Adaptive optics with a laser guide star - The ALFA system

All 8-metre class telescopes will be equipped with adaptive optics systems in order to reach their highest angular resolutions. With the additional requirement for maximal sky coverage comes the mandatory use of a laser guide star. ALFA is an adaptive optics system using a sodium laser guide star, which is installed on the 3.5-m telescope at Calar Alto observatory. It has shown that it is possible to reach Strehl ratios better than 20% in the K-band. In this contribution we describe the design of the system, the observational trade-off necessary to use a laser guide star, and the performance that might be expected.