How to Determine the Optimal Aperture Design for Large-Aperture Mirrors

Resolution Requirements

• Spatial Resolution: High spatial resolution Earth observation—such as urban monitoring and military reconnaissance—demands large-aperture mirrors to enhance resolution. According to the Rayleigh criterion, the angular resolution θ of a telescope relates to the wavelength λ and mirror aperture D as θ = 1.22λ / D. In the visible band (λ ≈ 550 nm), achieving high resolution requires increasing D. For instance, detailed monitoring of urban structures necessitates sufficiently large apertures to resolve fine features. When observing from geostationary orbit, the aperture must be precisely calculated based on distance and resolution requirements to achieve specific ground pixel resolution.

• Spectral Resolution: Applications involving spectral analysis of Earth’s surface (e.g., vegetation monitoring, resource exploration) prioritize spectral resolution. While spectrometers primarily determine spectral resolution, large-aperture mirrors collect more light, boosting signal strength and indirectly improving spectral resolution. For example, monitoring ocean chlorophyll concentrations benefits from enhanced light collection, enabling more accurate spectral analysis. Here, the trade-off between increased light-gathering capability and added system complexity must be balanced to determine the optimal aperture.

Observation Distance and Platform

• Low Earth Orbit (LEO) Platforms: At altitudes of several hundred kilometers, LEO observation requires relatively smaller apertures. Small LEO remote sensing satellites, constrained by platform capacity and cost, typically use apertures ranging from tens of centimeters to ~1 meter. However, high-resolution monitoring of specific areas may demand larger apertures (e.g., commercial satellites with multi-meter apertures for fine imaging).

• Geostationary Orbit (GEO) Platforms: At ~36,000 km altitude, effective Earth observation requires extremely large apertures. High-resolution imaging from GEO may demand apertures of several meters or more. For instance, Japan’s JAXA developed a GEO telescope with a 3.6 m aperture composed of six mirror segments to achieve high-resolution Earth observation.

Optical System Characteristics

• Optical System Type: Different systems (e.g., Cassegrain, Ritchey-Chrétien) impose varying aperture requirements. Design parameters like focal ratios and relative apertures of primary/secondary mirrors must be considered. Synthetic aperture optical systems, which combine smaller mirrors to emulate a large aperture, require optimization of sub-mirror apertures and equivalent synthetic aperture based on resolution and field-of-view needs.

• Aberration Correction: Large apertures are prone to aberrations (e.g., spherical, coma). Correcting these may involve complex elements or specialized mirror shapes, impacting aperture selection. For example, aspheric mirrors effectively correct aberrations in large apertures, but their manufacturing difficulty and cost scale with size. Thus, balancing correction efficacy and aperture design is critical for optimization.

Manufacturing Costs and Technical Feasibility

• Materials and Processes: Material and manufacturing constraints limit achievable aperture sizes. Traditional optical glass faces deformation under self-weight in large mirrors, compromising surface accuracy. Advanced materials (e.g., beryllium-aluminum alloys, ULE glass) offer superior performance but incur high costs and processing challenges. Precision manufacturing (grinding, polishing) and metrology for large apertures further increase complexity and expense. Aperture design must align with existing materials, processes, and budgets.

• Launch and Deployment Challenges: Larger apertures increase volume and mass, complicating satellite launch and on-orbit deployment. Limited launch vehicle capacity necessitates compact packaging and reliable in-orbit deployment. For example, deployable mirror designs must ensure stability and precision during launch and unfolding. Aperture decisions must integrate launch costs and deployment feasibility.