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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

  

Observing strategies

MOSAIC offers 3 observing strategies that enable different types of sky subtraction:

Stare: This observing strategy is recommended for the observations of bright targets in the VIS bands, thought is still accessible for the other MOS configurations. A fraction of the MOS subfields are allocated to sky position. A model sky spectrum is computed at the position of each science target using OH sky line modelling. This technique considers the absolute and relative variations of OH sky line intensities, as well as variations due to instrumental flexures, which can impact the wavelength scale.

Nodding: This sky subtraction strategy will be use in MOS-VIS mode for science cases requiring accurate sky subtraction and in MOS-NIR for relatively bright sources. The object and the sky are alternatively by a sub-field following a sequence ABBA or ABAB, obtained by nodding either the telescope or the sub-field. 


Cross beam switching (Xswitch). This strategy provides the state-of-art sky subtraction for faint targets observed in near-IR. Sub-fields are placed in pairs separated by 15” and forming a dual aperture. The object is observed in both fibers following a sequence ABBA or ABAB, obtained by nodding either the telescope or the sub-field. During the consecutive A-B sequences, a given object is always observed by one of the fibres bundle of the pairs alternately. This method has the advantage to be similar to the nodding along slit and thus is 100% of the time on the scientific targets and allows a very accurate instrumental response subtraction. 

observing configurations

  Mode
 MOS-VIS-LR
 MOS-VIS-HR
 MOS-NIR
 MIFU
Stare 
 x
 x
 x
 
 Nod
 x
 x
 x
 x
 Xswtich
 x
 
 x
 

{/AF}

 

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 GLAO, and second, it will include shared-slit spectrographs for which the spectrograph optics and detectors can be used either for the MOS mode or for the mIFU 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 MOS apertures and for the IFU pick-off mirror.  Figure below shows a conceptual design for the MOSAIC focal plane and the implementation of the 2 observational modes and associated instrument configuration:

  • MOS-VIS-LR: 200 sub-fields of 0.70” in diameter to be allocated within the full field of view. Each sub-field consists in bundles of 7 microlens, directly attached to the fiber link feeding the spectrograph. 

  • MOS-VIS-HR: 130 sub-fields of 0.70” in diameter to be allocated within the full field of view but with a lower density than MOS-VIS-LR. The sub-field is fragmented in 19 microlenses. 

  • MOS-NIR: 200 sub-fields of 0.60” in diameter (7 microlenses)

  • mIFU: 8 mIFU receivers at the edge of the field re-image into a microlend-fibre array the light coming from pick-off mirror located in the field of view. 

FoV configuration v2.001 min

The figure below shows the optical path of the Instrument, with the two independent paths: the Visible science path (in blue shades) and the NIR science path (in red shades).


Optical path 

Product Breakdown Structure

MOSAIC is made of 7 system and 19 sub-system. The figure below shows the top-level product tree. The product tree is, where possible, split into functionally coherent units, with the aim of easing the design and integration of the whole instrument across the consortium. System level units should be, in most cases, capable of being tested and verified independently before integration with other systems.

PBS l1 extended

Front-End system

This system interfaces with the ELT prefocal station and stands on the Nasmyth plateform. It is made of:

  •         The Pre-focal sub-system, hosts the calibration modules and instrument shutter.

  • The Focal Plane sub-system, aka positioner, allow us to allocate the MOS apertures and the mirror pickoffs (for NGS and mIFUs)  in the field of view

  •         The Optical Relay sub-system, 12 optical devices located at the edge of the field, that re-image the light pick-off from the mirrors in the focal plane. Eight Optical Relays are used for the path compensation and re-imaging of targets for the mIFU mode. Four of them are used for the Natural Guide Stars and are equiped with wavefront sensors. 

  • Instrument Core sub-system, a rotating structure on the top of the Nasmyth support plateform that compensate for field rotation during observation. It supports the focal plane, the optical relays and the AO modules.

  • Nasmyth Support sub-system, a mechanical structure that interfaces with the ELT prefocal station and Nasmyth plateform. It support the other sub-system of the front-end and provides routine path for fibre links and cables. 

Front end

VIS Science Light Path system (VIS path)

The VIS path has 3 sub-systems: the two fiber links for the low resolution and high resolution mode of the MOS-VIS mode; and the VIS spectrograph. The two set of fiber link share the same spectrograph via a slit exchanger. 

 

VIS Path

NIR Science Light Path system (NIR path)

The NIR path has 3 sub-systems: the two fiber links for the MOS-NIR and mIFU mode; and the NIR spectrograph. The two set of fiber link share the same spectrograph via a slit exchanger. 

Ground Layer AO system (GLAO)

Electronic and. Instrument Control System (EICS)

The Instrument Control System is physically distributed between the Nasmyth platform (local control units which need to be close to their hardware, e.g. detector controllers), the computer room (LCU computing), and the control room (User software on the instrument workstation).

Science Software (SSO)

AIV system tools (S-AIV)

System AIV tools (AIV) includes all tools required to build and maintain the instrument, but not required during normal operation. They will be stored when not in use. 

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