Overview

In the realm of modern communication, Earth observation, and scientific research, satellites play an indispensable role. The optical systems utilized in on-orbit satellites are paramount for capturing high-quality images and data. However, designing optical systems for space applications presents numerous challenges, including size constraints, the necessity for a large field of view, and demanding relative aperture requirements that traditional optical designs struggle to address.

Historically, coaxial systems were employed in satellites, but they faced limitations such as a restricted field of view, central obscuration, and diminished ideal transfer functions, rendering them inadequate for capturing comprehensive images of expansive areas. Conversely, off-axis systems, while offering a larger field of view, encountered challenges related to central obscuration and constraints on relative aperture. The imperative to develop an innovative optical system that could surmount these challenges became a pivotal goal in advancing satellite imaging capabilities.

Challenges

Traditional optical systems utilized in satellites confront various impediments that hinder their optimal performance. Coaxial systems, like the on-axis Three-Mirror Anastigmat (TMA), grapple with a limited field of view, substantial central obscuration, and diminished ideal transfer functions. These limitations compromise imaging performance, especially in scenarios requiring extensive coverage.

Off-axis optical systems, although capable of providing a larger field of view and reduced central obscuration, encounter limitations when aiming for a large relative aperture. This limitation impacts the system’s light-gathering ability and, consequently, its effectiveness in low-light conditions. Furthermore, optimizing off-axis systems for space applications while maintaining a compact design proves to be a complex challenge.

Solutions: The Success Story of the Off-Axis Three-Mirror System

To surmount the challenges posed by traditional optical designs, Shanghai Optics developed the Off-Axis Three-Mirror System, a groundbreaking solution revolutionizing satellite imaging capabilities. The system employs an innovative off-axis intermediate imaging three-mirror optical architecture to address challenges related to size limitations, field of view, and relative aperture requirements.

The off-axis design of the system enables it to maintain a fully symmetrical structure with a concave-convex-concave mirror arrangement. This design effectively corrects axial aberrations, ensuring a high-quality, diffraction-limited imaging performance. The system’s off-axis configuration achieves a large field of view while maintaining a short overall length, crucial for space applications where size and weight must be minimized.

Specifications:

Optical System Specifications:

  • Effective Aperture: 150mm for efficient light collection and superior image resolution.
  • Relative Aperture: 1/10 for excellent light-gathering capabilities in challenging lighting conditions.
  • Focal Length: 1500mm, striking a balance between field of view and image resolution.
  • Field of View (FOV): 0.61° for comprehensive coverage during on-orbit observations.
  • Working Wavelength: Operating within the range of 400nm to 900nm, capturing a broad spectrum for various scientific and observational applications.

Optical System Selection:

  • After evaluating four total internal reflection system structures, Shanghai Optics determined that Plan 3, an off-axis three-mirror system, best suits the requirements of the system. Plan 3 offers advantages such as distortion aberration correction, a large field of view, and a short optical tube length, making it the optimal choice for on-orbit satellite imaging missions.

Optical System Design:

  • Meticulous optical design and precise aberration correction, utilizing ellipsoidal primary and tertiary mirrors, along with a hyperbolic secondary mirror, optimize imaging performance across the field of view. The transfer function curve demonstrates the system’s surpassing quality requirements, ensuring sharp and detailed imagery.

Opto-mechanical Design:

  • The opto-mechanical design ensures precise alignment and integration of optical components, utilizing titanium alloy and glass-ceramic materials for lightweight construction and thermal stability. Backside single-point support enhances component stability, ensuring reliable operation in space’s harsh environment.

Assembly Size and Weight:

  • The Off-Axis Three-Mirror System boasts a compact assembly size of 380mm x 233mm x 450mm and a total mass of 32.59kg. Design choices and material selections contribute to the system’s overall efficiency, enabling optimal performance in on-orbit satellite imaging missions.

The Off-Axis Three-Mirror System signifies a remarkable advancement in space-based optical technology, enhancing satellite missions with superior imaging capabilities and improved data acquisition for various scientific and observational applications. By overcoming obstacles faced by traditional optical designs, the system sets a new standard for on-orbit satellite imaging, contributing to our understanding of the universe and enabling critical space missions.

Do not hesitate to contact Shanghai Optics today. We’d be more than happy to discuss your projects and how best they can become a success.

Learn More