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 :

MOSAIC is the proposed multiple-object spectrograph for the ELT that will utilise the widest possible field of view provided by the telescope. In terms of adaptive optics, there are two distinct operating modes required to meet the top-level science requirements. The MOSAIC High Multiplex Mode (HMM) requires either seeing-limited or GLAO correction within a 0.6 (NIR) and 0.9 (VIS) arcsecond sub-fields over the widest possible field for a few hundred objects. To achieve seeing limited operation whilst maintaining the maximum unvignetted field of view for scientific observation will require recreating some of the functionality present in the Pre-Focal Station relating to control of the ELT active optics. MOSAIC High Definition Mode Control (HDM) requires a 25% Ensquared Energy (EE) within 150mas in the H-band element for approximately 10 targets distributed across the full ELT field, implying the use of Multiple Object AO (MOAO). Initial studies have shown that to meet the EE requirements whilst maintaining high-sky coverage will require the combination of wavefront signals from both high-order NGS and LGS to provide a tomographic estimate for the correction to be applied to the open-loop MOAO DMs. In this paper we present the current MOSAIC AO design and provide the first performance estimates for the baseline instrument design. We then report on the various trade-offs that will be investigated throughout the course of the Phase A study, such as the requirement to mix NGS and LGS signals tomographically. Finally, we discuss how these will impact the AO architecture, the MOSAIC design and ultimately the scientific performance of this wide-field workhorse instrument at the ELT.

More information here (SAO/NASA ADS Astronomy Abstract Service)

Positioner System Architecture

Baseline for the full architecture is a stepped tiled focal plane. This is the only configuration which can compensate adequately for the non-telecentricity of the ELT and provide support for all required modes. With 7.4’, and a tile size of 0.5’ (~100mm), we can achieve the 200 multiplex including LGS vignetting (TBD)


REM : this is just a concept representative of the type of tile we envision for MOSAIC.

  • Total system bandwidth is 0.37 to 1.8μm
  • Cross over between Vis optimised system and NIR optimised system is 0.8μm
  • Coverage at R5000 is achieved with 3 VPH in Vis and 3 VPH in NIR


  • Initial optical concept for the visible spectrograph, with the final optical architecture, indicates that the beam size will be around 200-230 mm
  • Camera speed is ~ F/1.6
  • ie a feasible design
  • [The concept is actually quite close in terms of parameters to the WEAVE spectrograph that Oxford, GEPI and NOVA are involved in]
  • More detailed concept is currently under investigation...

Both HMM and HDM modes use microlenses and fibres to transmit light to the spectrographs, the principle being to make a pupil image on the fibre cores. Microlenses and fibres are directly on the tile in the focal plate for HMM, while they are offset from the focal plate, after an optical relay including a deformable mirror for HDM. In the case of HDM, the microlens surface is conjugated to the image of the sky, and acts as a high-definition sampler.

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?