Mastering Large-Aperture Mirror Accuracy: Techniques for Higher Imaging Resolution

1. Optimization of Manufacturing Processes

• Gravity Unloading-Based Rotation Testing Process: In terrestrial manufacturing environments, gravity affects the surface figure of large-aperture space aspheric mirrors. To achieve zero-gravity surface figure manufacturing, a high-precision rotation testing method based on gravity unloading can be established. For example, using the N-step equal-interval rotation method:

  • First, clarify its fundamental principles. In a specific manufacturing case (e.g., a Ф1290mm ULE Aspheric Mirror), strictly control rotation angle and eccentricity errors (actual angle error < 0.1°, eccentricity error < 0.1mm).

  • During the low-precision phase, use the 3-step rotation method to process test results, rapidly converging mirror surface figure accuracy to 0.029λ RMS.

  • Address the cumulative amplification of symmetric errors caused by the rotation method through targeted removal, further converging surface figure accuracy to 0.023λ RMS.

  • Finally, use the 6-step rotation method to process test results and guide optical manufacturing, achieving high surface figure accuracy. After removing gravity-induced deformation error, the surface figure accuracy reaches 0.010λ RMS, approximating the mirror's zero-gravity surface figure in orbit.

  • This method applies to meter-class and larger space aspheric mirrors.

• Optimized Grinding & Polishing Techniques: Grinding and polishing are critical for mirror surface figure accuracy. Over the past half-century, techniques for large-aperture aspheric mirrors have evolved:

  • Traditional grinding is being replaced by CNC grinding, enabling precise material removal via controlled toolpath and pressure (e.g., Computer-Controlled Optical Surfacing - CCOS).

  • Deterministic polishing techniques like Ion Beam Figuring (IBF) and Magnetorheological Finishing (MRF) are widely adopted:

    • IBF uses high-energy ion beams for nanoscale material removal.

    • MRF uses magnetorheological fluid to improve surface roughness and correct figure errors.

  • Combining these advanced techniques significantly enhances surface figure accuracy.

2. Improvements in Surface Metrology

• High-Precision Detection Algorithms: For large-aperture optical component testing:

  • A "double segmentation" method effectively locates laser spots with large intensity variations.

  • Gray centroid method provides stable spot centroid extraction.

  • Feature-based classification identifies front-surface reflection spots.

  • These algorithms improve metrology accuracy, providing reliable data for surface correction.

• Advanced Metrology Methods:

  • Scanning Pentaprism Method: Measures large flat mirrors by scanning a pentaprism and autocollimator to detect tilt angle differences. Surface figure is represented as a linear combination of Zernike polynomials, solved via least-squares fitting. Achieves 7.6nm RMS accuracy. Verified against Ritchey-Common method (difference: 7.1nm RMS for 1.5m mirror).

  • Ritchey-Common Method:

    • Requires spherical reference mirrors. Analyzes eccentricity and tilt errors via optical modeling.

    • Simulations for 2m mirrors show: with eccentricity <5% aperture and tilt <1° within 11°-30° Ritchey angle range, surface recovery error is ~10⁻³λ RMS.

    • Practical application achieved 0.0238λ RMS and 0.1629λ PV for a Φ2m mirror (λ=632.8nm).

3. Support Structure Design Optimization

• High-Tolerance Support Structures: Address stress-induced degradation:

  • Example: 1.5m high-precision space mirror (RB-SiC material) with triangular back-open lightweight design and three-point flexure mounts.

  • Optimized using Isight software to minimize RMS change under 9 assembly error scenarios (0.01mm error).

  • Results:

    • Lightweight ratio: 82.1% (mass: 170.23kg)

    • 1g gravity: <0.016λ RMS

    • 0.02mm forced displacement: 0.016λ RMS

    • 20℃±5℃: ΔRMS <0.002λ

    • First natural frequency: 101.3Hz

• Adhesive Impact Mitigation:

  • Modeled adhesive curing shrinkage using thermal-load FEM. Analyzed effects of adhesive volume, location, distribution, and parameters.

  • Optimized design for rectangular mirror:

    • Six side-mounted flexible adhesive rings

    • Non-uniform near-uniform distribution

    • Adhesive: Ø10mm × 0.1mm thickness

    • Result: PV=53.26nm, RMS=10.98nm, max stress=0.04MPa

  • Topology-optimized frame reduced weight by 62.12% (7.93kg).

4. Reducing Environmental Micro-Vibration Effects
As space remote sensors increase in aperture and lightweight design, mirror stiffness decreases, making surface figures susceptible to micro-vibrations (e.g., from stepper motors, reaction wheels, cryocoolers).

• Dynamic Response Analysis Method:

  • Combines modal superposition and Zernike polynomial fitting.

  • Expresses each mode shape as a linear combination of Zernike polynomials.

  • Computes overall dynamic surface error via modal superposition.

  • Analyzes optical aberrations from micro-vibrations via Zernike coefficients.

• Enables targeted mitigation of vibration-induced surface errors to improve imaging resolution.