The Schmidt telescope at Calar Alto Observatory, Spain, was installed there in 1980 by the Max-Planck-Institut für Astronomie, Heidelberg, and has been operated since then by this institute; originally it had been operational at the Hamburg Observatory since 1955. The corrector plate and mirror have 80 cm and 120 cm diameter respectively; focal length is 240 cm. A field 5.5 deg by 5.5 deg is photographed on 24 cm by 24 cm curved plates. Two objective prisms of 1.7 deg and 4.0 deg refractive angle are available (Heckmann 1955; Birkle 1984). Thousands of direct and spectral plates have been taken, mainly for the Hamburg Quasar Survey (Engels et al. 1988). Other observing programs referred e.g. to astrometry of comets, giving support to ESA's Giotto space encounters with comets Halley and Grigg-Skjellerup (Roeser et al. 1986; Morley 1992). Gradually, however, the number of plates increased showing image quality which did not correspond to the excellent performance of the telescope optics. Several technical improvements have therefore been installed to remove some obvious mechanical instability.