1 PERFORMANCE AND RESULTS OF THE NAOS VISIBLE WAVEFRONT SENSOR Philippe Feautrier 1 , Reinhold J. Dorn 2 , Gérard Rousset 3 , Cyril Cavadore 2 , Julien Charton 1 , Claudio Cumani 2 , Thierry Fusco 3 , Norbert Hubin 2 , Pierre Kern 1 , Jean-Louis Lizon 2 , Yves Magnard 1 , Pascal Puget 1 , Didier Rabaud 3 , Patrick Rabou 1 and Eric Stadler 1 . 1 LAOG: Laboratoire d’Astrophysique de Grenoble, BP 53, 38041 Grenoble Cedex 9, France2 2 ESO: European Southern Observatory, Germany and Chile 3 ONERA: Office National d’Etudes et de Recherches Aérospatiales, France Abstract: The NAOS adaptive optics system was installed in December 2001 on the Nasmyth focus of the ESO VLT. It includes two wavefront sensors: one is working at IR wavelengths, the other at visible wavelengths. This paper describes the NAOS Visible Wave Front Sensor based on a Shack-Hartmann principle. This wavefront sensor unit includes: 1) a continuous flow liquid nitrogen cryostat and a low noise fast readout CCD camera controlled by the ESO new generation CCD system FIERA using a fast frame rate EEV/Marconi CCD-50. This 128 x 128 pixels split frame transfer device has a readout noise of 3 e- at 50 kilopixel/sec/port. FIERA provides remotely controlled readout modes with optional binning, windowing and flexible integration time. 2) two remotely exchangeable micro-lens arrays (14x14 and 7x7 micro-lenses) cooled to the CCD temperature ( -100 ˚C). The CCD array is directly located in the micro lenses focal plane, only a few millimeters apart without any relay optics. Additional opto-mechanical functions are also provided (atmospheric dispersion compensator, flux level control, field of view limitation). On sky performances of the wavefront sensor are presented. Adaptive Optics corrections was obtained with a reference star as faint as visible magnitude 17. The maximum achievable band-path is 35 Hz at 0 dB for the open loop transfer function. Key words: CCD, low noise, camera, adaptive optical system, wavefront sensor, Strehl Ratio, microlenses, cryostat.