Compact high-resolution wide-field freeform surface off-axis four-reflector optical system

Abstract

This invention relates to a compact, high-resolution, wide-field-of-view freeform surface off-axis four-mirror optical system, belonging to the field of optical system design technology. It addresses the problems of current imaging systems used in high-resolution space remote sensing cameras, which cannot simultaneously achieve a large field of view, small size, high imaging performance, and are bulky. This invention employs an off-axis system reflection structure, including a first mirror, a second mirror, an aperture stop, a third mirror, a fourth mirror, and an image plane arranged sequentially along the optical path. The aperture stop is located on the second mirror. The first and third mirrors utilize Zernike fringe surfaces, while the second and fourth mirrors utilize standard quadric surfaces and even-order aspherical surfaces, respectively. The four mirrors are arranged in a trapezoidal structure, sequentially in a positive-negative-positive-positive order of optical power. This invention can achieve near-diffraction-limited imaging in the visible and near-infrared bands, achieving both high resolution and a wide field of view while maintaining a compact structure and low distortion.

Description

Technical Field

[0001] This invention relates to the field of optical system design technology, and in particular to a compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system, mainly used in the field of remote sensing imaging in the visible and near-infrared bands. Background Technology

[0002] Resolution is a crucial factor influencing the commercial value of satellite remote sensing data. Currently, commercial satellite optical remote sensing has fully entered the "sub-meter" era. High-resolution images can replace airborne remote sensing images in many applications, offering significant economic benefits. However, with a fixed detector pixel size, achieving higher resolution requires a longer focal length. Reflective systems, with their advantages of chromatic aberration-free operation, long focal length, and wide imaging spectral range, are widely used in space remote sensing cameras. However, with the increase of focal length and field of view, the envelope size of traditional off-axis three-mirror optical systems increases dramatically, making aberration correction difficult and distortion control challenging, significantly impacting image quality and limiting their application in space remote sensing cameras.

[0003] In 2016, Wang Yunqi et al. published "Design of a Wide Field-of-View Off-Axis Three-Mirror Optical System Based on Transfer Matrix" in Infrared and Laser Engineering. The article designed an off-axis three-mirror optical system with a wide rectangular field of view of 17°×2°. The system has a focal length of 1440mm and an F number of 4.8, but does not use freeform surfaces, so the system size is relatively large.

[0004] Chinese patent CN114035309A discloses a wide field-of-view, long-wavelength off-axis three-mirror optical system based on a freeform surface. The system uses a freeform surface, has a focal length of 500mm, and a total length of 513mm, which is relatively long.

[0005] Chinese patent CN113031238A discloses a multi-lens integrated large field-of-view long focal distance axis four-reflector optical system. Although its assembly and adjustment are relatively easy, the system cannot further expand the field of view, and the system size is still relatively large.

[0006] Chinese patent CN102087407A discloses an ultra-large field-of-view off-axis total internal reflection optical system. Although the system has a large field of view, its optical power distribution scheme of negative-positive-negative-positive results in a long system, which is not conducive to the compact design of high-resolution spatial remote sensing cameras.

[0007] Therefore, designing a compact, high-resolution, wide-field-of-view optical system has become an urgent problem to be solved in the field of space remote sensing imaging. Summary of the Invention

[0008] To address the issues of current imaging systems for high-resolution space remote sensing cameras failing to balance large field of view, small size, and high imaging performance, as well as the bulky size resulting from the increased focal length and field of view of high-resolution space remote sensing cameras, such as off-axis three-mirror systems, this invention proposes a compact high-resolution wide-field-of-view freeform surface off-axis four-mirror optical system. This system utilizes four mirrors and makes reasonable use of freeform surfaces, which can better achieve system aberration correction and balance, and has loose tolerances for easy implementation.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A compact, high-resolution, wide-field-of-view freeform surface off-axis four-mirror optical system is disclosed. This optical system employs an off-axis system reflection structure, which includes a first mirror, a second mirror, an aperture stop, a third mirror, a fourth mirror, and an image plane arranged sequentially from the object side to the image side along the optical path direction. The aperture stop is disposed on the second mirror, and the first mirror and the third mirror are located on the same vertical plane close to the rotational symmetry axis of the second mirror.

[0011] The second reflector has negative optical power, the first reflector, the third reflector, and the fourth reflector have positive optical power, and the surface shape of the first reflector and the third reflector is a Zernike fringe surface, the surface shape of the second reflector is a standard quadratic surface, and the surface shape of the fourth reflector is an even-order aspherical surface.

[0012] The four mirrors are arranged in a trapezoidal structure, with the rotational symmetry axis of the second mirror as the optical axis. The distance between the optical axes of the centers of the first and second mirrors is the height d of the trapezoid. The ratio of the height d of the trapezoid to the focal length f of the optical system satisfies: The ratio of the length TL of the longest base of the trapezoid to the focal length f of the optical system satisfies: The relationship between the long side width PML of the first reflector and the maximum field of view HFOV satisfies: The volume V of the optical system satisfies: V ≤ 0.064f 3 ;

[0013] In the global coordinate system, with the second reflector as the reference plane, the first reflector rotates about -3° to 3° about the X-axis, the third reflector rotates about -3° to 3° about the X-axis, and the fourth reflector rotates about -10° to 10° about the X-axis.

[0014] The advantages of the compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system of the present invention are as follows:

[0015] (1) Small F-number, large field of view:

[0016] The optical system of this invention adopts an off-axis four-mirror optical system. According to the positive-negative-positive-positive optical power pattern, the first and third mirrors adopt a freeform surface design, while the second and fourth mirrors adopt an aspherical surface design. With a certain tilt and eccentricity, the optical system with a small F number and large aperture can achieve near-diffraction-limited imaging in a field of view of 16.2°×0.5° or even larger.

[0017] (2) Compact structure, easy to design:

[0018] The compact high-resolution wide field-of-view freeform surface off-axis four-mirror optical system of the present invention contains only four mirrors, with a small axial length and compact structure. The axial distance and angle around the X-axis of the first and third mirrors are close, and the axial distance of the second and fourth mirrors is close, which facilitates the design of a common reference and common support structure.

[0019] (3) Good imaging quality, high transfer function, small distortion, and small field curvature:

[0020] The compact, high-resolution, wide-field-of-view freeform off-axis four-mirror optical system of the present invention utilizes the rational allocation and design of the optical power of the four mirrors to achieve imaging quality close to the diffraction limit in the visible and near-infrared bands, with relative distortion of less than 0.3% and field curvature of less than 0.01mm across the entire field of view. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0022] Figure 1 This is a schematic diagram of the compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to an embodiment of the present invention;

[0023] Figure 2 This is a comparison diagram of the structural dimensions of an off-axis three-mirror optical system based on freeform surfaces and a compact high-resolution wide-field-of-view off-axis four-mirror optical system based on freeform surfaces as described in the embodiments of the present invention;

[0024] Figure 3 This is a graph of the modulation transfer function (MTF) of the compact high-resolution wide-field freeform surface off-axis four-reflector optical system described in the embodiments of the present invention.

[0025] Figure 4This is a grid distortion diagram of the compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system described in an embodiment of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be understood that the specific embodiments described are merely used to explain this application and are not intended to limit this application.

[0027] This invention provides a compact, high-resolution, wide-field-of-view freeform surface off-axis four-mirror optical system. The initial structure of this optical system can be designed by combining Seidel aberration theory and vector aberration theory. By using four mirrors with optical power, a volume envelope evaluation function is established, and successive optimizations are performed to obtain an optimized result with high space utilization and ultra-compact structure.

[0028] See Figure 1 This embodiment provides a compact, high-resolution, wide-field-of-view freeform surface off-axis four-mirror optical system. The optical system adopts an off-axis system reflection structure. Along the optical path direction from the object side to the image side, a first mirror M1, a second mirror M2, an aperture stop k, a third mirror M3, and a fourth mirror M4 are arranged sequentially. The aperture stop k is set on the second mirror M2. By using an off-axis field of view, the central obstruction is avoided, and there is no intermediate image plane.

[0029] Specifically, the reflecting surfaces of the first reflecting mirror M1 and the second reflecting mirror M2 are arranged opposite each other, and the reflecting surfaces of the third reflecting mirror M3 and the fourth reflecting mirror M4 are arranged opposite each other. Furthermore, the first reflecting mirror M1 and the third reflecting mirror M3 are located near the same perpendicular plane to the optical axis (i.e., the axis of rotational symmetry of the second reflecting mirror M2). Incident light rays are reflected from the first reflecting mirror M1 to the second reflecting mirror M2, then from the second reflecting mirror M2 to the third reflecting mirror M3, then from the third reflecting mirror M3 to the fourth reflecting mirror M4, and finally from the fourth reflecting mirror M4 to the image plane.

[0030] exist Figure 1 In the right-handed coordinate system (i.e., the global coordinate system) shown, the four mirrors are arranged in a trapezoidal structure. The ratio of the height d of the trapezoid (i.e., the optical axis distance between the centers of the first mirror M1 and the second mirror M2) to the focal length f of the optical system satisfies: The ratio of the length TL of the longest base of the trapezoid (i.e., the length of the envelope of the optical system in the Y-axis direction) to the focal length f of the optical system satisfies: The relationship between the width of the long side PML of the first reflecting mirror M1 (i.e., the length of the first reflecting mirror M1 in the X-axis direction) and the maximum field of view HFOV satisfies: The volume V of the optical system satisfies: V ≤ 0.064f 3 .

[0031] The second reflector M2 has negative optical power, while the first reflector M1, the third reflector M2, and the fourth reflector M4 have positive optical power. The four reflectors are arranged in sequence in a positive-negative-positive-positive manner.

[0032] The surface of the first reflector M1 and the third reflector M3 is a Zernike fringe surface, the surface of the second reflector M2 is a standard quadratic surface, and the surface of the fourth reflector M4 is an even-order aspherical surface.

[0033] In the global coordinate system, with the second reflector M2 as the reference plane, the rotation angle of the first reflector M1 about the X-axis is -3° to 3°; the rotation angle of the third reflector M3 about the X-axis is -3° to 3°; and the rotation angle of the fourth reflector M4 about the X-axis is -10° to 10°.

[0034] In this embodiment, the surface shape of the first reflecting mirror M1 and the third reflecting mirror M3 adopts the Zernike fringe elevation surface, and its surface shape expression is as follows:

[0035]

[0036] This polynomial consists of two parts: the first part is the even-degree aspherical basis term, and the second part is the Zernike polynomial. Here, z is the freeform surface elevation of the first reflecting mirror M1 or the third reflecting mirror M3, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, and α... i A is the coefficient of the even-degree term. i These are the coefficients of the Zernike polynomial. For the Zernike polynomial on the unit circle, Let ρ be the angle in polar coordinates, ρ be the normalized radial coordinate, and N be the number of terms in the Zernike polynomials in the sequence. In short, the first term represents the base profile of the entire freeform surface, and the second term represents the detailed extension of the freeform surface.

[0037] The surface of the second reflecting mirror M2 is a standard quadratic surface, and its surface shape expression is as follows:

[0038]

[0039] Where z is the freeform surface elevation of the second reflecting mirror M2, c is the curvature, r is the radial coordinate, and k is the quadratic surface coefficient.

[0040] The fourth reflecting mirror M4 has an even-order aspherical surface, and its surface shape expression is as follows:

[0041]

[0042] Where z is the freeform surface elevation of the fourth reflecting mirror M4, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, and α i The coefficient of the even-degree term.

[0043] To further improve the resolution of the optical system, the surface shapes of the second mirror M2 and the third mirror M3 can be replaced with freeform surfaces.

[0044] The compact, high-resolution, wide-field-of-view freeform surface off-axis four-mirror optical system obtained according to this embodiment has a field of view ≥16.2°×0.5°, F# ≥4, and a spectral range covering the visible and near-infrared bands. Furthermore, the relative distortion across the entire field of view is less than 0.3%, and the field curvature is less than 0.01mm. Due to the small field curvature, a multispectral detector array can be placed on the focal plane, which is beneficial for achieving wide-swath pushbroom imaging.

[0045] The optical system in this embodiment uses four mirrors to form an off-axis reflection structure. The first mirror M1 and the third mirror M3 are free-form surfaces characterized by Zernike polynomials, the second mirror M2 is a standard quadratic surface, and the fourth mirror M4 is an even-order aspherical surface. They are arranged in a positive-negative-positive-positive optical power manner, which enables the optical system to achieve near-diffraction-limited imaging in the visible and near-infrared bands. It has both high resolution and wide field of view, while being compact and having low distortion.

[0046] The technical solution of the present invention will be further described below with reference to a specific embodiment.

[0047] In this embodiment, the technical specifications of the optical system are as follows:

[0048] Operating wavelength: Visible and near-infrared band 400nm~800nm;

[0049] Entrance pupil diameter: 340mm;

[0050] Focal length: 2m;

[0051] Field of view: 16.2° × 0.5°.

[0052] In this embodiment of the compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system, the optical system structural dimensions are as follows: Figure 2 As shown in (b), the optical axis distance d between the first reflecting mirror M1 and the second reflecting mirror M2 is 740 mm, that is... The longest base of the trapezoid, TL, is 740mm, that is... The width of the long side of the first reflecting mirror M1, PML, is 800mm, that is... The total volume of the system is less than or equal to 0.438 m³. 3 .

[0053] In global coordinates, with the second reflecting mirror M2 as the reference plane, and the rotational symmetry axis of the second reflecting mirror M2 as the optical axis, perpendicular to... Figure 1 The X-axis is shown in the paper. The first reflector M1 is rotated 0° about the X-axis; the third reflector M3 is rotated 0° about the X-axis; and the fourth reflector M4 is rotated 6° about the X-axis.

[0054] The structural dimensions of a freeform surface off-axis three-mirror optical system with superior image quality, designed based on the same optical system specifications, are as follows: Figure 2 As shown in (a), the comparison shows that the compact high-resolution wide field-of-view freeform surface off-axis four-reflector optical system designed in this embodiment is smaller in size.

[0055] like Figure 3 As shown, the compact, high-resolution, wide-field-of-view freeform off-axis four-mirror optical system of this embodiment achieves diffraction-limited image quality in the visible and near-infrared bands, while also... Figure 3 As can be seen, the optical system in this embodiment has a small field curvature, and near-diffraction-limited image quality can be obtained even with a planar image plane. Mesh distortion is as follows... Figure 4 As shown, from Figure 4 It can be seen that the optical system of the present invention has relatively small imaging distortion.

[0056] The advantages of the compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system of the present invention are as follows:

[0057] (1) Small F-number, large field of view:

[0058] The optical system of this invention adopts an off-axis four-mirror optical system. According to the positive-negative-positive-positive optical power pattern, the first and third mirrors adopt a freeform surface design, while the second and fourth mirrors adopt an aspherical surface design. With a certain tilt and eccentricity, the optical system with a small F number and large aperture can achieve near-diffraction-limited imaging in a field of view of 16.2°×0.5° or even larger.

[0059] (2) Compact structure, easy to design:

[0060] The compact high-resolution wide field-of-view freeform surface off-axis four-mirror optical system of the present invention contains only four mirrors, with a small axial length and compact structure. The axial distance and angle around the X-axis of the first and third mirrors are close, and the axial distance of the second and fourth mirrors is close, which facilitates the design of a common reference and common support structure.

[0061] (3) Good imaging quality, high transfer function, small distortion, and small field curvature:

[0062] The compact, high-resolution, wide-field-of-view freeform off-axis four-mirror optical system of the present invention utilizes the rational allocation and design of the optical power of the four mirrors to achieve imaging quality close to the diffraction limit in the visible and near-infrared bands, with relative distortion of less than 0.3% and field curvature of less than 0.01mm across the entire field of view.

[0063] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0064] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system, characterized in that: An off-axis system reflection structure is adopted, which includes a first reflecting mirror, a second reflecting mirror, an aperture stop, a third reflecting mirror, a fourth reflecting mirror and an image plane arranged sequentially from the object side to the image side along the optical path direction. The aperture stop is set on the second reflecting mirror, and the first reflecting mirror and the third reflecting mirror are on the same vertical plane close to the rotational symmetry axis of the second reflecting mirror. The second reflector has negative optical power, the first reflector, the third reflector, and the fourth reflector have positive optical power, and the surface shape of the first reflector and the third reflector is a Zernike fringe surface, the surface shape of the second reflector is a standard quadratic surface, and the surface shape of the fourth reflector is an even-order aspherical surface. The four mirrors are arranged in a trapezoidal structure, with the rotational symmetry axis of the second mirror as the optical axis. The distance between the optical axes of the centers of the first and second mirrors is the height d of the trapezoid. The ratio of the height d of the trapezoid to the focal length f of the optical system satisfies: The ratio of the length TL of the longest base of the trapezoid to the focal length f of the optical system satisfies: The relationship between the long side width PML of the first reflector and the maximum field of view HFOV satisfies: The volume V of the optical system satisfies: V ≤ 0.064f 3 ; In the global coordinate system, with the second reflector as the reference plane, the first reflector rotates about -3° to 3° about the X-axis, the third reflector rotates about -3° to 3° about the X-axis, and the fourth reflector rotates about -10° to 10° about the X-axis.

2. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to claim 1, characterized in that, The optical system has a field of view of ≥16.2°×0.5°, an F number of ≥4, and a spectral range of visible and near-infrared bands.

3. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to claim 2, characterized in that, The spectral range is 400nm to 800nm.

4. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The optical system has a focal length f of 2m, a trapezoidal height d of 740mm, a longest base length TL of 740mm, and a long side width PML of 800mm for the first reflector.

5. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The first reflector has a rotation angle of 0° about the X-axis, the third reflector has a rotation angle of 0° about the X-axis, and the fourth reflector has a rotation angle of 6° about the X-axis.

6. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The surface shape expressions of the first reflector and the third reflector are as follows: Where z is the freeform surface elevation of the first or third reflecting mirror, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, and α i A is the coefficient of the even-degree term. i These are the coefficients of the Zernike polynomial. For the Zernike polynomial on the unit circle, ρ is the angle in polar coordinates, ρ is the normalized radial coordinate, and N is the number of terms in the Zernike polynomial in the sequence.

7. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The surface shape expression of the second reflecting mirror is: Where z is the freeform surface elevation of the second reflector, c is the curvature, r is the radial coordinate, and k is the quadratic surface coefficient.

8. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The surface shape expression of the fourth reflecting mirror is: Where z is the freeform surface elevation of the fourth reflecting mirror, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, and α i The coefficient of the even-degree term.

9. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The relative distortion of the optical system across the entire field of view is less than 0.3%, and the field curvature is less than 0.01 mm.

10. The compact, high-resolution, wide-field-of-view freeform surface off-axis four-reflector optical system according to any one of claims 1 to 3, characterized in that, The entrance pupil diameter of the optical system is 340 mm.

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