  The DKIST is designed to deliver accurate spectropolarimetric calibrations
across a wide wavelength range and large field of view for solar observations.
Instruments deliver spectral resolving powers up to 300,000 in multiple cameras
of several instrument. We require detailed knowledge of optical coatings on all
optics to ensure we can predict and calibrate the polarization behavior of the
system. Optical coatings can be metals protected by many dielectric layers or
several-micron thick dichroics. Strong spectral gradients up to 60deg
retardance per nanometer wavelength and several percent diattenuation per
nanometer wavelength are observed in such coatings. Often, optical coatings are
not specified with spectral gradient targets for polarimetry in combination to
both average and spectral threshold type specifications. DKIST has a suite of
interchangeable dichroic beam splitters using up to 96 layers. We apply the
Berreman formalism in open-source Python scripts to derive coating polarization
behavior. We present high spectral resolution examples on dichroics where
transmission can drop 10% with associated polarization changes over a 1 nm
spectral bandpass in both mirrors and dichroics. We design dichroic coatings
that pass polarization spectral gradient requirements in addition to
reflectivity. We now can fit multi-layer coating designs to predict system
level polarization properties of mirrors, anti-reflection coatings and
dichroics at arbitrary incidence angles, high spectral resolving power on
curved surfaces through optical modeling software packages. Polarization
predictions for large astronomical telescopes require significant metrology
efforts on individual optical components combined with systems-level modeling
efforts. We show our custom-built laboratory spectropolarimeter and metrology
efforts on protected metal mirrors, anti-reflection coatings and dichroic
mirror samples.
