Abstract
Interference fringes are a major source of systematic error in astronomical spectropolarimeters. We apply the Berreman formalism with recent spatial fringe aperture averaging estimates to design and fabricate new fringe-suppressed polarization optics for several Daniel K. Inouye Solar Telescope (DKIST) use cases. We successfully performed an optical contact bond on a 120-mm-diameter compound crystal retarder for calibration with wavelength-dependent fringe suppression factors of one to three orders of magnitude. Special rotational alignment procedures were developed to minimize spectral oscillations, which we show here to represent our calibration stability limit under retarder thermal perturbation. We developed a fabrication technique to deliver low beam deflection for our large aperture polycarbonate (PC) retarders. Modulators are upgraded in two DKIST instruments with minimal beam deflection and bandpass-optimized antireflection coatings for fringe suppression factors of hundreds. We confirm that PC retarders do fringe as expected when low deflection is achieved. We show that increased retardance spatial variation from PC does not degrade modulation efficiency.
Highlights
We have recently investigated spatial variation of retardance across multilayer retarders made of polished crystals, stretched polycarbonate (PC), and Ferroelectric liquid crystals (FLCs) in HS18b
We show in Appendix F that the clocking oscillations for this optic are similar in magnitude between the SiO2 retarders and the MgF2 retarders
The primary fringe anticipated from the original six-crystal visible spectropolarimeter (ViSP) modulator is caused by the air-to-oil interface in a single 2-mm-thick crystal and is at magnitudes of roughly 1% to 2%
Summary
In H18,43 we applied Berreman calculus[44,45] to polarization fringes formed in multilayer crystals with predictions and data collected in the lab and at a solar telescope We extended this calculus in HS18a43 to include fringe magnitude estimates of converging and diverging beams. We have recently investigated spatial variation of retardance across multilayer retarders made of polished crystals, stretched polycarbonate (PC), and Ferroelectric liquid crystals (FLCs) in HS18b.46 This variation was included in the DKIST optical model to show polarization calibration errors as functions of field angle and wavelength. We apply the Berreman calculus for collimated beams with our beam focal ratio fringe scaling relation[43] to estimate fringe magnitudes from every component internal to each retarder in both the DKIST calibration optics and the three first-light instruments ViSP, DL-NIRSP, and Cryo-NIRSP. The first column of the Mueller matrix elements are II, IQ, IU, and IV
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