Cassegrain Lens Diffractive Surface Aberration Correction
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Solution Overview
Problem
Cassegrain optics used in target-seeking missiles face challenges in achieving high resolution and correcting image errors when imaging small targets at long distances, leading to disruptive image quality.
Innovation Solution
Incorporating a main mirror with a diffractive surface and a detector optics system comprising four lenses, where the first, third, and fourth lenses are converging, and the second lens is diverging, to correct longitudinal color errors and image aberrations, while the optical joint features a prism joint with aspherical surfaces for enhanced mobility and image correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional Cassegrain optics is used, then the structure is compact and lightweight, but image errors occur when imaging small targets at long distances
Solution Approach 1:
The patent applies parameter changes by introducing a diffractive surface on the main mirror with specific groove depths (0.5-2.0 times the design wavelength) and spatial frequencies (50-500 lines/mm) to correct longitudinal chromatic aberration. This modifies the optical parameters of the existing Cassegrain structure without fundamentally changing its compact design
Solution Approach 2:
The patent uses composite optical structures by combining refractive lenses (detector optics with 2-4 lenses) and diffractive surfaces on the same optical path. This composite approach allows simultaneous correction of different types of aberrations (chromatic and spherical) while maintaining the compact Cassegrain configuration
2Measurement precision
If the detector optics include multiple lenses for correction, then image errors are reduced, but the device complexity increases
Solution Approach 1:
The patent optimizes the detector optics by selecting specific lens counts (2-4 lenses) with particular focal lengths and spacing parameters. The diffractive surface parameters (groove depth, spatial frequency) are also optimized to work synergistically with the lens system, achieving aberration correction with minimal added complexity
Solution Approach 2:
The optical correction function is segmented between different components: the diffractive surface on the main mirror handles longitudinal chromatic aberration, while the detector optics lenses (2-4 elements) correct spherical and other monochromatic aberrations. This segmentation allows each component to be optimized for its specific correction task
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enables high-resolution imaging with reduced image errors, allowing for reliable target detection at long distances and maintaining a compact, lightweight design suitable for missiles, with improved temperature compensation and low sensitivity to scattered light.
Implementation Method 1
the main mirror has a diffractive surface
Implementation Method 2
the first mirror - i.e. the mirror of the Cassegrain optics on which rays from the object scene first impinge - is a concave, in particular aspherical, main mirror
Implementation Method 3
the second mirror is a convex, in particular an aspherical secondary mirror
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
The lens (8) has a detector lens (16), and a mirror lens (12) movable relative to the detector lens, where the mirror lens has a concave aspherical main mirror (18) and a convex aspherical secondary mirror (20). An optical joint (14) is provided in an area between the main and secondary mirrors for tracking an optical path on a detector during rolling-pitching-movement of the mirror lens. The detector lens has a set of lenses (50, 52, 54, 56) e.g. collective lens and divergent lens, for forming an object scene on the detector, where the lenses are movably relative to each other.