NAOS: from an AO system to an astronomical instrument

Observing at high angular resolution from the ground is not made possible with Adaptive Optics alone, and besides the turbulence residuals, atmospheric refraction, thermal background or instrument's mechanical flexures may also severely limit the gain of optical quality that AO techniques are supposed to provide. We describe here how NAOS, the newly installed AO system on the VLT, has been designed to accommodate for these unavoidable effects. In particular, beam chopping, flexures compensation and AO tracking on reference objects with a significant relative motion will be addressed. It will thus be shown how long term astronomical observations at the diffraction limit can be carried out with an AO system under regular ground level conditions, thanks to the implementation of original technical solutions.