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The DKIST is designed to deliver accurate spectropolarimetric calibrationsacross a wide wavelength range and large field of view for solar observations.Instruments deliver spectral resolving powers up to 300,000 in multiple camerasof several instrument. We require detailed knowledge of optical coatings on alloptics to ensure we can predict and calibrate the polarization behavior of thesystem. Optical coatings can be metals protected by many dielectric layers orseveral-micron thick dichroics. Strong spectral gradients up to 60degretardance per nanometer wavelength and several percent diattenuation pernanometer wavelength are observed in such coatings. Often, optical coatings arenot specified with spectral gradient targets for polarimetry in combination toboth average and spectral threshold type specifications. DKIST has a suite ofinterchangeable dichroic beam splitters using up to 96 layers. We apply theBerreman formalism in open-source Python scripts to derive coating polarizationbehavior. We present high spectral resolution examples on dichroics wheretransmission can drop 10% with associated polarization changes over a 1 nmspectral bandpass in both mirrors and dichroics. We design dichroic coatingsthat pass polarization spectral gradient requirements in addition toreflectivity. We now can fit multi-layer coating designs to predict systemlevel polarization properties of mirrors, anti-reflection coatings anddichroics at arbitrary incidence angles, high spectral resolving power oncurved surfaces through optical modeling software packages. Polarizationpredictions for large astronomical telescopes require significant metrologyefforts on individual optical components combined with systems-level modelingefforts. We show our custom-built laboratory spectropolarimeter and metrologyefforts on protected metal mirrors, anti-reflection coatings and dichroicmirror samples.

Polarization Modeling and Predictions for DKIST Part 5: Impacts of enhanced mirror and dichroic coatings on system polarization calibration