Nanostructured optical coatings for solar physics observations from space

METIS is a coronagraph onboard of Solar Orbiter. It will perform simultaneous observation at HeII Lyman-α line, HI Lyman-α line and in visible. To achieve such capability, instrument mirrors need to be coated by multilayer structures with high efficiency at all three spectral ranges. Coatings with higher performances respect than standard coating based on Mo/Si couple are desirable. SCORE, an instrument prototype of METIS, has just flown on board of a NASA sounding rocket: in this case, optics were coated with Mg/SiC couple. Better performances have been obtained in term of reflectivity, but long term stability of this coating is an open problem. Moreover the harsh conditions of the environment met during the Solar Orbiter mission given by plasma particles and high temperature could affect the lifetime of the optical components on the long term. We present the design and reflectivity tests of new multilayer structure in which performances improvement is obtained by the use of novel capping layers. All multilayers are tuned at 30.4nm line but the design also maximize the performances at 121.6nm and 500 - 650 nm visible range. Analysis of Solar Orbiter environment have been carried on in order to point out the main damaging sources for the nanostructures. Experimental tests for investigating the effects of the thermal heating and particles bombardments in the reflectivity performances have been planned