Polarization modeling and predictions for DKIST, part 9: flux distribution with FIDO

Abstract. Astronomical instruments greatly improve wavelength multiplexing capabilities by using beam splitters. In the case of the 4-m National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) solar telescope, over 70 W of optical power is distributed simultaneously to four instruments, each with multiple cameras. Many DKIST observing cases require simultaneous observations of many narrow bandpasses combined with an adaptive optics system. The facility uses five dichroic optical stations to allow at least 11 cameras and two wavefront sensors to simultaneously observe ultraviolet to infrared wavelengths with flexible reconfiguration. The DKIST dichroics required substantial development to achieve very tight specifications over very large apertures of 290 mm diameter. Coating spectral variation occurs over <1  nm wavelength, comparable with instrument bandpasses. We measure retardance spectral variation of up to a full wave and diattenuation varying over ±10  %   per nm. Spatial variation of Mueller matrix elements for coatings in both transmission and reflection requires careful metrology. We demonstrate coatings from multiple vendors exhibit this behavior. We show achievement of 5-nm root mean square (RMS) reflected wavefront and 24-nm RMS power with coatings over 8  μm thick. We show mild impacts of depolarization and spectral variation of polarization on modulation efficiency caused by the dichroic coatings. We show an end-to-end system polarization model for the visible spectropolarimeter instrument, including the dichroics, grating, analyzer, and all coated optics. We show detailed performance for all DKIST dichroics for community use in planning future observations.