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Measurement of polarization assemblies for the Daniel K. Inouye Solar Telescope

We present here methodology and instrumentation for the precise measurement of retardance and optic axis orientation of retarder assemblies for the Daniel K. Inouye Solar Telescope. This solar telescope will perform broadband polarimetry of the sun. Each Meadowlark assembly is made up of three compound zero order retarders that must have a retardance variation of less than 6.33 nanometers across the greater than 110 millimeter clear aperture. The retardation of each component was measured using a combination of spectral transmission scans and ellipsometry, with test wavelengths of less than a 0.45 nanometer bandwidths and yielding a standard deviation in measurements of less than 0.001 waves. A technique for the measurement of the near zero window (Infrasil® and CaF2) retardance is shown, in addition to retardance measurements of the component waveplates. An average retardance of 0.63 nm for CaF2 and 0.28 nm for Infrasil® was found. Finally, a technique for determining the optic axis tilt of each crystal waveplate using laser ellipsometry is discussed.