AO concepts for multi-object 3D spectroscopy on ELT
F. Hammer¹, P. Jagourel¹, B. Neichel^1,2, T. Fusco², E. Gendron¹, J.G. Cuby³, M. Puech¹, H. Flores^1
1 GEPI, Observatoire de Paris ² ONERA ³ Observatoire de Marseille
A major goal of modern astrophysics is to understand our origin, i.e. the physical processes of the formation of stars and metal in galaxies, which ultimately lead to the formation of telluric planets and life. The concept of Extremely Large Telescopes (ELTs) uniquely complements detections made from space, by accommodating high spectral resolution, 3D spectroscopy devices and relatively large fields of view (about 10 arc minutes). The major challenge for ground based telescopes is atmospheric turbulence, which allows image qualities of 0.5 arc second at best for long exposures. It can be corrected by the mean of Adaptive Optics with, nevertheless, specific difficulties linked to cosmological fields which lie far from the plane of the Galaxy (i.e. with a low density of natural guide stars and resulting poor sky coverage). We will present the science requirements concerning the required spatial resolution (in the 0.05 to 0.15 arc second range) as well as first estimations of the AO system characteristics that will allow fulfilling the need. GLAO (Ground Layer Adaptive Optics) should allow a first stage enhancement to achieve typically a 0.30-0.35 arc second resolution which, nevertheless, is not consistent with the ultimate foreseen performance. Emphasis will be made on a specific Multi-Object AO concepts (FALCON or Distributed Adaptive Optics) which is extremely promising for this kind of scientific application. Limitations linked to Natural Guide Stars systems will be discussed in terms of sky coverage and resolution (simulations show that a 0.1-0.15 arc second resolution seems to be achievable with NGS with reasonable sky coverage). Novel ideas concerning the implementation of rather simple Laser Guide Stars systems will be presented. Should some critical issues be solved (cone effect), this last solution would allow the aim of 0.05 arc second resolution to be met. A discussion of a better compromise between science and AO requirements will be presented.

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