The 4MOST facility will be running spectroscopic Public Surveys on VISTA nearly full time, and consists next to the instrument also of all the software to schedule and observe many surveys in parallel and the Data Management system to reduce, analyze, validate and publish all the data including higher-level data products (radial velocities/redshifts, stellar parameters, abundances, etc.).

High- and low-resolution spectrographs will be located near the telescope underneath the azimuth floor with short, very efficient fibre runs. The spectrographs are fixed configuration, three-armed spectrographs, equipped with VPH gratings and large CCDs with 6k x 6k pixels in each arm.


The critical components of the hardware of the facility are the telescope with a wide field corrector, the fibre positioner system with a fibre run to the spectrographs.


The Focal Surface Unit for VISTA contains the Wide Field Corrector with an Atmospheric Dispersion Corrector, providing a 2.5-degree diameter field-of-view in the Cassegrain focus. The Unit furthermore contains the Low- and High-order Wave Front Sensors and the Acquisition and Guiding systems.

The positioner system selected for 4MOST is based on the tilted spine principle and draws on the experience obtained with Echidna system of FMOS on the Subaru telescope.

The positioner system selected for 4MOST is based on the tilted spine principle and draws on the experience obtained with Echidna system of FMOS on the Subaru telescope. It can simultaneously reconfigure ~2400 fibres with fibre separations as small as 20 arcsec.


Optical layout of one of the Low Resolution Spectrographs. The three arms in fixed configuration provide continues wavelength coverage from 390 nm to 950 nm at resolution R>5000. Each arm contains a CCD detector with 6k x 6k pixels.


To achieve the main science goals, 4MOST shall be able to obtain:

  • radial velocities of ≤2 km/s accuracy of the faintest stars observed by Gaia by obtaining spectra of 19 (goal 19.5) r-mag stars with S/N=10 per Angstrom at resolution R=5000 in a ≤1 hour exposure,
  • abundances of up to 15 chemical elements of ≤16 V-mag stars by obtaining spectra with S/N=140 per Angstrom at resolution R=20000 in a ≤2 hour exposure,
  • redshifts of 22 r-mag galaxies and AGN in a ≤2 hour exposure in dark time.

The science requires that, within a 5 year survey, 20 (goal 30) million targets shall be observed at R~5000 and 2.0 (goal 3.0) million objects at R~20,000 (these numbers include community targets). In this period at least a 15,000 (goal 20,000) degreearea on the sky shall be covered at least two times (goal three times).

This table summarizes the main instrument specifications and current design values.

Specification Requirement Goal Design
Field-of-View in hexagon >4 degree2 >5 degree2 4.1 degree2
Fibre multiplex per pointing >1500 >2400 2436
Smallest target separation <30″ <15″ <17″

Low-Resolution Spectrograph (LRS)

Fibre multiplex >800 >1500 1624
Spectral resolution R> 4000 R>7500 @800nm R>4000–7800
Wavelength coverage 400–885 nm 390–950 nm 370–950 nm
High-Resolution Spectrograph (HRS)
Fibre multiplex >800 >800 812
Spectral resolution R>18,000 R>20,000 R>18,500
Wavelength coverage 392.8–435.5,
521–571 &
610–675.5 nm
521–571 &
606–683 nm
516–573 &
610–679 nm
Photon detecting percentage (in 1.0 arcsec seeing) >15% >20% >15%
Spectral crosstalk (after data reduction) <0.05% <0.02% TBD
Fibre aperture diameter 1.4″±0.1″ 1.4″±0.1″ 1.45″±0.05″
Observing efficiency Overhead < 20% Overhead < 10% <18%
Available sky area (Zenith angle) 2–55 degrees 2–70 degrees 2–55 degrees