The Calar Alto Observatory, or `Centro Astronomico Hispano-Aleman', is located at an altitude of 2168 m in the `Sierra de los Filabres', in southern Spain. Its construction on Calar Alto mountain began in 1973. It is operated jointly by the MAX-PLANCK-INSTITUT FÜR ASTRONOMIE in Heidelberg (MPIA), Germany, and the `Comision Nacional de Astronomia'. The MPIA provides four telescopes of diameters 3.