4MOST is a wide-field, high-multiplex spectroscopic survey facility
under development for the VISTA telescope of the European Southern
Observatory (ESO). Its main science drivers are in the fields of
galactic archeology, high-energy physics, galaxy evolution and
cosmology. 4MOST will in particular provide the spectroscopic
complements to the large area surveys coming from space missions like
Gaia, eROSITA, Euclid, and PLATO and from ground-based facilities like
VISTA, VST, DES, LSST and SKA. The 4MOST baseline concept features a 2.5
degree diameter field-of-view with ~2400 fibres in the focal surface
that are configured by a fibre positioner based on the tilting spine
principle. The fibres feed two types of spectrographs; ~1600 fibres go
to two spectrographs with resolution R<5000 (λ~390-930 nm) and
~800 fibres to a spectrograph with R>18,000 (λ~392-437 nm and
515-572 nm and 605-675 nm). Both types of spectrographs are
fixed-configuration, three-channel spectrographs. 4MOST will have an
unique operations concept in which 5 year public surveys from both the
consortium and the ESO community will be combined and observed in
parallel during each exposure, resulting in more than 25 million spectra
of targets spread over a large fraction of the southern sky. The 4MOST
Facility Simulator (4FS) was developed to demonstrate the feasibility of
this observing concept. 4MOST has been accepted for implementation by
ESO with operations expected to start by the end of 2020. This paper
provides a top-level overview of the 4MOST facility, while other papers
in these proceedings provide more detailed descriptions of the
instrument concept[1], the instrument requirements development[2], the
systems engineering implementation[3], the instrument model[4], the
fibre positioner concepts[5], the fibre feed[6], and the
spectrographs[7].
