Polarization modeling and predictions for Daniel K. Inouye Solar Telescope part 5: impacts of enhanced mirror and dichroic coatings on system polarization calibration

Abstract

The Daniel K. Inouye Solar Telescope (DKIST) is designed to deliver accurate spectro-polarimetric calibrations across a wide wavelength range and large field of view for solar disk, limb and coronal observations. DKIST instruments deliver spectral resolving powers up to 300,000 in multiple cameras of multiple instruments sampling nanometer scale bandpasses. 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 60 • 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 worked with a vendor to design dichroic coatings with relatively benign polarization properties that pass spectral gradient requirements and polarization requirements in addition to reflectivity. We now have the ability to fit multi-layer coating designs which allow us to predict system level polarization properties of mirrors, anti-reflection coatings and dichroics at arbitrary incidence angles, high spectral resolving power and on curved surfaces through optical modeling software packages. Performance predictions for polarization at large astronomical telescopes requires 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, antireflection coatings and dichroic mirror samples.

Polarization modeling and predictions for Daniel K. Inouye Solar Telescope, part 7: preliminary NCSP system calibration and model fitting

“…1.1 of H19. 67 We outline an example here with the CN instrument optical path. We use this optical path as NCSP also uses these same optics for models presented in this paper.…”

The Daniel K. Inouye Solar Telescope – Observatory Overview

“…Preliminary system model calibrations were successful and >100-variable fits are well within tolerance expectations for the properties of DKIST optics (Harrington et al, 2020b). The suite of facility dichroics that feed the AO-assisted instruments have open-source coating designs that have been characterized for their polarization performance in both reflection and transmission and also for spatial uniformity of coatingpolarization properties (Harrington, Sueoka, and White, 2019).…”

“…Systems engineering also included development of imaging and wavefront-error budgets and active tracking and management of error budgets throughout the process. A new approach to polarimetry system error budgets was developed and is described in a series of publications Harrington et al, , 2021Harrington and Sueoka, 2018a,b;Harrington, Sueoka, and White, 2019). The process concludes with final verification of SRD, OCD, and ISRD requirements and hand-over of the facility to operations.…”

“…The configurable facility instrument distribution system (FIDO) enables multiple instrument configurations (Elmore et al, 2014;Harrington, Sueoka, and White, 2019). Multi-instrument experiments are required by many science use-cases, for example, the sampling of the magnetic field at multiple heights in the solar atmosphere.…”

“…New coatings were developed in collaboration with vendors to mitigate interference fringes (see Harrington et al (2021) and the references therein). The mirror-coating polarization was measured for every coated surface in the system including all instrument optics and facility dichroics Harrington, Sueoka, and White, 2019). The coatings and their impact on system calibration were modeled to understand how to use the AO system and field-scanning techniques to keep polarization errors below user-defined thresholds.…”