How to optimize the cryogenic large-format free-off-axis three-mirror optical system

1. Optimization of Initial Optical System Design
1.1 Theory-based initial system construction:
Utilizing vector aberration theory and Fermat's principle enables direct acquisition of unobscured freeform initial systems with good imaging quality. For instance, when designing wide-field freeform off-axis reflective optical systems, this method establishes initial frameworks that only require simple optimization to achieve final systems, effectively reducing design complexity.

1.2 Gradual field expansion design:
Starting from smaller initial fields, the field of view is progressively expanded using equal-length increments until reaching the target full field. During each expansion step, error sensitivity is recalculated and controlled to levels lower than previous stages. For example, in designing wide-field freeform off-axis three-mirror systems with low error sensitivity, the field is gradually expanded while employing freeform surfaces for aberration correction to achieve low error sensitivity targets.

2. Application and Optimization of Freeform Surfaces
2.1 Freeform aberration correction:
Freeform surfaces effectively correct aberrations in off-axis three-mirror systems. When converting from coaxial to off-axis configurations introduces new aberrations, freeform surfaces can compensate accordingly. For instance, in designing compact off-axis three-mirror systems with astigmatism correction, freeform surfaces compensate newly generated aberrations to achieve near-diffraction-limited performance.

2.2 Field expansion through freeform surfaces:
In wide-field system designs, conventional aspheric optimization often proves inadequate. Applying Zernike polynomial freeform surfaces to tertiary mirrors significantly increases design freedom and expands imaging fields. For example, in spatial optical imaging systems, this approach achieves sagittal fields up to 20°.

2.3 Volume compression via freeform surfaces:
Leveraging freeform surfaces' aberration balancing and volume compression capabilities enables compact off-axis three-mirror system designs. Guided by nodal aberration theory during optimization and following specific optimization rules, highly compact systems can be realized.

3. Refrigeration and Cold Stop Efficiency Optimization
3.1 Refrigerated detectors and cold stop configuration:
In refrigerated infrared off-axis three-mirror systems, using the detector's cold stop as the aperture stop achieves 100% cold stop efficiency. Example implementations demonstrate significant system performance improvements.

3.2 Mirror imaging of aperture stop:
Imaging the aperture stop at the primary mirror position through secondary and tertiary mirrors substantially reduces primary mirror size while maintaining performance, achieving compact designs.

4. System Alignment and Precision Control
4.1 Field curvature analysis and compensation:
Based on vector wavefront aberration theory, analyzing field curvature characteristics during small-misalignment states enables compensation through focal plane tilting. Simulation studies clarify relationships between subfield quantities and mirror alignment accuracy, informing optimized alignment procedures to enhance imaging precision.

4.2 Alignment process optimization:
Continuous refinement of alignment methodologies improves efficiency and accuracy. For example, testing camera MTF for field curvature characteristics and compensating through focal plane tilt adjustments enhances edge-field MTF performance across all fields.

5. Toolpath Generation and Machining Optimization
5.1 Freeform polishing path planning:
Effective toolpath generation methods are proposed for freeform mirror fabrication. For primary and tertiary mirrors in off-axis systems, NURBS-based polishing strategies (concentric circular, quasi-concentric, and spiral paths) with tool posture analysis ensure machining accuracy.

5.2 Process-equipment matching:
Continuous optimization of machining processes combined with high-precision equipment improves freeform surface fabrication accuracy and efficiency, thereby enhancing overall optical system performance.