  Aims: To propose a method for the polarimetric calibration of large
astronomical mirrors that does not require use of special optical devices nor
knowledge of the exact polarization properties of the calibration target.
  Methods: We study the symmetries of the Mueller matrix of mirrors to exploit
them for polarimetric calibration under the assumptions that only the
orientation of the linear polarization plane of the calibration target is known
with certainty.
  Results: A method is proposed to calibrate the polarization effects of single
astronomical mirrors by the observation of calibration targets with known
orientation of the linear polarization. We study the uncertainties of the
method and the signal-to-noise ratios required for an acceptable calibration.
We list astronomical targets ready for the method. We finally extend the method
to the calibration of two or more mirrors, in particular to the case when they
share the same incidence plane.
