Breakthrough in Diffractive Space Telescope Technology

With the rapid advancement of space-based Earth observation technology, both military and civilian applications demand optical systems that simultaneously achieve dual challenges: near-diffraction-limited high-resolution imaging across a broad spectral range (e.g., 0.65–0.75 μm), while meeting stringent requirements for lightweight construction, compactness, and cost-effectiveness. Traditional reflective telescopes, though capable of correcting aberrations through multi-mirror configurations and aspheric designs, face critical bottlenecks such as the need for primary mirror surface accuracy better than λ/20 (visible band) and difficulties in controlling deformations of thin-film structures. These limitations significantly increase manufacturing complexity and costs.

Technical Breakthrough: Synergistic Innovation of Diffractive Optics and Reflective Systems

1. Design Principles
The primary challenge in designing diffractive telescopes lies in the strong chromatic dispersion of diffractive elements, which can only focus light precisely within an extremely narrow spectral range. To enable broadband applications of diffractive lenses, chromatic aberration correction is essential. Conventional refractive lenses typically use cemented structures combining glasses with different dispersion properties to correct chromatic aberrations over specific spectral ranges. However, this approach cannot be directly applied to diffractive lenses, as all diffractive elements share identical dispersion characteristics—i.e., the Abbe number of a diffractive element depends solely on wavelength:

 V0=λ0/（λ1-λ2）

2. Planar Diffractive Objective: Lightweight Core

A planar diffractive lens with micron-scale relief structures serves as the objective, integrated with an ultra-thin substrate (total thickness <20 μm). This enables a super-lightweight design with a 1000 mm aperture, 8 m focal length (f/#=100).

Compared to traditional reflectors, mass is reduced by over 80%, and surface figure tolerance is relaxed to λ/5, significantly lowering manufacturing difficulty.

The transmissive design cancels dual-surface path delays, rendering surface figure errors negligible to optical path differences—breaking the precision limitations of conventional reflective systems.

3. Off-Axis Three-Mirror Eyepiece: Chromatic Correction and Compactness

A coaxial off-axis three-mirror system with conic aspheric surfaces eliminates alignment eccentricity errors. 

Integrated diffractive surface compensation achieves full chromatic correction across 0.65–0.75 μm within a 0.02°×0.035° field of view (FOV), with spot diameters <8 μm.

The system delivers MTF >0.5 at 30 lp/mm spatial frequency, approaching diffraction-limited imaging performance.

Key Technical Validation

Spectral Coverage: Achromatic performance across 0.65–0.75 μm continuous band

Resolution: MTF >0.5 at 30 lp/mm

Alignment Tolerance: Mirror surface accuracy requirement reduced to λ/5

Scalability: Harmonic diffractive lens designs may extend coverage to full spectrum (ongoing research)

Future Development
Current designs are limited by eyepiece aperture, resulting in a small FOV (0.02°×0.035°). Optimization pathways include:

Harmonic Diffractive Objective: Extend operational bandwidth to 0.5–1.2 μm

Freeform Mirror Integration: Expand FOV to 0.1°×0.15°

Modular Optical Design: Enable efficient alignment for larger-aperture systems (>2 m)

Conclusion
This diffractive telescope solution resolves the longstanding conflict between lightweight design and high resolution in space optical systems through the innovative integration of planar diffractive objectives and off-axis three-mirror eyepieces. It provides a viable technical pathway for next-generation Earth observation satellites, deep-space exploration, and related missions. With relaxed surface tolerance requirements and modular architecture, the design dramatically reduces manufacturing costs, accelerating the scalable application of high-precision space optical systems.