (MOAO) in the near-IR. A pick- off mirror in the focal plane directs light via an MOAO adaptive system (receiver) and fibre bundle to the spectrograph
  • IGM: Inter-Galactic Medium mode, light bucket IFS operating in seeing limited conditions. A pick-off mirror redirects the light via a path compensator and a fibre bundle to the spectrograph.

Conceptual design :

Who are we? Infos on the MOSAIC consortium.


Scientific goals and milestones: why MOSAIC?


How do we get there? All the technology behind MOSAIC.


What performance can we expect from MOSAIC?


How will MOSAIC fit in the instrumental landscape?