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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 is conceived as a multi-purpose MOS for the ELT, covering the Visible and Near Infrared bandwidth (0.45 – 1.8 μm) with two modes: multi-object spectroscopy (MOS) and spatially resolved spectroscopy (mIFU).

MOSAIC will be located at the Nasmyth B port of the ELT. The instrument will work with Ground Layer Adaptive Optics image quality over the full field of view of the ELT ~40 arcmin2 and over a nearly full sky coverage. GLAO combines 4 laser guide stars with fainter natural guide to achieve 40% ensquared energy within 0.4 arcsec in H-band.  

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

MOS and IFU modes

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 multi-object spectroscopy 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).

MOS modes min

The IFU mode 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

iFU mode

 

Each mode offers a number of instrument configurations with different spectral resolution and aperture on sky summarized in the table below:

Instrument configuration name
 MOS-VIS-LR
 MOS-VIS-HR
 MOS-NIR-LR
 MOS-NIR-HR
 mIFU-LR
 mIFU-HR
 Mode
 MOS   
 MOS
 MOS
 MOS
mIFU  
 mIFU
 Operating wavelength (μm)
 0.45 – 0.7
0.45 – 0.877 
 0.77 – 1.80  
 0.77 – 1.80
 0.77 – 1.80
 0.77 – 1.80
Multiplex 
 200
 100
 200 
 200
 8 
 8
  Aperture on sky (arcsec)
 0.7
 0.7
 0.6
 0.6
 2.5
 2.5
 Spectral resolution  (λ/Δλ)
 R ~ 4,000
 R ~ 18,000
 R ~ 4,000
 R ~ 18,000
 R ~ 4,000
 R ~ 18,000

Spectral configurations

Nine spectral resolution settings of 4000, 9000 and 18000 are provided in all instrumental modes, covering the wavelength range of 0.45–1.80 micrometres. In the visible channel, the operating wavelength will be covered by the 2 LR bands but not simultaneously.  

  Band
 VIS LR1
 VIS LR2
 VIS LR3 (Goal)
VIS HR1
VIS HR2
Resolution   (λ/Δλ)
  4000 (5000 goal)
 4 000 (5 000 goal)
 4 000
18 000 
18 000
Bandwidth (μm)
 0.45 - 0.59
 0.59 -0.77
0.38-0.45 
0.51-0.57
0.61-0.67

The full near-IR band spanning from 0.77 to 1.8μm is covered simultaneously at low spectral resolution. The two HR bands in the near-IR can also be observed simultaneously.

  Band
 NIR LR
 NIR HR1
 NIR HR2 
Resolution (λ/Δλ)  
  4 000 (5 000 goal)
 9 000
 18 000
Bandwidth (μm)
 0.77 - 1.8
 0.77-0.89
1.53-1.62

  

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