Compact high-resolution wide-field-of-view free-form surface off-axis four-mirror optical system

Abstract

The invention relates to a compact high-resolution wide-field-of-view free-form surface off-axis four-mirror optical system, belongs to the technical field of optical system design, and solves the problem that an imaging system for a high-resolution space remote sensing camera at present cannot give consideration to a large field angle, a small size, high imaging performance and a huge volume. An off-axis system reflection structure is adopted and comprises a first reflector, a second reflector, an aperture diaphragm, a third reflector, a fourth reflector and an image plane which are sequentially arranged in the light path direction, the aperture diaphragm is arranged on the second reflector, the first reflector and the third reflector adopt Zernike fretting rise planes, and the fourth reflector is arranged on the image plane. The second reflector and the fourth reflector respectively adopt a standard quadric surface and an even-order aspheric surface, and the four reflectors are arranged in a trapezoidal structure and are sequentially arranged in a mode of positive-negative-positive-positive focal power. According to the invention, near-diffraction limit imaging can be realized in visible light and near-infrared bands, high resolution and wide field of view are realized, the structure is compact, and distortion is small.

Description

Technical Field

[0001] The present invention relates to the technical field of optical system design, and in particular to a compact high-resolution wide-field free-form surface off-axis four-mirror optical system, which is mainly used in the field of visible and near-infrared band remote sensing imaging. Background Art

[0002] Resolution is an important factor affecting the commercial value of satellite remote sensing data. At present, commercial satellite optical remote sensing has fully entered the "sub-meter" era. High-resolution images can replace airborne remote sensing images in many applications and have great economic benefits. However, when the size of the detector pixel is certain, achieving higher resolution means a longer focal length is required. Reflective systems are widely used in space remote sensing cameras due to their advantages such as no chromatic aberration, long focal length, and large imaging spectrum. However, for traditional off-axis three-mirror optical systems, as the focal length and field of view increase, the envelope size will increase dramatically, aberration correction will be difficult, and distortion will be difficult to control, which will greatly affect the image quality and limit its application in space remote sensing cameras.

[0003] In 2016, Wang Yunqi et al. published "Design of wide-field 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 no free-form surface is used, so the system size is relatively large.

[0004] The Chinese patent with publication number CN114035309A discloses a wide-field and long-waveband off-axis three-mirror optical system based on a free-form surface. The system uses a free-form surface, has a system focal length of 500mm, and a total length of 513mm, and the total length of the system is relatively long.

[0005] The Chinese patent with publication number CN113031238A discloses a multi-mirror integrated large field of view long focal distance axis four-mirror optical system. Although its installation and adjustment difficulty is relatively low, the system cannot further expand the field of view, and the system volume is still relatively large.

[0006] The Chinese patent with publication number CN102087407A discloses an ultra-large field of view off-axis total reflection optical system. Although the field of view of the system is large, its optical focal length is distributed in a negative-positive-negative-positive manner, which will result in a longer system and is not conducive to the compact design of high-resolution space 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] In order to solve the problem that the current imaging system for high-resolution space remote sensing cameras cannot take into account a large field of view, a small size and high imaging performance, and the problem that the high-resolution space remote sensing cameras currently represented by off-axis three-mirror cameras have a large size due to the increase in imaging focal length and field of view, the present invention proposes a compact high-resolution wide-field-of-view free-form surface off-axis four-mirror optical system. The system utilizes four mirrors and reasonably uses free-form surfaces, which can better realize system aberration correction and balance, and has a loose tolerance for easy implementation.

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

[0010] A compact high-resolution wide-field free-form surface off-axis four-mirror optical system, the optical system adopts an off-axis system reflection structure, the off-axis system reflection structure comprises a first reflector, a second reflector, an aperture stop, a third reflector, a fourth reflector and an image plane arranged in sequence from the object side to the image side along the optical path direction, and the aperture stop is arranged on the second reflector, and the first reflector and the third reflector are close to the same vertical plane of the rotational symmetry axis of the second reflector;

[0011] The second reflector has a 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 adopts a Zernike fringe sag surface, the surface shape of the second reflector adopts a standard quadratic surface, and the surface shape of the fourth reflector adopts an even-order aspheric surface;

[0012] The four reflectors are arranged in a trapezoidal structure, with the rotational symmetry axis of the second reflector as the optical axis, the optical axis distance between the center of the first reflector and the center of the second reflector is the height d of the trapezoid, and the ratio of the height d of the trapezoid to the focal length f of the optical system satisfies: The ratio of the length of the longest base of the trapezoid TL 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 optical system volume V satisfies: V≤0.064f 3 ;

[0013] In the global coordinate system, with the second reflector as the reference plane, the rotation angle of the first reflector with respect to the X-axis is -3° to 3°, the rotation angle of the third reflector with respect to the X-axis is -3° to 3°, and the rotation angle of the fourth reflector with respect to the X-axis is -10° to 10°.

[0014] The compact high-resolution wide-field free-form surface off-axis four-mirror optical system of the present invention has the following beneficial effects:

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

[0016] The optical system of the present invention adopts an off-axis four-mirror optical system. According to the positive-negative-positive-positive mode of optical power, the first reflector and the third reflector adopt a free-form surface design, the second reflector and the fourth reflector adopt an aspheric surface design, and with a certain tilt eccentricity, the optical system with a small F number and a large aperture can achieve near-diffraction-limited imaging within a field of view of 16.2°×0.5° or even larger.

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

[0018] The compact high-resolution wide-field free-form surface off-axis four-mirror optical system of the present invention contains only four reflectors, has a small axial length, a compact structure, and the axial distance and angle around the X-axis of the first reflector and the third reflector are close, and the axial distance of the second reflector and the fourth reflector are close, which is convenient for 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 free-form surface off-axis four-mirror optical system of the present invention utilizes the reasonable distribution and design of the optical focal length of four reflectors, and the imaging quality is close to the diffraction limit in the visible and near-infrared bands, the relative distortion of the whole field of view is less than 0.3%, and the field curvature is less than 0.01mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

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

[0023] Figure 2 This is a structural size comparison diagram of an off-axis three-mirror optical system based on a free-form surface and a compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to an embodiment of the present invention;

[0024] Figure 3 is a modulation transfer function (MTF) curve of the compact high-resolution wide-field free-form surface off-axis four-mirror optical system described in an embodiment of the present invention;

[0025] Figure 4It is a grid distortion diagram of the compact, high-resolution, wide-field-of-view free-form surface off-axis four-mirror optical system described in an embodiment of the present invention. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. It should be understood that the specific embodiments described are only used to explain the present application and are not used to limit the present application.

[0027] The present invention provides a compact high-resolution wide-field free-form surface off-axis four-mirror optical system. The initial structure of the optical system can be designed by combining Seidel aberration theory and vector aberration theory. By adopting four reflectors with optical power and establishing a volume envelope evaluation function, successive optimizations are performed to obtain an optimization result with high space utilization and ultra-compact structure.

[0028] See also Figure 1 This embodiment provides a compact high-resolution wide-field free-form surface off-axis four-mirror optical system. The optical system adopts an off-axis system reflection structure. A first reflector M1, a second reflector M2, an aperture stop k, a third reflector M3 and a fourth reflector M4 are arranged in sequence along the optical path from the object side to the image side, and the aperture stop k is arranged on the second reflector M2. The field of view is off-axis to avoid central obstruction, and there is no intermediate image plane.

[0029] Specifically, the reflection surfaces of the first reflector M1 and the second reflector M2 are arranged opposite to each other, the reflection surfaces of the third reflector M3 and the fourth reflector M4 are arranged opposite to each other, and the first reflector M1 and the third reflector M3 are close to the same vertical plane of the optical axis (i.e., the rotational symmetry axis of the second reflector M2) and are located near the vertical plane. The incident light is reflected from the first reflector M1 to the second reflector M2, then from the second reflector M2 to the third reflector M3, then from the third reflector M3 to the fourth reflector M4, and finally from the fourth reflector M4 to the image plane.

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

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

[0032] The surface shapes of the first reflector M1 and the third reflector M3 are Zernike fringe sagittal surfaces, the surface shape of the second reflector M2 is a standard quadratic surface, and the surface shape 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 with respect to the X-axis is: -3°~3°; the rotation angle of the third reflector M3 with respect to the X-axis is: -3°~3°; the rotation angle of the fourth reflector M4 with respect to the X-axis is: -10°~10°.

[0034] In this embodiment, the surface shapes of the first reflector M1 and the third reflector M3 adopt Zernike fringe sag surface, and the surface shape expression is:

[0035]

[0036] This polynomial is divided into two parts, the first part is the even-order aspheric basis term, and the second part is the Zernike polynomial. Among them, z is the free surface sag of the first reflector M1 or the third reflector M3, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, α i is the coefficient of the even term, A i are the Zernike polynomial coefficients, is a Zernike polynomial on the unit circle, is the angle of the polar coordinate, ρ is the normalized radial coordinate, and N is the number of terms in the Zernike polynomial sequence. In short, the first term is the base profile of the entire free-form surface, and the other term represents the detail expansion of the free-form surface.

[0037] The surface shape of the second reflector M2 adopts a standard quadratic surface, and its surface shape expression is:

[0038]

[0039] Wherein, z is the free surface vector height of the second reflector M2, c is the curvature, r is the radial coordinate, and k is the quadratic surface coefficient.

[0040] The surface of the fourth reflector M4 is an even-order aspheric surface, and its surface expression is:

[0041]

[0042] Among them, z is the free surface sag of the fourth reflector M4, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, α i is the even-order coefficient.

[0043] In order to further improve the resolution of the optical system, the surface shapes of the second reflector M2 and the third reflector M3 may be changed to free-form surfaces.

[0044] According to the compact high-resolution wide-field free-form surface off-axis four-mirror optical system obtained in this embodiment, the field angle is ≥16.2°×0.5°, F#≥4, the spectral range is visible light and near-infrared band, and the relative distortion of the whole field of view will be less than 0.3%, and the field curvature is less than 0.01mm. Due to the small field curvature, a multi-spectral detector array can be placed on the focal plane, which is conducive to realizing large-width push-scan imaging.

[0045] The optical system of this embodiment uses four reflectors to form an off-axis system reflection structure. The first reflector M1 and the third reflector M3 use free-form surfaces characterized by Zernike polynomials, the second reflector M2 uses a standard quadratic surface, and the fourth reflector M4 uses an even-order aspheric surface. They are arranged in a positive-negative-positive-positive manner of optical focal length, so that the optical system can achieve near-diffraction limit imaging in the visible light and near-infrared bands, and has a compact structure and small distortion while having both high resolution and a wide field of view.

[0046] The technical solution of the present invention is further described below in conjunction with a specific embodiment.

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

[0048] Working band: visible near infrared band 400nm~800nm;

[0049] Entrance pupil diameter: 340mm;

[0050] Focal length: 2m;

[0051] Field of view: 16.2°×0.5°.

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

[0053] In the global coordinate system, the second reflector M2 is used as the reference surface, the rotational symmetry axis of the second reflector M2 is used as the optical axis, and the rotational symmetry axis of the second reflector M2 is used as the optical axis. Figure 1 The inward side of the paper is the X-axis, and the rotation angle of the first reflector M1 with respect to the X-axis is 0°; the rotation angle of the third reflector M3 with respect to the X-axis is 0°; and the rotation angle of the fourth reflector M4 with respect to the X-axis is 6°.

[0054] The structural dimensions of the free-form surface off-axis three-mirror optical system with better image quality designed based on the same optical system technical indicators are as follows: Figure 2 As shown in (a), by comparison, it can be seen that the compact high-resolution wide-field free-form surface off-axis four-mirror optical system designed in this embodiment is smaller in size.

[0055] like Figure 3 As shown, the compact high-resolution wide-field free-form surface off-axis four-mirror optical system of this embodiment has an image quality reaching the diffraction limit in the visible and near-infrared bands. Figure 3 It can also be seen that the field curvature of the optical system of this embodiment is small, and the image quality close to the diffraction limit can be obtained even if the image plane is flat. Figure 4 As shown, from Figure 4 It can be seen that the imaging distortion of the optical system of the present invention is small.

[0056] The compact high-resolution wide-field free-form surface off-axis four-mirror optical system of the present invention has the following beneficial effects:

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

[0058] The optical system of the present invention adopts an off-axis four-mirror optical system. According to the positive-negative-positive-positive mode of optical power, the first reflector and the third reflector adopt a free-form surface design, the second reflector and the fourth reflector adopt an aspheric surface design, and with a certain tilt eccentricity, the optical system with a small F number and a large aperture can achieve near-diffraction-limited imaging within a field of view of 16.2°×0.5° or even larger.

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

[0060] The compact high-resolution wide-field free-form surface off-axis four-mirror optical system of the present invention contains only four reflectors, has a small axial length, a compact structure, and the axial distance and angle around the X-axis of the first reflector and the third reflector are close, and the axial distance of the second reflector and the fourth reflector are close, which is convenient for 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 free-form surface off-axis four-mirror optical system of the present invention utilizes the reasonable distribution and design of the optical focal length of four reflectors, and the imaging quality is close to the diffraction limit in the visible and near-infrared bands. The relative distortion of the entire field of view will be less than 0.3%, and the field curvature will be less than 0.01mm.

[0063] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described 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 above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. Compact high-resolution wide-field free-form off-axis four-mirror optical system, characterized by: An off-axis system reflection structure is adopted, and the off-axis system reflection structure includes a first reflector, a second reflector, an aperture stop, a third reflector, a fourth reflector and an image plane which are sequentially arranged along the optical path direction from the object side to the image side, and the aperture stop is arranged on the second reflector, and the first reflector and the third reflector are close to the same vertical plane of the rotational symmetry axis of the second reflector; The second reflector has a 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 adopts a Zernike fringe sag surface, the surface shape of the second reflector adopts a standard quadratic surface, and the surface shape of the fourth reflector adopts an even-order aspheric surface; The four reflectors are arranged in a trapezoidal structure, with the rotational symmetry axis of the second reflector as the optical axis, the optical axis distance between the center of the first reflector and the center of the second reflector is the height d of the trapezoid, and the ratio of the height d of the trapezoid to the focal length f of the optical system satisfies: The ratio of the length of the longest base of the trapezoid TL 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 optical system volume V satisfies: V≤0.064f 3 ; In the global coordinate system, with the second reflector as the reference plane, the rotation angle of the first reflector with respect to the X-axis is -3° to 3°, the rotation angle of the third reflector with respect to the X-axis is -3° to 3°, and the rotation angle of the fourth reflector with respect to the X-axis is -10° to 10°.

2. The compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to claim 1, characterized in that: The field of view of the optical system is greater than or equal to 16.2°×0.5°, the F number is greater than or equal to 4, and the spectral range is visible light and near infrared bands.

3. The compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to claim 2, characterized in that: The spectrum ranges from 400nm to 800nm.

4. The compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to any one of claims 1 to 3, characterized in that: The focal length f of the optical system is 2 m, the height d of the trapezoid is 740 mm, the length TL of the longest bottom side of the trapezoid is 740 mm, and the long side width PML of the first reflector is 800 mm.

5. The compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to any one of claims 1 to 3, characterized in that: The rotation angle of the first reflector with respect to the X-axis is 0°, the rotation angle of the third reflector with respect to the X-axis is 0°, and the rotation angle of the fourth reflector with respect to the X-axis is 6°.

6. The compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to any one of claims 1 to 3, characterized in that: The surface expressions of the first reflector and the third reflector are: Wherein, z is the free surface vector height of the first reflector or the third reflector, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, α i is the coefficient of the even term, A i are the Zernike polynomial coefficients, is a Zernike polynomial on the unit circle, is the angle of the polar coordinate, 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 free-form surface off-axis four-mirror optical system according to any one of claims 1 to 3, characterized in that: The surface expression of the second reflector is: Wherein, z is the free surface vector height 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 free-form surface off-axis four-mirror optical system according to any one of claims 1 to 3, characterized in that: The surface expression of the fourth reflector is: Wherein, z is the free surface vector height of the fourth reflector, c is the curvature, r is the radial coordinate, k is the quadratic surface coefficient, α i is the even-order coefficient.

9. The compact high-resolution wide-field free-form surface off-axis four-mirror optical system according to any one of claims 1 to 3, characterized in that: The full field relative distortion of the optical system is less than 0.3%, and the field curvature is less than 0.01 mm.

10. The compact high-resolution wide-field free-form surface off-axis four-mirror 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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