1.

INTRODUCTION

In the context of ESA’s Cosmic Vision 2015-2025 scientific program, Solar Orbiter (SolO, see figure 1) represents the first M-class mission, dedicated to solar and heliospheric physics, currently under implementation for a launch planned in October 2018.⁽¹⁾

Figure 1.

Artist’s impression of Solar Orbiter (Credit ESA)

2.

METIS OVERVIEW

As one of the six SolO solar remote-sensing instruments (see figure 2), the Multi Element Telescope for Imaging and Spectroscopy (METIS) is proposed by an international scientific consortium led by Istituto Nazionale di Astrofisica (INAF, Astronomical Observatory of Turin), funded by the Italian Space Agency (ASI) and under development and implementation by the Italian consortium ATI composed by Compagnia Generale per lo Spazio (CGS) and Thales Alenia Space – Italia (TAS-I).

Figure 2.

Payload accommodation onboard Solar Orbiter (Credit ESA)

Figure 3.

CAD view of METIS (Credit: CGS, ASI)

An example is represented by the METIS Shield Entrance Aperture (SEA), as shown in figure 4.

Figure 4.

CAD view of the METIS SEA (Credit: TAS-I, ASI)

Magic Black™ is the coating of choice where the absolute lowest reflectance / highest emissivity is required in the FUV – UV,VIS up to SWIR regions - typically for stray-light suppression. In addition to its superior optical performance (see Figs 5-7). Magic Black™ was thoroughly tested and qualified for thermal cycling, thermal vacuum cycling, cryogenic temperature performance, outgassing, space atomic oxygen and reflectance after environmental exposure.

Figure 5.

Total Hemispherical Reflectance of Acktar’s Magic Black™ (Credit: Acktar Ltd.) ⁽²⁾

Figure 6.

BRDF of Acktar’s Magic Black™ at 532 nm wavelength (Credit: Acktar Ltd.) ⁽³⁾

Figure 7.

BRDF of Acktar’s Magic Black™ at 640 nm wavelength (Credit: Acktar Ltd.) ⁽³⁾

Magic Black™ delivers many other attributes important in space applications as it has essentially zero outgassing, it is vacuum and thermal-vacuum cycling compatible, suitable for operating temperatures from -269°C to +350°C, has no particulation, it is only a few microns thick and compatible with virtually all substrate materials.

Magic Black™ has been extensively tested and qualified for space applications and is applied in a variety of space programs including among others:

The METIS Inverted External Occulter (IEO, figure 8) is the front part of the instrument: its surface will be exposed to the solar flux so experiencing very challenging environmental conditions: 380° C was considered as maximum qualification temperature by dedicated thermal analysis conducted by CGS and TAS-I (figure 9). Acktar’s Magic Black™ was previously required to be qualified for 350°C as maximum temperature.

Figure 8.

CAD view of the METIS IEO (Credit: CGS, ASI)

Figure 9.

Thermal distribution obtained by thermal analysis on the IEO (Credit: CGS, ASI)

3.

QUALIFICATION TEST

A dedicated thermal-vacuum cycling test was planned by CGS, TAS-I and ACKTAR on several titanium coated samples (Figure 10) to qualify the Magic Black™ coating stability up to 380°C.

Figure 10.

Coated test sample (Credit: Acktar Ltd.)

3.2.

Investigated physical properties

The purpose of the test was to investigate the degradation of the following characteristics:

• • optical performances: reflectance

• • thermal performances: absorptance α and emittance ε values, by means of which it is possible to correctly calculate radiative heat transfer of the METIS parts during mission

• • mechanical performances: adhesion test

• • surface resistivity

• • visual inspection

Figure 11.

Surface resistivity test sample (Credit: Acktar Ltd.)

The characterization of the samples was carried out:

• • at the beginning;

• • after 25 cycles, on a first set of samples;

• • at the end (after 100 cycles), on a second set of samples, in order to identify a possible settlement on the investigated physical properties during the cycling test.

3.4.

Test set-up

Thermal Vacuum cycling was conducted using a Test Facility (ITL Inc.) comprising a stainless steel vacuum chamber accommodating a sample holder with a heating/cooling system (figure 12).

The vacuum pumping system consists of an oil-free mechanical scroll pump and a turbomolecular pump with 2,050 l/s pumping speed – capable of achieving the ultimate pressure of < 10^-8 Torr.

The test samples were placed on the surface of the sample holder plate.

Figure 12.

ITL thermal-vacuum chamber (Credit: ITL inc.)

4.

CONCLUSION

Acktar Magic black coatings deposited on Ti6Al4V substrates fully meet the thermal-vacuum cycling qualification requirements for the METIS program as they relate to reflectance, absorptance, emittance, adherence, surface resistivity, and visual appearance.

Moreover, due to a IEO hardware design modification, a further thermal-vacuum cycling test will be required to qualify the Acktar™ Magic Black coating up to +450 °C. Further validation of thermo-optical test will be conducted soon on the overall METIS instrument (Thermal Model).