  Accurate astrophysical polarimetry requires a proper characterization of the
polarization properties of the telescope and instrumentation employed to obtain
the observations. Determining the telescope and instrument Muller matrix is
becoming increasingly difficult with the increase in aperture size of the new
and upcoming solar telescopes. We have carried out a detailed multi-wavelength
characterization of the Dunn Solar Telescope (DST) at the National Solar
Observatory/Sacramento Peak as a case study and explore various possibilites
for the determination of its polarimetric properties. We show that the
telescope model proposed in this paper is more suitable than that in previous
work in that it describes better the wavelength dependence of aluminum-coated
mirrors. We explore the adequacy of the degrees of freedom allowed by the model
using a novel mathematical formalism. Finally, we investigate the use of
polarimeter calibration data taken at different times of the day to
characterize the telescope and find that very valuable information on the
telescope properties can be obtained in this manner. The results are also
consistent with the entrance window polarizer measurements, opening very
interesting possibilities for the calibration of future large-aperture solar
telescopes such as the ATST or the EST.
