Spectropolarimetry of magnetic stars

New determinations of the mean longitudinal magnetic field, of the crossover, and of the mean quadratic magnetic field of Ap stars are presented. They are based on spectra recorded simultaneously in both circular polarizations at ESO with the CASPEC spectrograph fed by the 3.6 m telescope. This paper discusses 95 observations of 44 stars. A major result of this study is the discovery that HD 137509 has a predominantly quadrupolar magnetic field, a strucuture previously found in only a couple of stars. Improvement or revision of the determination of the rotation period has been achieved for 3 stars. The stars studied in this work include 14 rapidly oscillating Ap stars (for 6 of which no previous attempt to detect a magnetic field had ever been made) and 21 Ap stars with spectral lines resolved into their magnetically split components when observed at high enough dispersion in unpolarized light (for 9 of these stars, no determination of the longitudinal field had been performed before). The observations discussed in this paper have been performed between 1989 and 1994, a period during which CASPEC and its Zeeman analyzer have progressively undergone various configuration changes. The results reported here demonstrate that the polarimetric performance of the instrument has remained unaltered through these modifications. Thanks to the latter, the achieved resolving power was increased, which resulted in improved magnetic measurement accuracies