Polarization modeling and predictions for DKIST, part 8: calibration polarizer spatial variation impacts

Abstract

Astronomical spectropolarimeters require high accuracy polarizers with large aperture and stringent uniformity requirements. In solar applications, wire grid polarizers are often used as performance is maintained under high heat loads and temperatures over 200°C. DKIST is the NSF’s new 4-m aperture solar telescope designed to deliver accurate spectropolarimetric solar data across a wide wavelength range, covering a large field of view simultaneously using multiple facility instruments. Polarizers at 120 mm diameter are used to calibrate DKIST instruments but vary spatially in transmission, extinction ratio, and orientation of maximum extinction. We combine new spatial and spectral metrology for polarizers and retarders to simulate the accuracy losses with field angle and wavelength caused simultaneously by spatial variation of several optical parameters including beam decenter from misalignments. We also present testing of a new crystal sapphire substrate polarizer designed and fabricated to improve DKIST long wavelength calibrations. We assess spatial thickness variation of sapphire and fused silica wafer substrates using spectral interference fringes.