New challenges for Adaptive Optics: Extremely Large Telescopes

The performance of an adaptive optics (AO) system on a 100 m diameter ground based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 % is achieved at 0.5 μm with a limiting magnitude of the AO guide source near R∼ 10, at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use 3 or 4 reference sources and up to 3 deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 at 0.5 μm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 % with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faint (R ∼ 22) NGS for low-order correction, which results in a sky coverage of 40 % at the Galactic poles in the visible.