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A multi-object spectroscopic survey facility for the European Southern Observatory

A new optical instrument design can be used to study the evolution of Milky Way stars, galaxies, black holes, and the universe.
28 May 2013, SPIE Newsroom. DOI: 10.1117/2.1201305.004898

Several European strategic documents have outlined the need for a multi-object spectroscopic (MOS) survey instrument to complement several upcoming space missions—such as Gaia, eROSITA (extended ROentgen Survey with an Imaging Telescope Array), and Euclid—and large ground-based projects (e.g., the Large Synoptic Survey Telescope and the Square Kilometre Array).1–3 These projects will locate billions of astronomical objects at a range of different wavelengths. However, subsequent characterization of the physical conditions, chemical composition, and velocity of the identified objects requires a dedicated full-sky spectroscopic survey.

The appropriate MOS instrument must have a large field-of-view to enable repeated observations of most of the southern sky within a five-year time frame. It must also be able to observe thousands of targets simultaneously in order to observe millions of targets within five years. Finally, the wavelength coverage and spectral resolution must be sufficient to measure stellar abundances and radial velocities, and the instrument sensitivity must be high enough to characterize stars in the center of the Milky Way and distant galaxies.

We have designed the 4-meter Multi-Object Spectroscopic Telescope (4MOST) facility for the Visible and Infrared Survey Telescope for Astronomy (VISTA) of the European Southern Observatory (ESO) to meet the MOS requirements outlined above (see Figure 1). The telescope's main characteristics are listed in Table 1.

Figure 1. The main instrument components of the 4m Multi-Object Spectroscopic Telescope (4MOST). AESOP: Australian–European Southern Observatory Positioner.

Our instrument features a new wide-field corrector for the VISTA telescope that creates a focal surface, with a 540mm diameter, that captures a 2.5°-diameter field-of-view. At this surface, fibers capture light using our AESOP (Australian-ESO Positioner) fiber positioning system. The fibers are on tilting spines that move the fiber tips around, using a principle similar to that employed on the Echidna fiber positioner of the Fiber Multi Object Spectrograph (FMOS), on the Subaru Telescope.4 The positioner can arrange its 2436 fibers to match the target positions in the focal surface within two minutes and with an accuracy of 10μm. The light collected by the fibers is transmitted to the two medium-resolution (resolving power R ∼ 5000) and two high-resolution (R ∼ 20,000) spectrographs. These spectrographs have fixed configurations and two-channel designs, and use refractive cameras. The expected sensitivity of our instrument is illustrated in Figure 2.

Figure 2. Instrument sensitivity for four typical science cases during a 1-hour exposure at new moon, with a 1.1′′(90th percentile) full-width at half-maximum seeing. mAB: Luminosity.
Table 1.Main characteristics of 4MOST. R: Resolving power. S/N: Signal-to-noise. λ: Wavelength. AB-mag: AB-magnitude, unit of luminosity.
Wide Field Corrector, 4 lens-elements
Field-of-View 2.5° diameter
Effective fiber area 4.05°2 hexagon layout
AESOP Fiber Positioner, tilting spine concept
Multiplex 2436
Fill factor >3 fibers per target
Instrument sensitivity, for >90% of observing conditions
R∼20,000, S/N>150/Å, 2 hours 15.5–16 AB-mag in full moon
R∼5000, S/N>30/Å, 1 hour 18.3 AB-mag in full moon
R∼5000, S/N>4/Å, 1 hour 22.5–21 AB-mag in new moon
Medium Resolution Spectrographs, 2 channels
Resolution Δλ/λ∼5000–8000
Wavelength 390–630 & 600–930nm
Multiplex 812 science fibers
Detectors two 3k×8k-pixel CCDs per channel
High Resolution Spectrographs, 2 channels
Resolution Δλ/λ∼20,000–23,000
Wavelength 395–456.5 & 587–673nm
Multiplex 406 science fibers
Detectors one 3k×8k-pixel CCD per channel

Usually, telescopes are operated for one science program at a time. However, for the high-multiplex (2436 fibers used simultaneously) 4MOST system this would be highly inefficient as many programs have sky target densities that are too low to fill all of the fibers. Our scheduling software for 4MOST therefore merges all the science programs and objectives into a single master catalog. This allows us to conduct optimized contemporaneous observations to maximize our output.

To demonstrate the capabilities of our instrument and the feasibility of our operations concept, we have developed a full 4MOST facility simulator. We tested the simulator using seven science projects, each with millions of targets distributed over the sky. The simulator assigns targets for the fibers and creates observation sequences for a five-year survey. We examine the simulated spectra, which include atmospheric and instrumental effects, to determine if the science requirements are met. Our simulations show that in five years 4MOST can observe more than 2 million stars at high resolution and more than 12 million stars at medium resolution for Milky Way evolution studies; more than 2 million x-ray cluster and active galaxies to study the growth of large-scale structures and black holes in the universe; and more than 12 million galaxies in the redshift 0.3–1.4 range for investigations into galaxy evolution and cosmology.

ESO's Science Technical Committee has recently recommended the 4MOST facility for implementation. We should complete our final design by early 2016. Full integration and testing in Europe should be finished by the end of 2018. We aim to have instrument first light and begin a five-year survey in 2019. Once realized, 4MOST will be the premier MOS facility in the southern hemisphere and will be fully competitive with similar instruments that are planned for the northern hemisphere.

The author gratefully acknowledges the outstanding support received from 4MOST team members at the supporting institutes: Leibniz Institute for Astrophysics Potsdam, Max Planck Institute for Extraterrestrial Physics, Center for Astronomy of Heidelberg University (ZAH), Institute of Astronomy (Cambridge), Rutherford Appleton Laboratory, The Paris Observatory (GEPI), Netherlands Institute for Radio Astronomy (ASTRON), Australian Astronomical Observatory, University of Lund, University of Uppsala, and University of Groningen.

Roelof S. de Jong
Leibniz Institute for Astrophysics Potsdam (AIP)
Potsdam, Germany

Roelof de Jong is the head of the Milky Way and Local Volume section at AIP and is the principal investigator of the 4MOST survey facility. His main research interests are the structure and formation of galaxies.

1. P. de Zeeuw, F. Molster, A Science Vision for European Astronomy, ASTRONET, 2007. Available at http://www.astronet-eu.org/
2. M. Bode, M. Cruz, F. Molster, The ASTRONET Infrastructure Roadmap, ASTRONET, 2008. Available at http://www.astronet-eu.org/
3. J. Drew, J. Bergeron, J. Bouvier, M. Cunha, A. Diaz, G. Kovacs, A. Quirrenbach, C. Tadhunter, M. Turatto, P. Vilchez, Report by the ETSRC on Europe's 2–4m Telescopes Over the Decade to 2020, ASTRONET, 2010. Available at http://www.astronet-eu.org/
4. M. Akiyama, S. Smedley, P. Gillingham, J. Brzeski, T. Farrell, M. Kimura, R. Muller, N. Tamura, N. Takato, Performance of Echidna fiber positioner for FMOS on Subaru, Proc. SPIE 7018, p. 70182V, 2006. doi:10.1117/12.788968