mission control A spectroscopy facility for many The 4-metre Multi-Object Spectroscopic Telescope (4MOST) is a versatile optical survey instrument that will be installed on the VISTA telescope in 2022, ofering medium- and high-resolution spectra and an innovative operations mode, explains Principal Investigator Roelof de Jong on behalf of the 4MOST Consortium.

he 4MOST project is creating a (R ≈ 20,000) covering three optical Calibration units Wide Field new spectroscopic survey facility wavelength bands. Each spectrograph has Corrector Tthat will provide the highest target three fixed-wavelength channels equipped multiplex on the largest field-of-view in with high quantum efficiency CCD Metrology the Southern Hemisphere. The instrument detectors with 6,144 × 6,160 pixels. cameras is to be installed on the 4-m-class Visible Running as a dedicated survey facility and Infrared Survey Telescope for at Paranal with its many Fibre Astronomy (VISTA) of the European clear nights and high image quality, the positioner Southern Observatory (ESO), located at productivity of 4MOST is predicted to be the Paranal Observatory in . Expected very good. The instrument system consists to start its first survey at the end of 2022, of state-of-the-art components for high it will have the capacity to obtain about throughput and in a total exposure time

15 million spectra per year at medium of two hours is expected to deliver high- High-resolution and high spectral resolution in the optical resolution spectra with a signal-to-noise spectrograph –1 wavelength range. ratio S/N = 100 Å for objects with a flux of Medium-resolution spectrographs As the instrument name suggests, mAB = 16 mag, yielding high-quality stellar 4MOST is foreseen to address a large range elemental abundances. In the same exposure of science cases. In particular its aim is to time, the medium-resolution spectrographs –1 complement and enhance space missions reach S/N = 30 Å for mAB = 18 mag objects Fig. 1 | Schematic of the 4MOST components and ground-based facilities covering large to obtain good stellar parameters and –1 integrated with VISTA. areas of the sky such as Gaia, eROSITA, S/N = 3 Å for mAB = 22 mag objects, Euclid, PLATO, VISTA, LSST, and SKA. sufficient to measure the redshifts of The 4MOST Consortium will conduct galaxies and AGNs. verification is expected to be completed at a survey of faint Galactic stellar objects An innovative operations mode is the end of 2022, after which 4MOST will that will determine the Galactic 3D foreseen for 4MOST. Normally one science start its first five-year survey. gravitational potential to unprecedented project is scheduled at a time on a telescope, 4MOST is being developed by a team detail and measure the formation history but 4MOST has such a high multiplex of more than 350 scientists, engineers, and dynamical evolution of the Milky that many programmes will not have managers, and support staff. Consortium Way. A survey of galaxy clusters and active enough targets to fill all fibres, and hence partners are Leibniz-Institut für Astrophysik galactic nuclei (AGNs) discovered by the 4MOST will observe many science cases Potsdam (AIP; lead institute), Australian eROSITA X-ray satellite will provide very simultaneously. This also allows science Astronomical Optics — Macquarie, competitive constraints on cosmological cases that have sparse targets spread over the Centre de Recherche Astrophysique de parameters and dark energy evolution and entire sky to be mixed in with other projects Lyon, European Southern Observatory, determine the role of black hole and host that have sufficient target densities to fill Institute of Astronomy — Cambridge, galaxy co-evolution. Extensive galaxy, AGN, most fibres. ESO will issue a call for Letters Max-Planck-Institut für Astronomie, Max- and type-Ia supernova surveys will provide of Intent in the fourth quarter of 2019 to Planck-Institut für extraterrestrische Physik, strong constraints on dark energy, dark solicit proposals from its astronomical NOVA/ASTRON Dwingeloo, Zentrum für matter, and galaxy evolution. community, to be mixed in with the Astronomie der Universität Heidelberg, The facility consists of a number of programme that will be carried out by the École polytechnique fédérale de Lausanne, distributed components that will be attached 4MOST consortium. All 4MOST data and Lunds Universitet, Rijksuniversiteit to the VISTA telescope (Fig. 1). A new derived analysis products will be distributed Groningen, Universität Hamburg, University Wide Field Corrector will provide a 2.6º to the entire astronomical community. of Western Australia, and Uppsala field of view. At the focal surface of 535-mm The 4MOST subsystems are currently in Universitet. ❐ diameter, a fibre positioner system based on construction with full system integration the tilting spine principle moves 2,436 glass starting at the Leibniz Institute for Roelof S. de Jong fibres to astronomical targets of interest with Astrophysics in Potsdam at the end of 2019. Leibniz-Institut für Astrophysik Potsdam (AIP), an accuracy of 5 μm. The fibres transport Full integration and testing in Europe is Potsdam, Germany. the target’s light to two medium-resolution expected to be completed by early 2022, e-mail: [email protected] spectrographs (R = λ/Δλ ≈ 5,000) with a after which the instrument will be shipped wavelength coverage of 370–950 nm and to Paranal Observatory to be integrated Published online: 7 June 2019 to one high-resolution spectrograph and commissioned at the telescope. Science https://doi.org/10.1038/s41550-019-0808-x

574 Nature Astronomy | VOL 3 | JUNE 2019 | 574 | www.nature.com/natureastronomy