  We present mathematical methods, based on convex optimization, for correcting
non-physical coherency matrices measured in polarimetry. We also develop the
method for recovering the coherency matrices corresponding to the smallest and
largest values of the degree of polarization given the experimental data and a
specified tolerance. We use experimental non-physical results obtained with the
standard polarimetry scheme and a commercial polarimeter to illustrate these
methods. Our techniques are applied in post-processing, which complements other
experimental methods for robust polarimetry.
