The Magdalena Ridge Observatory Interferometer: custom near-IR beamsplitter and AR coatings

This report focuses on the design, application, and testing of custom beamsplitter and anti-reflection coatings for use in the Magdalena Ridge Observatory Interferometer (MROI) beam combiners. The coatings were designed in collaboration with Optical Surface Technologies, and the University of Cambridge. The fringe tracker and science combiners will operate across the J, H, and K bands. The coatings were designed to achieve three optical characteristics critical to optical interferometry: 1) minimized stress of the substrate (leading to induced wavefront errors), 2) high throughput, and 3) high visibilities in broadband unpolarized light. The AR coating has mean reflection losses of less than 0.5%. Beamsplitter coatings experienced visibility losses less than 1% due to group delay dispersion and s and p phase differences.