LGS implementation for NAOS

After more than two years of very successful operation in NGS mode, the VLT Shack-Hartmann AO system NAOS will be upgraded to be operational with the VLT LGS in late 2004. The implementation concerns a new STRAP tip-tilt sensor, an optical path including the trombone to accommodate for LGS height variations, a LIDAR device to measure the initial LGS height, and many high and low level software changes (real-time computer, instrument control, templates, preparation software, etc.). The paper presents this upgrade concept as well as some analysis of the predicted performance of NAOS-LGS.