The sodium laser guide star system of ALFA

ALFA is an adaptive optics system with laser guide star for the 3.5 m telescope on Calar Alto, Spain. ALFA has been developed jointly by the Max-Planck-Institutes for Astronomy (MPIA, Heidelberg) and for Extraterrestrial Physics (MPE, Garching). All core components of ALFA were installed at the telescope in the course of 1996; the system is currently undergoing integration and testing. ALFA uses a continuous-wave dye laser pumped by an argon ion laser to create an artificial guide star in the mesospheric sodium layer. The laser is stabilized to the sodium D-2 line at 589nm with a sodium vapor cell. In normal operation the frequency is locked to the Lamb dip in the absorption cell, but it is also possible to scan the laser under computer control over several GHz for diagnostic purposes. The dye laser has been optimized for stable operation at high power output; 3.75 W are regularly achieved in single-mode operation. The laser system is located in the coude room of the telescope; the laser beam is transmitted through the coude path to a 50 cm beam expansion telescope attached to the mirror cell of the 3.5 m telescope. The first measurements of the sodium spot created with the ALFA laser were made in 1996 with the guide camera of the 3.5 m telescope and with a 1.23 m telescope located 276 m away.