Data Set Overview : A long term project of polarimetric observtions of asteroids is being conducted at the 2.15-m telescope of the El Leoncito Observatory in Argentina, using the Torino photopolarimeter. The results from 1995 through 2005 were reported in Cellino et al. (2005) and are archived in this data set.  The Torino photopolarimeter, described in Scaltriti et al. (1989), is capable of simultaneous polarimetric observations in UBVRI colors, making it possible to obtain an automatic elimination of the spurious polarization of the background skylight. Since the observations are simultaneous, they are not affected by possible changes of polarization due to rotation of the target.  For all the observations included in this data set, the instrument was used in the linear polarization configuration. Data reduction is based on standard routines developed for this kind of instrument, with some adaptation for the needs of asteroid observation. Specifically, the reduction routines included standard procedures to derive the Stokes parameters from photon countings obtained at eight different positions of a half-wave plate located in front of the polarizer (a calcite block). From these photon countings the Stokes parameters Q an U (also called Px and Py by some authors) are derived, which fully describe the state of linear polarization. Then, the Pr parameter, which is the scientific result of polarization observations of asteroids, is computed as P (square root of Q**2 + U**2) times cosine(2*theta), theta being the angle between the plane of linear polarization of the incoming beam, and the plane perpendicular to the scattering plane (defined as the Earth - target - Sun plane). Theta is computed from trivial geometrical considerations, being known from the coordinates of the Sun and of the asteroid at the epoch of observation.  The observations were carried out between 1995 and 2005, with typically one or two observing runs per year and 4-7 nights per run. Observing nights were usually selected around the new moon to minimize the polarization contamintaion by moonlight.  In the paper, the results are divided into Tables 1-4, representing different polarimetric investigations, plus one summary table. Here, the data in Tables 1-4 are combined into one data file, torinopol.tab. Table 5, the summary table, is included here in the file table5.tab.  The file torinopol.tab contains object number and phase angle, followed by proper polarization and theta in each of the five bandpasses UBVRI, and their uncertainties.  In addition, the file table5.tab (Table 5 in the published paper) contains a summary of overall results for the IRAS-observed asteroids with basic data in Table 2.  References:  Cellino, A., R. Gil Hutton, M. DiMartino, Ph. Bendjoya, I.N. Belskaya, and E.F. Tedesco, Asteroid polarimetric observations using the Torino UBVRI photopolarimeter, Icarus 179, 304-324, 2005.  Scaltriti, F., V. Piirola, A. Cellino, E. Anderlucci, L. Corcione, G. Massone, F. Racioppi, and F. Porcu, The UBVRI photopolarimeter of the Torino Astronomical Observatory, Mem. Soc. Astron. It. 60, 243-246, 1989.

Confidence Level Overview : See the published paper, Cellino et al. (2005) for detailed information about the reliability of the measurements.