The MICADO first light imager for the ELT: manufacturing of the large dichroic plate of its SCAO module

MICADO, a first-light instrument on the European Extremely Large Telescope, operates in the NIR, offering imaging, astrometry, spectroscopy, and high-contrast capabilities. The optical design of its single conjugate adaptive optics (SCAO) module features a 305 mm CaF₂ dichroic plate that splits light between AO and the scientific camera, achieving over 97% transmission from 960–2450 nm and >92% reflectivity from 589–960 nm. The front surface has a thin-film optical filter optimized to minimize coating-induced stress and sensitivity to manufacturing errors. Deposition was performed via ion-beam sputtering with careful control of thermal gradients and post-deposition annealing to release internal stress. The back side has a 16-layer broadband anti-reflective coating ensuring >98.5% transmission. Post-coating characterization confirmed uniform transmission over a 290 mm aperture and reflected wavefront errors within specifications, demonstrating high optical performance.