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

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... /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.