4MOST will be a wide-field, high-multiplex, fibre-fed, optical spectroscopic survey facility to be mounted on ESO’s 4-m-class telescope VISTA and operated in ESO’s Paranal Observatory operational environment. 4MOST will be the only instrument installed at VISTA and will be entirely dedicated to observations for ESO Public Surveys. Apart from the instrument itself, the facility will also include the Operations and Data Management Systems necessary for the planning, scheduling and execution of the observations, and for the reduction, analysis, validation and publication of the data and and high-level data products.

4MOST will feature a 2.5-degree diameter field-of-view with 2436 science fibres in the focal plane. The fibres will be configured by a fibre positioner based on the tilting spine principle, enabling extremely short reconfiguration times. The fibres feed three, fixed-configuration spectrographs: one third of the fibres (i.e. 812) will go to a single high-resolution spectrograph (HRS) with resolution R ≈ 18000 – 21000, while the remaining 1624 fibres will feed two low-resolution spectrographs (LRS) with resolution R ≈ 4000 – 7500. In each spectrograph the light is split into three different wavelength channels (blue, green and red) before being dispersed by VPH gratings and recorded by identical 6k × 6k E2V CCDs with low readout noise. The instrument will also feature a Wide Field Corrector with an Atmospheric Dispersion Corrector, an Acquisition & Guiding and Wavefront Sensor system, as well as Metrology and Calibration systems.


Overview of 4MOST on VISTA. The high and low-resolution spectrographs will be located near the telescope underneath the azimuth floor with short, very efficient fibre runs.


AESOP prototype with 64 spines, showing the fibre positioner in action. Credit: Scott Smedley

As mentioned above, the positioner system selected for 4MOST is based on the tilting spine principle and draws on the experience obtained with the Echidna system of FMOS on the Subaru telescope. It can simultaneously reconfigure ~2400 fibres, and thus achieves extremely short reconfiguration times, while allowing for fibre separations as small as 20 arcsec. The video to the left shows a prototype of the 4MOST AESOP positioner in action.

Optical layout of one of the low-resolution spectrographs. The three arms in fixed configuration provide continuous wavelength coverage from 390 to 950 nm at a resolution of R > 4000. Each arm is equipped with a 6k × 6k CCD detector.


To achieve its main science goals, a 2-hour observation with 4MOST shall provide data with sufficient quality to obtain:

  • radial velocities with a precision of better than 2 km/s for any Gaia source (GVega < 20.5 mag)
  • stellar parameters and abundances of up to 15 chemical elements for GVega < 15.5 mag, and abundances with a precision of better than 0.15 dex for some key elements down to GVega < 18 mag
  • redshifts of rAB < 22 mag galaxies and AGN

In a 5-year survey, 4MOST shall be able to cover >17,000 deg2 at least twice, and to obtain spectra of more than 20 million sources at low resolution and more than 3 million spectra at high resolution.


Parameter Design value
Field of view Shape: hexagonal
Size: 4.2 deg2
Diameter: 2.5 deg
Accessible sky (zenith angle < 55°) 30,000 deg2
Multiplex fibre positioner 2436
On-sky fibre aperture Shape: circular
Size: 1.65 arcsec2
Diameter: 1.45 arcsec
Minimum distance between fibres 15 arcsec
Expected on-target fibre-hours per year LRS: 3.2 Mh/yr
HRS: 1.6 Mh/yr
Overheads 3.5 min for the field acquisition (telescope preset), 4.4 min for each science exposure including attached calibrations
Detectors Readout noise: < 2.3 e
Readout modes: normal (100 kHz), fast (400 kHz)
Binning modes (cross-dispersion × spectral): 1×1, 2×1, 1×2, 2×2, 1×4
Low-resolution spectrographs
Multiplex 812 (× 2)
Wavelength range 370 – 950 nm
Spectral resolving power 4000 – 7700 (see figure below)
Spectral sampling ≥ 2.5 pixel/FWHM
Radial velocity accuracy < 1 km/s
Sensitivity Limiting AB magnitudes for S/N = 10 Å-1, 6 × 20 min exposures, mean seeing, new moon (also see figure below):

Wavelength (nm) 400 500 600 700 800 900
Limit (AB mag) 20.2 20.4 20.4 20.2 20.2 19.8
High-resolution spectrograph
Multiplex 812
Wavelength range 392.6 – 435.5, 516 – 573, 610 – 679 nm
Spectral resolving power 18,000 – 21,000 (see figure below)
Spectral sampling ≥ 2.5 pixel/FWHM
Radial velocity accuracy < 1 km/s
Sensitivity Limiting AB magnitudes for S/N = 100 Å-1, 6 × 20 min exposures, mean seeing, 80% moon (also see figure below):

Wavelength (nm) 420 540 650
Limit (AB mag) 15.7 15.8 15.8
Spectral resolution

Left (click to enlarge): spectral resolving power of the low-resolution spectrographs in the blue, green and red wavelength channels. The dashed line shows the requirement. Right: Same for the high-resolution spectrograph.


Expected 4MOST point-source sensitivity for the S/N per Å values and sky conditions indicated in the legend. The solid lines are for a total exposure time of 120 min, whereas the dashed lines are the limits for 20 min exposures. The approximate conversion to S/N per pixel is obtained by dividing the HRS values by 3.3 and the LRS numbers by 1.7. This plot is representative for an entire 4MOST survey, not just for the optimal conditions. The sensitivities shown are those required by typical science cases, i.e. for obtaining detailed elemental abundances of stars (orange), stellar parameters and some elemental abundances (dark blue), stellar radial velocities (light blue), and galaxy and AGN redshifts (black: 90% completeness, grey: 50% completeness).