Satellites are crucial in modern communication, Earth observation, and scientific research, with their optical systems playing a key role in capturing high-quality images and data.
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Designing optical systems for space applications, however, presents numerous challenges. These include size constraints, the need for a large field of view, and demanding relative aperture requirements—issues that traditional optical designs often struggle to address.
Historically, satellites used coaxial optical systems, which had limitations such as a restricted field of view, central obscuration, and diminished ideal transfer functions. These issues made them inadequate for capturing comprehensive images of expansive areas.
In contrast, off-axis systems offered a broader field of view but still faced challenges related to central obscuration and constraints on relative aperture. Consequently, the development of an innovative optical system that could overcome these challenges became a critical objective in advancing satellite imaging capabilities.
Challenges
Conventional optical systems used in satellites face several challenges that impede their optimal performance. Coaxial systems, like the on-axis Three-Mirror Anastigmat (TMA), have a narrow field of view, significant central obscuration, and reduced ideal transfer functions.
These restrictions degrade imaging performance, particularly in scenarios demanding extensive coverage.
While capable of offering a broader field of view and less central obscuration, off-axis optical systems are limited when attempting to achieve a large relative aperture.
This constraint impacts the system’s light-gathering capability and effectiveness in low-light settings. In addition, optimizing off-axis systems for space applications while maintaining a compact design is a significant problem.
Solutions: The Success Story of the Off-Axis Three-Mirror System
To address the challenges posed by conventional optical designs, Shanghai Optics created the Off-Axis Three-Mirror System, a game-changing technology revolutionizing satellite imaging capabilities.
The system incorporates a novel off-axis intermediate imaging three-mirror optical architecture to resolve issues related to size constraints, field of view, and relative aperture requirements.
The system’s off-axis design allows it to maintain a completely symmetrical structure with a concave-convex-concave mirror layout.
This design effectively corrects axial aberrations, resulting in high-quality, diffraction-limited imaging performance. The system’s off-axis design provides a large field of view while maintaining a short overall length, which is critical for space applications where size and weight must be minimized.
Specifications
Optical System Specifications
- Effective Aperture: 150 mm for optimal light collection and excellent image resolution.
- Relative Aperture: 1/10 for good light-gathering capabilities in difficult lighting conditions.
- Focal Length: 1500 mm, achieving a balance between field of view and image resolution.
- Field of View (FOV): 0.61° for complete coverage during on-orbit observations.
- Working Wavelength: Operating within the range of 400 nm to 900 nm, covering a wide spectrum for different scientific and observational applications.
Optical System Selection
- After assessing four total internal reflection system structures, Shanghai Optics concluded that Plan 3, an off-axis three-mirror system, best meets the system’s requirements. Plan 3 provides benefits such as distortion aberration correction, a broad field of view, and a short optical tube length, making it the best option for on-orbit satellite imaging missions.
Optical System Design
- Thorough optical design and accurate aberration correction, using ellipsoidal primary and tertiary mirrors, along with a hyperbolic secondary mirror, improve imaging performance across the field of view. The transfer function curve indicates how the system exceeds quality criteria, resulting in sharp and detailed images.
Opto-Mechanical Design
- The opto-mechanical design enables exact alignment and integration of optical components, while titanium alloy and glass ceramic materials provide lightweight construction and thermal stability. Backside single-point support improves component stability, guaranteeing consistent operation in space’s hostile environment.
Assembly Size and Weight:
- The Off-Axis Three-Mirror System has a small assembly size of 380 mm x 233 mm x 450 mm and weighs 32.59 kg. Design decisions and material selections contribute to the system’s overall efficiency, allowing for optimal performance in on-orbit satellite imaging missions.
The Off-Axis Three-Mirror System marks a significant advancement in space-based optical technology, enhancing satellite missions with its superior imaging capabilities and improved data acquisition for various scientific and observational applications.
By overcoming the limitations of traditional optical designs, this system sets a new standard for on-orbit satellite imaging. It plays a crucial role in expanding our understanding of the universe and enabling critical space missions, thereby pushing the boundaries of what is possible in space exploration and monitoring.
This information has been sourced, reviewed and adapted from materials provided by Shanghai Optics.
For more information on this source, please visit Shanghai Optics.