Black coatings for combined stray light and thermal passive management for the challenging environmental conditions of Solar Orbiter

Abstract

In the context of ESA’s Cosmic Vision 2015-2025 scientific program, Solar Orbiter (SolO) represents the first M-class mission, currently under implementation for a launch planned in October 2018. One of the SolO solar remote-sensing instruments is the Multi Element Telescope for Imaging and Spectroscopy (METIS), under development by an Italian consortium ATI composed by CGS and TAS-I. METIS is a coronagraph that will perform simultaneously broad-band imaging in visible light and narrow-band imaging in UV of the Sun corona with unprecedented spatial resolution. The surfaces of several Ti-6Al-4V components of the METIS Optical Unit must have a very low reflectance in order to minimize straylight and therefore ACKTAR Magic BlackTM coating was specified. An example is represented by the internal surface of the Inverted External Occulter (IEO) - the front part of the instrument - which is exposed to the direct solar flux and experiences challenging environmental conditions. Qualification for 450°C was required whereas Acktar's Magic BlackTM qualified maximum temperature was 380°C. A dedicated thermal-vacuum cycling test (100 total cycles, −120°C − 450°C) was conducted to qualify Magic BlackTM to quantify any performance degradation under these conditions: • optical (reflectance) • thermal (∝ and e values) to allow calculation of the radiative heat transfer of the METIS parts during mission • mechanical performances The following success criteria were established: • total reflectance at wavelengths of 400−1000nm <;5% • degradation of absorptance ∝ and emissivity e to be <10% • surface resistivity <5 kΩ/square