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.
Liquid crystal (LC) technology, a critical component in a diverse range of optics for visible wavelengths, has recently been adapted into devices for the mid-wave infrared (MWIR). Optics designs, including variable retarders, attenuators, linear polarization rotators, and tunable filters, have been modified for optimal performance over the range of 3.6 to 5.7 microns.
We constructed these designs using material selected for optimal optical behavior in this wavelength range. Description and characterization of these chosen component materials is included along with the performance of each device. We present design challenges, along with future plans and possibilities for MWlR LC technology.