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.

Figure 1. The 42m (diameter) European Extremely Large Telescope, planned for completion in 2018 (European Southern Observatory).

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 an optical integral-field spectrograph for ESO's 8m (diameter) Very Large Telescope (VLT). The instrument is scheduled for commissioning in 2012.

Figure 2.The first of 24 Multi-Unit Spectroscopic Explorer spectrographs during acceptance testing (Centre de Recherche Astronomique de Lyon).

Encouraged by the superb image quality and high throughput of the spectrograph optics, we propose a scalable configuration of such modules that can be manufactured in a small industrial series and modified for use with fiber-optical mini-integral-field units (IFUs) instead of image slicers. The fiber-based IFUs can be deployed over the telescope's full field of view and feature innovative photonics devices, so-called hexabundles.⁶ These fused fiber bundles (see Figure 3) offer large filling factors compared to classical bundles and a much simpler and more compact package than microlens-array-fed fiber approaches. The system design profits from the Anglo-Australian Observatory's experience in developing robotic fiber positioners, such as the successful OzPoz facility at the VLT.

Figure 3. Fused hexabundle consisting of 61 fibers (Univ. of Sydney).

An appealing feature of ERASMUS-F is the idea to deploy a proof-of-concept E-ELT pathfinder instrument at the VLT, which—at the same time—is fully justified as a highly competitive common-user facility. The primary goal is to extend the science remit from integrated to resolved properties of a large sample of ~100,000 galaxies. We presented a summary of the science case for the proposed MOS instrument, dubbed ‘FIREBALL,’ to the ESO community in 2009.⁷

On behalf of the German Verbundforschung (‘collaborative research’), the participating institutes in Potsdam and Göttingen have taken the lead, including the optomechanical design of the pathfinder instrument and the manufacturing of a laboratory demonstrator. The Centre de Recherche Astronomique de Lyon designed and tested⁸ the MUSE spectrograph module, whose overall performance and excellent image quality were recently validated.⁹ The hexabundles were developed and tested at the University of Sydney (Australia) and will soon demonstrate their scientific feasibility at the Anglo-Australian Telescope.

In addition, our concept using deployable fiber IFUs and replicable spectrographs allows combining proven technologies with novel developments in astrophotonics¹⁰ into a highly modular instrument layout that can be scaled from current telescopes to the new generation of ELTs with minimum technological and financial risks. By 2011, we will present our ERASMUS-F study, which addresses the priority identified to develop a high-multiplex MOS for the E-ELT. In parallel, new photonics devices, such as fiber-Bragg and arrayed-waveguide gratings, are subjects of research and development¹¹ for possible implementation in ERASMUS-F.

The participating partners in the ERASMUS-F study include the Astrophysikalisches Institut Potsdam (Germany), the Centre de Recherche Astronomique de Lyon (France), the University of Sydney (Australia), the Institut für Astrophysik Göttingen (Germany), the Anglo-Australian Observatory (Australia), and the innoFSPEC-Potsdam innovation center (Germany). The German partners acknowledge funding from BMBF (Federal Ministry of Education and Research) Verbundforschung.