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 \mu m with a limiting magnitude of the AO guide source near R \sim 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 \mu m ) .
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 \sim 22 ) NGS for low-order correction , which results in a sky coverage of 40 % at the Galactic poles in the visible .