Design of a Cooled Large-Format Freeform Off-Axis Three-Mirror Optical System

1. Compatibility with Large-Format Detectors: With the increasing demand for ultra-large-format infrared remote sensing, the Optical System must be designed to accommodate high-resolution imaging requirements, such as those of 4K-resolution large-format infrared detectors.

2. High Cold Stop Efficiency: Utilize the cold stop of the cooled infrared detector as the system’s aperture stop, aiming for 100% cold stop efficiency to enhance the detector’s radiation collection capability and improve imaging quality.

3. Wide Field of View (FOV) and Unobstructed Configuration: Achieve a broader observation range while avoiding light loss and stray light caused by obstructions, ensuring imaging integrity and clarity.

4. Superior Imaging Quality: The system’s Modulation Transfer Function (MTF) must meet specified criteria across all fields of view to guarantee sharp imaging for practical applications.

Structural Configuration

1. Mirror Combination: A secondary imaging structure typically employs one even-order Aspheric Mirror and two freeform mirrors. This configuration effectively corrects aberrations and enhances imaging performance. For example, the primary mirror adopts an even-order aspheric surface, while the secondary and tertiary mirrors use XY polynomial freeform surfaces. The flexibility of freeform surfaces enables the correction of aberrations generated under large FOVs.

2. Aperture Stop and Exit Pupil: A real exit pupil is aligned with the cold stop to achieve 100% cold stop efficiency. In some designs, the secondary and tertiary mirrors image the aperture stop onto the primary mirror’s position, not only fulfilling the cold stop efficiency goal but also significantly reducing the primary mirror’s aperture and optimizing the system’s compactness.

Key Technologies

1. Application of Freeform Surfaces: Freeform surfaces play a critical role in expanding the FOV and correcting aberrations. For instance, XY polynomial freeform surfaces on the secondary and tertiary mirrors allow flexible adjustment of light paths to compensate for aberrations under large FOVs, ensuring high imaging quality across all fields.

2. Athermalization Design: Address the impact of environmental temperature fluctuations on imaging quality through athermalization. For example, ensure the MTF across all fields remains above a threshold within a temperature range of -40°C to 60°C, guaranteeing stable performance under varying conditions and improving system adaptability and reliability.

3. Aberration Correction: In addition to freeform surface correction, optimize the optical system’s layout and parameters for comprehensive aberration control. Techniques such as vector aberration theory and Fermat’s principle are used to establish an initial unobstructed freeform system with favorable imaging quality, followed by optimization to reduce design complexity and enhance correction.

Design Example
A system designed by Qian Zhuang, Mo Yan, Fan Rundong, et al. serves as a practical case. With a focal length of 150 mm, operating in the 1.5–5 μm wavelength range, an F-number of 5, and a 30°×25° FOV, the system employs an even-order aspheric primary mirror and XY polynomial freeform secondary and tertiary mirrors. The MTF at 25 lp/mm exceeds 0.4 across all fields, meeting the imaging requirements of large-format infrared detectors. This design successfully achieves a wide FOV, unobstructed configuration, high imaging quality, and compatibility with large-format detectors, validating the effectiveness of the proposed methodology.

Conclusion
The design of a cooled large-format freeform off-axis three-mirror optical system requires comprehensive consideration of multiple factors. By selecting appropriate structural configurations, applying key technologies, and optimizing through practical examples, the system can meet the growing demands for high-resolution, wide-FOV infrared remote sensing. As related technologies advance, such optical systems are expected to play a greater role in diverse fields, with future designs evolving toward higher efficiency, precision, and compactness.