Think big: multi-object spectroscopy with extremely large telescopes

Designing and building the next generation of extremely large telescopes (ELTs) is arguably the greatest challenge ever undertaken by astronomers. ELTs will address the major science issues of the next two decades, enabled by huge gains in sensitivity resulting from collecting areas that are more than 25 times larger than those of the largest telescopes today. Using current technologies, the size, complexity, and cost of instrumentation increases with the square of a telescope’s diameter.1 Building scaled-up versions of the current generation of instruments seems beyond reasonable limits of feasibility for the planned 42m (diameter) European ELT (E-ELT: see Figure 1). New and innovative approaches in instrument layout, system engineering, and manufacturing strategies are required. In the European astronomical community, the highest priority for ground-based optical and near-IR instrumentation has been identified as high-multiplex, multi-object spectroscopy (MOS).2 For the E-ELT, the European Southern Observatory (ESO) has awarded support for several concept studies,3 some of which address MOS approaches. In addition, as members of an international consortium, we are investigating an alternative that offers various advantages, including significant science gains because of a high multiplex factor and flexible deployment, low cost associated with small sizes of individual units, easy scaling because of a modular approach, and minimum risk using proven technologies. Our proposed MOS instrument concept, ERASMUS-F,4 builds on the consortium’s combined expertise. Technologically, it is based on optical-fiber systems and the spectrograph subsystems of the Multi-Unit Spectroscopic Explorer (MUSE: see Figure 2),5 Figure 1. The 42m (diameter) European Extremely Large Telescope, planned for completion in 2018 ( c European Southern Observatory).