New challenges for adaptive optics: extremely large telescopes

The performance of an adaptive optics (AO) system on a 100-m diameter groun d-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is ach ieved at 0.5 mu m with a limiting magnitude of the AO guide source near R m agnitude similar to 10, at the cost of an extremely low sky coverage. To al leviate this problem, the concept of tomographic wavefront sensing in a wid er field of view using either natural guide stars (NGS) or laser guide star s (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to X-fold the correct ed field size (up to 60 arcsec at 0.5 mu m). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefr ont correction in the visible using several bright NGS is discussed. The LG S approach involves the use of a faint (R similar to 22) NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.