Nighttime polarimetric measurements are often obtained very close to the limits of the instrumental capabilities. It is important to be aware of the possible sources of spurious polarization, and to adopt data reduction techniques that best compensate for the instrumental effects intrinsic to the design of the most common polarimeters adopted for nighttime observations. We define a self-consistent framework starting from the basic definitions of the Stokes parameters, and we present an analytical description of the data reduction techniques commonly used with a polarimeter (consisting of a retarder wave plate and a Wollaston prism) to explore their advantages and limitations. We first consider an ideal polarimeter in which all optical components are perfectly defined by their nominal characteristics. We then introduce deviations from the nominal behavior of the polarimetric optics, and develop an analytical model to describe the polarization of the outgoing radiation. We study and compare the results of two different data reduction methods, one based on the differences of the signals, and one based on their ratios, to evaluate the residual amount of spurious polarization. We show that data reduction techniques may fully compensate for small deviations of the polarimetric optics from their nominal values, although some important (first-order) corrections have to be adopted for linear polarization data. We include a detailed discussion of quality checking by means of null parameters. We present an application to data obtained with the FORS1 instrument of the ESO VLT, in which we have detected a significant amount of cross talk between circular and linear polarization. We show that this cross-talk effect is not due to the polarimetric optics themselves, but is most likely caused by spurious birefringence due to the instrument's collimator lens.

Abstract. We present the results of a statistical study of the magnetic structure of upper main sequence chemically peculiar stars. We have modelled a sample of 34 stars, assuming that the magnetic morphology is described by the superposition of a dipole and a quadrupole field, arbitrarily oriented. In order to interpret the modelling results, we have introduced a novel set of angles that provides one with a convenient way to represent the mutual orientation of the quadrupolar component, the dipolar component, and the rotation axis. Some of our results are similar to what has already been found in previous studies, e.g., that the inclination of the dipole axis to the rotation axis is usually large for short-period stars and small for long-period ones -see Landstreet & Mathys (2000). We also found that for short-period stars (approximately P < 10 days) the plane containing the two unit vectors that characterise the quadrupole is almost coincident with the plane containing the stellar rotation axis and the dipole axis. Long-period stars seem to be preferentially characterised by a quadrupole orientation such that the planes just mentioned are perpendicular. There is also some loose indication of a continuous transition between the two classes of stars with increasing rotational period.

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Quod si tam celebris est apud omnes gloria Adamantis, atque varia ista opum gaudia, gemmae unionesque, ad ostentationem tantum placent, ut digitis colloque circumferantur; non minori afficiendos speraverim gaudio eos, quibus curiositatis conscientia quam deliciarum est potior, novitate corporis alicujus, instar crystalli translucidi, quod ex Islandia nuper ad nos perlatum est; cujus tam mira est constitutio, ut haud sciam, num alias magis naturae apparuerit gratia. Erasmus Bartholinus, Experimenta crystalli islandici disdiaclasticiApart from a few objects of our immediate neighborhood (the solar system), all the information on the physical phenomena taking place in the Universe comes from the radiation that the astronomical objects send into space and that is finally collected on earth by telescopes or other instruments. Among the different kinds of radiation, electromagnetic waves have by far played the most important role in the history of Astronomy -probably, it is not unrealistic to say that more than 99% of our present knowledge of the Universe derives from the analysis of the electromagnetic radiation.Such radiation contains three different kinds of information, encoded into as many physical characteristics typical of any oscillatory propagation phenomenon: the propagation direction, the frequency and amplitude of the oscillation, and the oscillation direction -or polarization.The first one is the most direct and the easiest to measure: the human eye is itself a suitable instrument, though of limited accuracy. As time passed, positional astronomy became more and more accurate thanks to the invention of the optical telescope, the introduction of photographic and digital techniques and, more recently, the development of technologies for producing images of a given region at different wavelengths via radio, infrared, X-ray and γ-ray telescopes, often operating on board of spacecrafts. Our present knowledge of the morphology and dynamics of the Universe, and of the different objects of which it is composed (from planets to stars, from nebulae to globular clusters, from galaxies to Active Galactic Nuclei and to clusters of galaxies) is based on the huge number of such observations that have been accumulating during several centuries.However, even if very accurate, the measurement of the propagation direction of the electromagnetic radiation is inadequate to study other fundamental aspects of the physical Universe such as the composition, structure, and evolution of the different objects. To this aim, a detailed analysis of the frequency (or wavelength) distribution of the energy carried by the electromagnetic radiation is required, which was made possible by the invention of the spectrograph. Only through the systematic use of spectroscopic methods it has been possible to obtain a direct comprehension of the physical mechanisms which govern the equilibrium of stars, their birth, evolution and death, and the complicated processes taking place in the interstellar medium and in the nuclei of galaxies. Spectroscopy,...

Aims. We inverted a spectropolarimetric scan of an active region and a filament (240 × 340 arcsec) achieved with THEMIS on 7 December 2003 in the two lines Fe i 6302.5 and 6301.5 Å.Methods. The inversion was achieved for each line separately by using the UNNOFIT code of Landolfi and Landi Degl'Innocenti, and was improved by introducing a magnetic filling-factor parameter. The magnetic and non-magnetic theoretical atmospheres, mixed in the proportion given by the filling factor, were derived from the same set of parameters, except for the presence (or absence) of a magnetic field. The fundamental ambiguity is not solved. Results. The tests run with UNNOFIT show that the magnetic field strength B and the magnetic filling factor α cannot be separately recovered by the inversion in Fe i 6302.5, but that their product αB, which is the local average magnetic field, is recovered. The magnetic flux is only its longitudinal component. In addition, the results make two regimes clearly appear, corresponding to two ranges of local average magnetic field strength as measured in 6302.5: (a) the network, having a field inclined of about 20• -30• from the vertical in 6302.5 (spread more but non-horizontal in 6301.5), with a homogeneous azimuth. In this zone the local average field strength in 6302.5 is higher than 45 Gauss; (b) the internetwork, where the field is turbulent (with a horizontal trend, spread more at lower altitudes), and the 6302.5 local average field strength is lower than 45 Gauss (about 20 Gauss). Conclusions. The two lines display coherent results, in particular for the magnetic-field azimuth. From this coherence we conclude that the turbulence of the 20 Gauss internetwork field has a solar origin.