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.
A complete description of the 4MOST facility is provided by the 4MOST User Manual.
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 (goal 1 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.
The design capabilities of 4MOST guarantee that these requirements will be met.