  The daytime sky has been recently demonstrated as a useful calibration tool
for deriving polarization cross-talk properties of large astronomical
telescopes. The Daniel K Inouye Solar Telescope (DKIST) and other large
telescopes under construction can benefit from precise polarimetric calibration
of large mirrors. Several atmospheric phenomena and instrumental errors
potentially limit the techniques accuracy. At the 3.67m AEOS telescope on
Haleakala, we have performed a large observing campaign with the HiVIS
spectropolarimeter to identify limitations and develop algorithms for
extracting consistent calibrations. Effective sampling of the telescope optical
configurations and filtering of data for several derived parameters provide
robustness to the derived Mueller matrix calibrations. Second-order scattering
models of the sky show that this method is relatively insensitive to
multiple-scattering in the sky provided calibration observations are done in
regions of high polarization degree. The technique is also insensitive to
assumptions about telescope induced polarization provided the mirror coatings
are highly reflective. Zemax-derived polarization models show agreement between
the functional dependence of polarization predictions and the corresponding
on-sky calibrations.

Instrumentation and Methods for Astrophysics