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4MOST - 4-metre Multi-Object Spectroscopic Telescope
Roelof S. de Jong, Olga Bellido-Tirado, Cristina Chiappini, Éric Depagne, Roger Haynes, Diane Johl, Olivier Schnurr, Axel Schwope, Jakob Walcher, Frank Dionies, Dionne Haynes, Andreas Kelz, Francisco S. Kitaura, Georg Lamer, Ivan Minchev, Volker Müller, Sebastián E. Nuza, Jean-Christophe Olaya, Tilmann Piffl, Emil Popow, Matthias Steinmetz, Uğur Ural, Mary Williams, Roland Winkler, Lutz Wisotzki, Wolfgang R. Ansorgb, Manda Banerji, Eduardo Gonzalez Solares, Mike Irwin, Robert C. Kennicutt, David King, Richard McMahon, Sergey Koposov, Ian R. Parry, Nicholas A. Walton, Gert Finger, Olaf Iwert, Mirko Krumpe, Jean-Louis Lizon, Mainieri Vincenzo, Jean-Philippe Amans, Piercarlo Bonifacio, Mathieu Cohen, Patrick Francois, Pascal Jagourel, Shan B. Mignot, Frédéric Royer, Paola Sartoretti, Ralf Bender, Frank Grupp, Hans-Joachim Hess, Florian Lang-Bardl, Bernard Muschielok, Hans Böhringer, Thomas Boller, Angela Bongiorno, Marcella Brusa, Tom Dwelly, Andrea Merloni, Kirpal Nandra, Mara Salvato, Johannes H. Pragt, Ramón Navarro, Gerrit Gerlofsma, Ronald Roelfsema, Gavin B. Dalton, Kevin F. Middleton, Ian A. Tosh, Corrado Boeche, Elisabetta Caffau, Norbert Christlieb, Eva K. Grebel, Camilla Hansen, Andreas Koch, Hans-G. Ludwig, Andreas Quirrenbach, Luca Sbordone, Walter Seifert, Guido Thimm, Trifon Trifonov, Amina Helmi, Scott C. Trager, Sofia Feltzing, Andreas Korn, Wilfried Boland
TL;DR
4MOST addresses the need for a wide-field, high-multiplex spectroscopic facility that complements Gaia, eROSITA, and Euclid. The conceptual design combines alternative fibre-positioner concepts, fixed dual-arm spectrographs, and a facility simulator to optimize the survey. Initial simulations indicate that a 2500-fibre system could observe about 20 million low-resolution and 2 million high-resolution targets in five years.
Problem
4MOST addresses the need for a wide-field, high-multiplex spectroscopic facility complementing Gaia, eROSITA, and Euclid.
Method
The design evaluates Phi-Theta and R-Theta fibre positioners, fixed dual-arm high-resolution spectrographs, and a facility simulator modeling operations, targeting, throughput, and signal-to-noise.
Results
About 20 million low-resolution and 2 million high-resolution targets can be observed in five years with a 2500-fibre system and a 1:4 high-to-low-resolution fibre ratio.
Takeaways & Limitations
The proposed facility is structured to deliver large public spectroscopic samples while supporting complementary Galactic, cluster, and active-galaxy studies.
Takeaways & Limitations
The stated stellar-velocity and abundance requirements assume specific signal-to-noise, resolution, and target-magnitude conditions.
Abstract
from arXiv · showhide
The 4MOST consortium is currently halfway through a Conceptual Design study for ESO with the aim to develop a wide-field (>3 square degree, goal >5 square degree), high-multiplex (>1500 fibres, goal 3000 fibres) spectroscopic survey facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on the telescope to perform a 5 year public survey yielding more than 20 million spectra at resolution R~5000 (λ=390-1000 nm) and more than 2 million spectra at R~20,000 (395-456.5 nm & 587-673 nm). The 4MOST design is especially intended to complement three key all-sky, space-based observatories of prime European interest: Gaia, eROSITA and Euclid. Initial design and performance estimates for the wide-field corrector concepts are presented. We consider two fibre positioner concepts, a well-known Phi-Theta system and a new R-Theta concept with a large patrol area. The spectrographs are fixed configuration two-arm spectrographs, with dedicated spectrographs for the high- and low-resolution. A full facility simulator is being developed to guide trade-off decisions regarding the optimal field-of-view, number of fibres needed, and the relative fraction of high-to-low resolution fibres. Mock catalogues with template spectra from seven Design Reference Surveys are simulated to verify the science requirements of 4MOST. The 4MOST consortium aims to deliver the full 4MOST facility by the end of 2018 and start delivering high-level data products for both consortium and ESO community targets a year later with yearly increments.
1. INTRODUCTION
4MOST is conceived as a wide-field, high-multiplex survey facility on VISTA, designed especially to complement Gaia, eROSITA, and Euclid. Its facility philosophy integrates continuous operation, survey planning, instrument capabilities, and data products for multiple science cases.
- 4MOST is intended as a VISTA fibre-fed spectroscopic facility with a field of view and multiplex suited to surveying a large fraction of the Southern sky within a few years.
- The design is especially intended to complement Gaia, eROSITA, and Euclid.
- 4MOST runs almost continuously during its main 5-year survey with minimal instrument changes.
- The facility combines target selection, operations, survey strategy, instrument capabilities, and high-level data delivery as one coordinated package.
- A general-purpose design supports many optical-spectroscopy science cases while running different science programs in parallel.
2. SCIENCE DRIVERS
4MOST is designed to extend spectroscopic coverage beyond Gaia and support Galactic, cluster, and active-galaxy science enabled by eROSITA and other wide-area surveys. Its science drivers rely on combining radial velocities, stellar characterization, and redshifts across broad populations and distances.
- Gaia: Gaia provides radial velocities and astrophysical characterization for about 150 million stars, but its spectroscopic sensitivity is limited to mV~12–16 mag and the CaII-triplet region.
- Gaia: 4MOST aims to match Gaia’s astrometric limits spectroscopically, measuring sun-like stars nearly to the Milky Way center and RGB stars to 100 kpc.
- Galactic science: Line-of-sight velocities reveal substantial spatial and radial-velocity substructure in simulated stellar halos.
- eROSITA: 4MOST will survey more than 50,000 Southern eROSITA galaxy clusters and measure 3–30 galaxies in each cluster.
- eROSITA: 4MOST will determine the nature of more than 1 million AGNs, constraining active-galaxy evolution to z=5.
- Additional science: Other planned applications include Euclid photometric-redshift calibration, large-scale-structure studies, galaxy evolution, radio-galaxy follow-up, and transient follow-up.
3. INSTRUMENT SPECIFICATION
The instrument specifications translate 4MOST’s science goals into requirements for stellar velocities, chemical abundances, galaxy redshifts, target counts, and sky coverage. These requirements define the technical design basis.
- 4MOST must obtain radial velocities with ≤2 km/s accuracy for faint Gaia stars using R=5000 spectra with S/N=10 per Ångström.
- 4MOST must measure abundances of up to 15 chemical elements in 16 V-mag stars using R=20000 spectra with S/N=140 per Ångström.
- 4MOST must obtain redshifts for 22 r-mag galaxies and AGN.
- The 5-year survey requirement is 20 million targets at R~5000 and 2 million at R~20,000, with at least 16,000 deg2 covered twice.
- These user requirements drive the main instrument specifications listed in Table 1.
4. OPERATIONS CONCEPT
4MOST’s operations concept uses a continuous public survey, parallel science programs, rapidly repositionable fibres, integrated calibration, and staged data-quality control. These elements are intended to maximize useful observing time and survey output.
- 4MOST will run continuously on VISTA for a 5-year public survey delivering at least 20 million spectra over 15,000–20,000 deg2.
- Multiple survey catalogues are observed simultaneously at each pointing, allowing sparse large samples to run in parallel with other science programs.
- Fields are the basic observational unit, and selected fibres must be repositioned between tiles within CCD readout time.
- The calibration plan minimizes night-time calibration while monitoring throughput variations that affect sky subtraction.
- Data-quality control operates on minute, day, and half-year timescales, with high-level products released publicly in yearly increments.
5. INSTRUMENT DESIGNS
The conceptual instrument design combines a VISTA wide-field corrector, alternative fibre-positioner architectures, and dedicated high- and low-resolution spectrographs. Trade-offs remain in field of view, fibre allocation, spectrograph count, and calibration flexibility.
- System overview: 4MOST’s instrument concept comprises a wide-field corrector, fibre positioner and feed, and high- and low-resolution spectrographs.These subsystems form the main conceptual designs presented for the facility.
- Wide-field correctors: VISTA corrector designs span 2.2°, 2.5°, and 3.0° diameter fields, corresponding to about 3.0, 4.25, and 6.1 deg2 focal areas.The optimal field of view is to be selected using the facility simulator together with cost and risk criteria.
- Positioners and fibre system: Two fibre-positioner concepts are considered: the MuPos Phi-Theta system and the PotsPos R-Theta system.MuPos uses two rotational axes, whereas PotsPos combines linear and rotational movement.
- Positioners and fibre system: PotsPos uses large patrol areas to improve fibre-target assignment for low- and high-resolution fibres despite their different densities.Its actuators reach about 1.2–1.5 grid separations, and each focal-surface point can be reached by at least seven actuators.
- Spectrographs: The spectrograph count remains a trade-off governed by fibres per spectrograph, total fibre count, spectral cross-talk, and the high-to-low-resolution fibre ratio.The spectrograph units include dedicated low- and high-resolution designs.
- Spectrographs: A variable-attenuator and two-to-one-combiner scheme is being investigated to route sky or calibration light flexibly to fixed pseudo-slits.Other options remain under consideration if this approach proves impractical.
6. FACILITY SIMULATOR
The 4MOST Facility Simulator validates science performance and optimizes instrument design, science cases, and survey strategy. It combines operational, throughput, exposure-time, targeting, and data-quality simulations using mock Design Reference Surveys to estimate survey outcomes.
- The 4MOST Facility Simulator validates system performance and optimizes the instrument design, science cases, and survey strategy.It is intended to evolve into the planning tool for executing 4MOST surveys.
- The simulator models night-by-night operations, fibre targeting, atmospheric and instrumental throughput, exposure-time signal-to-noise, and survey data quality.Its components account for observing conditions, fibre assignments, detected spectra, individual-target success, and survey progress.
- The tiling strategy responds to target-density variation across the sky, including a tenfold increase toward the Galactic disk, by using repeated coverage of suitable areas.The figure distinguishes dark- and bright-time tiling with red and blue hexagons.
- Seven Design Reference Surveys provide mock target catalogues with template spectra, sky distributions, individual-spectrum success criteria, and five-year Figures of Merit.These reference surveys are based on key science projects that impose strong constraints on the design.
- A 2500-fibre system is estimated to observe about 20 million low-resolution and 2 million high-resolution targets in five years using a 1:4 high-to-low-resolution fibre ratio.The estimate includes reasonable overheads and calibrations.
- The simulations include focal-plane fibre-to-target assignment in crowded fields and track individual fibre patrol offsets over the five-year survey.Figure 16 illustrates both the assignment process and the accumulated x,y offsets for one fibre.
7. SCHEDULE
The Conceptual Design study was scheduled for completion and submission to ESO in February 2013, followed by comparison with the MOONS concept. Further work focused on prototyping critical system elements before installation and commissioning on VISTA in late 2018.
- The consortium planned to submit the completed Conceptual Design study to ESO by February 1, 2013, after continuing design and feasibility work through the rest of 2012.
- The study was to be compared with the MOONS concept, with a project decision expected in spring 2013.Because the concepts use different telescopes, both projects could technically proceed.
- Further development targeted critical system elements, including positioner accuracy and reliability, fibre throughput under stress, and fibre-metrology accuracy.These elements were to be tested or characterized through prototyping where necessary.
- Installation and commissioning on VISTA were scheduled for late 2018 after completion of the ongoing VISTA infrared imaging surveys.The schedule was aligned with Gaia and eROSITA data releases for using well-defined catalogues in 4MOST surveys.