Image Credit: BAE Systems, Courtesy NASA/JPL-Caltech
NASA is targeting late February 2025 to launch the agency’s next astrophysics observatory, SPHEREx. The Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer—will take stock of the sky in optical and near-infrared light over two years. Scientists will use it to study about 450 million galaxies, with an additional haul of more than 100 million stars within our own Milky Way. By mapping the entire sky in 102 infrared color bands—far surpassing the color resolution of previous all-sky maps—SPHEREx will provide unprecedented insights and determine targets for further study by other missions, such as NASA’s James Webb Space Telescope and the Wide Field Infrared Survey Telescope.
Modern space telescopes such as SPHEREx are advancing the boundaries of instrumentation, which drives the adoption of highly specialized materials. Integrating these novel materials into aerospace applications can streamline fabrication and assembly processes but requires testing to safeguard mission success and reduce risk during implementation. Material and process engineers at the Jet Propulsion Laboratory (California Institute of Technology) relied on SOC’s reflectometers to validate the thermo-optical and stray light performance of a critical component of SphereX’s instrumentation.
TARGET MATERIALS – TZM AND ACKTAR FRACTAL BLACK
SPHEREx will use Linear Variable Filters (LVFs) to capture spectra by leveraging their position-dependent wavelength-selective properties. Each LVF is mounted in front of the detectors, with the passband (the range of wavelengths transmitted) varying linearly across one spatial axis of the filter.
The LVF mounts were manufactured from titanium-zirconium-molybdenum (TZM), an alloy engineered for high strength at high temperatures. It exhibits a low thermal expansion coefficient, high thermal and electrical conductivity, and excellent thermal shock resistance. TZM is used in applications at high temperatures in the nuclear, aerospace, and electronics industries due to suitable mechanical properties under demanding operating conditions.
JPL engineers Hatcher, Hwang, and Blank, in their paper “Acktar Fractal Black™ as a Dual-Function Thermal-Optical Control Coating and Structural Primer” presented at the 2024 IEEE Aerospace Conference, emphasized the need for a specialized coating for the LVF mounts, requiring it to:
- Adhere well to a TZM substrate
- Conform to the intricate geometries of the LVF frame
- Control stray light through high absorption
- Serve as a primer for structural bonding
Traditional spray coatings fell short of these requirements, prompting the evaluation of Acktar Fractal Black™ (AFB) as a potential solution. AFB, an inorganic vacuum-deposited thermal-optical control coating, offers several advantages:
- Compatibility with diverse substrates, including metals and glass
- Extremely low reflectance across the visible and infrared spectra
- Broad operational temperature range
- Minimal outgassing
- Exceptional adherence to complex geometries
THERMO-OPTICAL CHARACTERIZATION
Coating characterization was conducted on four AFB-coated TZM coupons with two coating thicknesses: 3 μm and 10 μm. These coupons were evaluated for geometric conformance, adhesion, thermo-optical characterization, coating thickness, and cleanability. The samples underwent an initial thermo-optical property evaluation using SOC-410 portable reflectometers to test whether the material met the manufacturer’s specifications.
Absorptivity measurements were performed using a 410-Solar, capable of measurements from 300 to 2500 nm, in accordance with ASTM E903. Emissivity measurements were conducted using the ET-100, with measurement ranges from 1.5 to 21 μm, in accordance with ASTM E408. In addition to initial testing, JPL used SOC’s instruments to evaluate the AFB coatings after various cleaning processes, revealing their impacts on the absorptivity and emissivity of the samples.
CONCLUSION
The evaluation of Acktar Fractal Black™ yielded insights into its capabilities as a dual-function thermo-optical coating and primer for structural bonding in aerospace applications. AFB’s conformance to complex geometries and substrate adhesion outperformed other optically black spray coatings. SOC’s reflectometers provided data on the coating’s initial performance as well as durability when subjected to various cleaning methods.
SOC’s reflectometers have consistently proven their value in assisting leading aerospace organizations to meet and exceed stringent performance standards while ensuring mission readiness. As a trusted partner to the world’s foremost space innovators, SOC provides indispensable tools for quality control at various process stages. If you’re ready to discover how SOC instruments can address your engineering challenges, let’s connect and start a conversation.
REFERENCES
Hatcher, N., Hwang, M., & Blank, R. “Acktar Fractal Black™ as a Dual-Function Thermal-Optical Control Coating and Structural Primer.” 2024 IEEE Aerospace Conference, pp. 1–7, DOI: 10.1109/AERO58975.2024.10521186.
