  We present the analysis of the observations of standard stars for linear
polarization obtained from 1999 to 2005 within the context of the calibration
plan of the FORS1 instrument of the ESO VLT. We have considered observations
carried out both in imaging polarimetric and in spectropolarimetric mode.
Broadband polarization was obtained in the Bessel BVRI filters;
spectropolarimetry was obtained with various grisms covering different optical
ranges and with a typical resolution of a few hundreds. Spectropolarimetric
data have been convolved with the transmission functions of the Bessel filters,
which enabled us to calculate polarization values equivalent to broadband
polarization measurements in imaging mode. Finally, for each star, instrument
mode, and for each Bessel filter band, we have calculated an average
polarization value obtained after filtering all available data with a k-sigma
clipping algorithm.
