High-Precision Aluminum Mirrors for Infrared Astronomy

Excellent Machinability: Aluminum exhibits outstanding machinability, enabling the fabrication of an entire instrument structure, including Optical Components, from the same material. This helps mitigate optical misalignment issues at low temperatures. In space infrared missions, cooling the entire instrument is critical to suppress infrared background and detector noise. This characteristic of aluminum mirrors gives them significant advantages in the manufacturing of future infrared astronomical satellites.
Good Thermal Conductivity: Aluminum’s high thermal conductivity allows efficient heat dissipation from optical components, maintaining low-temperature stability. For large infrared solar telescopes, mirror materials with good thermal conductivity can reduce temperature differences between the mirror surface and ambient air. Additionally, polishing aluminum mirrors for infrared wavelengths is relatively straightforward, making low-cost metal mirrors (such as aluminum) a practical choice for primary mirrors.

II. Optical Performance Meets Requirements

High Surface Precision: Aluminum mirrors manufactured via ultra-precision machining exhibit wavefront error (WFE) values that meet the requirements of space infrared missions. For example, measurements based on power spectral density confirm that the surface precision of aluminum mirrors satisfies the specifications for the SPICA Coronagraph Instrument. When integrated into an Optical System, the total WFE is estimated at 33 nm (RMS), with each mirror contributing 10–20 μm (RMS) in the central 14 mm region.
Reflectivity Suitable for Space Observations: Aluminum mirrors provide adequate reflectivity in specific bands for space-based infrared astronomy. In potential NASA flagship missions such as LUVOIR, aluminum is the preferred reflective coating for broadband telescopes. To maximize reflectivity across wide spectral ranges, the aluminum surface must remain unoxidized (free of the natural oxide layer formed in air), enabling coverage of the 11–15 eV band.

III. High Stability

Maintaining Surface Shape at Cryogenic Temperatures: Optimized aluminum mirrors demonstrate sufficient stability to retain surface shape under cryogenic conditions. Finite element modeling predicts gravity-induced sag, mounting errors, and cryogenic deformation, validated through room-temperature and cryogenic testing. Experimental results show that preload forces dominate surface shape changes, with total deformation at 100 K meeting optical requirements.

Conclusion
Aluminum mirrors offer significant advantages for cooled optics in future infrared astronomical satellites, including excellent machinability, thermal conductivity, optical performance, and stability. These attributes make aluminum mirrors highly promising for space-based infrared observations.

Optimization Strategies

1. Enhanced Surface Treatment Processes

• Improved Reactive Plasma Ion-Assisted Deposition: Depositing HfO₂/SiO₂ multilayer films on single-point diamond-turned (SPDT) aluminum substrates via modified reactive plasma ion-assisted deposition creates laser-resistant, environmentally stable dielectric-enhanced IR mirrors. This method achieves a laser-induced damage threshold (LIDT) of 11 J/cm² at 1064 nm.

• High-Precision Manufacturing: SPDT technology produces optical-grade surfaces with roughness of 8–13 nm and form accuracy of 0.28λ (λ = 632 nm). Selective laser melting (SLM) of AlSi10Mg aluminum alloy mirrors, combined with SPDT, enables lightweight, high-precision space optics.

2. Defect Reduction

• Surface Particle Control: Laser-induced damage often originates from nodular defects caused by embedded particles. Strict control of substrate surface quality minimizes these defects.

• Damage Mechanism Analysis: Scanning electron microscopy (SEM) reveals laser damage morphology, guiding defect mitigation strategies.

3. Enhanced Spectral Reflectivity and Environmental Durability

• Multilayer Film Structures: HfO₂/SiO₂ multilayers boost spectral reflectivity, laser resistance, and environmental durability from UV to mid-wave infrared. LIDT testing predicts thresholds for damage processes.

• Aluminum Coating: Aluminum coatings reduce surface scattering to <20 Å RMS (e.g., C. ELCAN’s VQ process) and improve environmental stability.

4. Optimized Design and Manufacturing

• Cryogenic-Compatible Design: Aluminum’s machinability enables monolithic instrument structures, reducing cryogenic misalignment. Ultra-precision machining ensures WFE compliance for space missions.

• 3D-Printed High-Performance Mirrors: Topology-optimized, umbrella-rib-inspired designs with tetrahedral lattice filling reduce weight, deformation, and improve stiffness/modality compared to traditional drilling methods.

Conclusion
Through optimized surface treatments, defect control, enhanced coatings, and advanced manufacturing (e.g., 3D printing), aluminum mirrors achieve improved laser resistance and environmental stability, positioning them as ideal candidates for infrared laser optics in space applications.