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 Black^TM 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 Black^TM qualified maximum temperature was 380°C.
A dedicated thermal-vacuum cycling test (100 total cycles, -120°C - 450°C) was conducted to qualify Magic Black^TM to quantify any performance degradation under these conditions:
• optical (reflectance)
• thermal ( and ɛ 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 ɛ to be <10%
• surface resistivity <5 kΩ/square