Reflectivity analysis of a proposed multilayer coating for the slit assembly of Solar-C EUVST

SOLAR-C EUVST is the fourth M-class mission of the Japan Aerospace Exploration Agency, planned to be launched in 2028 as a the next solar mission after the successful - and still operating - Solar-B (Hinode). The mission is led by Japan with the contributions from NASA, ESA, and several European countries; the goal of the mission is to study the solar atmosphere in the extreme UV spectrum, carrying on board as its main instrumentation a spectrometer, EUVST (EUV High-throughput Spectroscopic Telescope), which includes also a Slit Jaw Imager. One key component of both instruments is the Slit Assembly, provided by an Italian Consortium led by the Italian Space Agency. The slit substrate requires a specifically designed optical coating to minimize the absorption in the visible and near IR spectrum and, at the same time, to maximize the reflection in the narrow bandpass centered at 280 nm, which is the wavelength observed by the Slit Jaw Imager. Since a high reflectivity is required, we verified that an aluminum-based coating can fulfill all the requirements. We then analyzed several typologies of protective dielectric layers, from single layer to more complex multilayer, using a genetic algorithm to optimize the number and thickness of the layers. A comparative analysis was performed focusing on both reflectance curve and absorbed heat. We selected two of the best performance coatings, the single layer protected aluminum coating SiO2/Al and the multilayer protected aluminum coating SiO2/[ZrO2/SiO2]3 /Al, and we prepared some samples. We analyzed the reflectivity of the samples in a broadband spectrum, from near UV to mid IR, comparing the measurements with the theoretical models. Both the theoretical models and the reflectivity measurements are presented in this work.