EXTREME ULTRAVIOLET MULTILAYER COATINGS WITH IMPROVED SPECTRAL FILTERING FOR SOLAR OBSERVATION APPLICATIONS

In recent years telescopes based to multilayer mirror technology have been employed in missions dedicated to the Sun observation in EUV wavelengths. Performances of multilayer are mainly evaluated in term of peak reflectivity at working wavelength and rejection capability of unwanted lines. Mo/Si multilayer are conventionally used for their high stability, but, when the structures are optimized for the 28.4 nm, the reflectivity is broadband and include the unwanted strong HeII line. A more narrowband solution has been obtained through an a-periodic design or using other material combinations. We propose an innovative method for designing suitable capping layer covering the multilayer which do not affect the reflectivity peak while rejecting unwanted lines. The capping layer solution can be adopted both in case of periodic and a-periodic multilayer, made by different materials. Thank to its flexibility, the design tool can provide innovative solutions with important improvement that can strongly affect the capability of the future solar instruments. We show and discuss theoretical and experimental results of some coatings designed by the use of the new mathematical tool