A compact long focal distance catadioptric optical system

Abstract

This invention relates to a compact long-focal-length on-axis reflective optical system, belonging to the field of optical remote sensing technology. It addresses the problems of existing off-axis reflective optical systems, which cannot simultaneously achieve long focal length and wide field of view, and suffer from large system size, distortion, and low resolution. The invention comprises, from object side to image side, a first reflecting mirror, an aperture stop, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a fifth reflecting mirror, a sixth reflecting mirror, and a detector. The centers of all reflecting mirrors are on the same plane and are not collinear. The first, third, and fifth reflecting mirrors are Zernike freeform surfaces, while the second and fourth reflecting mirrors are even-order aspherical surfaces. The optical powers of the first, second, third, fourth, and fifth reflecting mirrors are arranged in a positive-negative-negative-positive-positive sequence. By employing an off-axis five-mirror configuration, this invention achieves high resolution and long focal length while maintaining a compact structure, low distortion, and a wider field of view.

Description

Technical Field

[0001] This invention relates to the field of optical remote sensing technology, and more particularly to a freeform surface imaging optical system in optical remote sensing technology, especially a compact long focal length axis-reflective optical system. 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" resolution 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 necessitates longer focal lengths. Reflective systems, with their advantages of chromatic aberration-free operation, long focal lengths, and wide imaging spectral range, are widely used in space remote sensing cameras.

[0003] For traditional off-axis reflective optical systems, the envelope size increases dramatically with the increase of focal length and field of view, which poses a challenge to the compact design of high-resolution cameras. Furthermore, aberration correction is difficult and distortion is hard to control, which greatly affects image quality and limits its application in space remote sensing cameras.

[0004] 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 system with a wide rectangular field of view of 17°×2°, but did not use freeform surfaces, and the system size was relatively large.

[0005] Chinese patent CN114035309A discloses a wide-field-of-view, long-wavelength off-axis three-mirror optical system based on a freeform surface. It belongs to the off-axis reflective optical system. Its primary mirror uses a freeform surface shape, while the secondary and tertiary mirrors use even-order aspherical surfaces. However, the focal length is only 500mm, resulting in low imaging resolution. Furthermore, the system uses a single freeform surface with a lateral dimension close to the length of a focal length, making the system relatively long and unfavorable for the compact design of space remote sensing cameras.

[0006] Chinese patent CN113031238A discloses a multi-lens integrated large field-of-view long focal length axis four-reflector optical system. Although its assembly and adjustment are relatively easy, its lateral dimension is close to the length of a focal length, resulting in a large system volume.

[0007] Chinese patent CN102087407A discloses an ultra-large field-of-view off-axis total internal reflection optical system. The optical power of this system is distributed in a negative-positive-negative-positive scheme. Although this reverse telephoto structure can achieve an ultra-large field of view, it will result in a longer system, which is not conducive to the compact and miniaturized design of high-resolution cameras. Summary of the Invention

[0008] To address the limitations of existing off-axis reflective optical systems, which cannot simultaneously achieve long focal lengths and wide fields of view, and suffer from large system size, distortion, and low resolution, this invention provides a compact long-focal-length on-axis reflective optical system. This system is suitable for optical systems with focal lengths of approximately 3 meters. Compared to traditional secondary imaging off-axis reflective systems, this invention's optical system boasts a larger field of view, with the full field of view potentially exceeding 5°. Furthermore, due to its longer backstop, the imaging distortion is extremely low, making it suitable for freeform surface total internal reflection optical systems in multispectral detectors. It also features a smaller envelope size, with the system volume satisfying V ≤ 0.017f. 3 .

[0009] The technical solution adopted by this invention to solve the technical problem is as follows:

[0010] A compact long focal length axis-reflective optical system includes a first mirror, an aperture stop, a second mirror, a third mirror, a fourth mirror, a fifth mirror, a sixth mirror, and a detector arranged sequentially from the object side to the image side. The first mirror and the aperture stop are positioned coincidentally, the centers of each mirror are on the same plane, and the centers of any two mirrors are not collinear.

[0011] The first reflector is a freeform surface reflector with positive optical power, the second reflector is a rectangular aspherical reflector with negative optical power, the third reflector is a rectangular freeform surface reflector with negative optical power, the fourth reflector is a rectangular aspherical reflector with positive optical power, the fifth reflector is a rectangular freeform surface reflector with positive optical power, and the sixth reflector is a rectangular plane reflector. Furthermore, the first, third, and fifth reflectors are Zernike freeform surfaces, and the second and fourth reflectors are even-order aspherical surfaces.

[0012] Compared with existing technologies, the compact long-focal-distance on-axis reflective optical system of the present invention achieves high resolution while maintaining a compact structure by employing an off-axis five-mirror configuration. Compared with traditional secondary imaging off-axis reflective systems, it can achieve a wider field of view, has a real exit pupil, and has stronger stray light reduction capabilities. Moreover, the imaging distortion of the optical system of the present invention is extremely small, and the structure is further compacted by folding the optical path through a sixth reflecting mirror. The freeform surface used in the optical system of the present invention provides more degrees of freedom than traditional spherical and aspherical surfaces, which helps to balance various off-axis aberrations, ultimately achieving excellent performance such as high resolution, wide field of view, low distortion, and compact design. Attached Figure Description

[0013] 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:

[0014] Figure 1 This is a schematic diagram of the structure of a compact long focal length axis-reflective optical system according to an embodiment of the present invention;

[0015] Figure 2 This is a schematic diagram of the xyz right-handed spatial coordinate system used in an embodiment of the present invention;

[0016] Figure 3 yes Figure 1 The diagram shows the modulation transfer function (MTF) curve of a compact long-focal-length axis-reflective optical system.

[0017] Figure 4 yes Figure 1 The figure shows the F-Tan (theta) distortion curve of a compact long focal length axis-reflective optical system.

[0018] Explanation of reference numerals in the attached figures: M1, first reflecting mirror; M2, second reflecting mirror; M3, third reflecting mirror; M4, fourth reflecting mirror; M5, fifth reflecting mirror; M6, sixth reflecting mirror; STOP, aperture stop; IMA, detector image plane. Detailed Implementation

[0019] 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.

[0020] The structure of the compact long focal length axis-reflective optical system of the present invention is as follows: Figure 1As shown. The optical system of this invention adopts an off-axis reflection type and mainly includes six mirrors: a first mirror M1, a second mirror M2, a third mirror M3, a fourth mirror M4, a fifth mirror M5, and a sixth mirror M6. It also includes an aperture stop (STOP) and a detector. The first mirror M1, third mirror M3, and fifth mirror M5 are freeform surfaces; the second mirror M2 and fourth mirror M4 are even-order aspherical surfaces; and the sixth mirror M6 is a plane mirror used to fold the optical path, thereby making the optical system structure more compact. The centers of the first mirror M1, second mirror M2, third mirror M3, fourth mirror M4, fifth mirror M5, and sixth mirror M6 are on the same plane, but not on the same straight line. The centers of any two mirrors are not collinear; that is, the centers of the six mirrors are on the same plane but not on the same line. The optical axes of the six mirrors do not coincide with the system's optical axis; the system's optical axis is deflected once with each reflection.

[0021] This optical system is arranged in an ordered manner according to the xyz right-handed spatial coordinate system, as shown in the figure below. Figure 2 As shown, the z-axis direction is defined as the optical axis direction, and the y-axis is... Figure 2 In the plane, the x-axis is perpendicular to the yz coordinate plane, which is the meridional plane of the optical system. The optical axis is deflected once each time it passes through a mirror system. The arrangement order of the mirrors and aperture stops is as follows: first mirror M1, aperture stop STOP (the positions of the first mirror M1 and the aperture stop STOP coincide), second mirror M2, third mirror M3, fourth mirror M4, fifth mirror M5, sixth mirror M6, and detector image plane IMA.

[0022] The first reflecting mirror M1 is a freeform surface reflecting mirror with positive optical power; the second reflecting mirror M2 is a rectangular aspherical reflecting mirror with negative optical power; the third reflecting mirror M3 is a rectangular freeform surface reflecting mirror with negative optical power; the fourth reflecting mirror M4 is a rectangular aspherical reflecting mirror with positive optical power; the fifth reflecting mirror M5 is a rectangular freeform surface reflecting mirror with positive optical power; and the sixth reflecting mirror M6 is a rectangular plane reflecting mirror.

[0023] The reflecting surfaces of the first reflecting mirror M1 and the second reflecting mirror M2 are arranged opposite each other; the reflecting surfaces of the second reflecting mirror M2 and the third reflecting mirror M3 are arranged opposite each other; the reflecting surfaces of the third reflecting mirror M3 and the fourth reflecting mirror M4 are arranged opposite each other; the reflecting surfaces of the fourth reflecting mirror M4 and the fifth reflecting mirror M5 are arranged opposite each other; the reflecting surfaces of the fifth reflecting mirror M5 and the sixth reflecting mirror M6 are arranged opposite each other; and the sixth reflecting mirror M6 and the detector image plane IMA are arranged opposite each other. To achieve a long focal length while making the optical system structure more compact, the optical system of this invention adopts an off-axis five-reflector system structure. The image is formed by reflecting the target from infinity through 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, then from the fourth reflecting mirror M4 to the fifth reflecting mirror M5, then from the fifth reflecting mirror M5 to the sixth reflecting mirror M6, and finally from the sixth reflecting mirror M6 to the detector image plane IMA.

[0024] Furthermore, the ratios of the center distances between the first reflector M1 and the second reflector M2, the center distances between the second reflector M2 and the third reflector M3, the center distances between the third reflector M3 and the fourth reflector M4, the center distances between the fourth reflector M4 and the fifth reflector M5, the center distances between the fifth reflector M5 and the sixth reflector M6, and the center distance between the sixth reflector and the detector are 1:0.22:0.19:0.87:0.89:1.26.

[0025] This invention rationally allocates the optical power of the five reflectors, employing a positive-negative-negative-positive-positive optical power distribution scheme. Compared to the positive-negative-positive optical power distribution scheme of traditional off-axis three-mirror optical systems, this invention splits the negative optical power of the second reflector into the shared responsibility of the second reflector M2 and the third reflector M3, and splits the positive optical power of the third reflector into the shared responsibility of the fourth reflector M4 and the fifth reflector M5. The advantage is that it can achieve higher resolution while having a smaller envelope size.

[0026] The first reflecting mirror M1, the third reflecting mirror M3, and the fifth reflecting mirror M5 all adopt Zernike-style freeform surfaces, and their surface shape expressions are as follows:

[0027]

[0028] This polynomial consists of two parts: the first part is the quadratic aspheric basis term, and the second part is the Zernike polynomial. Here, c is the curvature, k is the quadratic surface coefficient, and A... i These are the coefficients of the Zernike polynomial. Let ρ be the Zernike polynomial on the unit circle, and ρ be the polar radius. Let be the angle in polar coordinates, N be the number of terms in the Zernike polynomial in the sequence, and x and y be the x-coordinate and y-coordinate of the surface, respectively. 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.

[0029] Furthermore, the first reflecting mirror M1, the third reflecting mirror M3, and the fifth reflecting mirror M5 can adopt a Zernike fringe surface with 19 terms, i.e., N=19, or a Zernike fringe surface with more terms, to further improve the resolution of the optical system.

[0030] The second reflecting mirror M2 and the fourth reflecting mirror M4 adopt an even-order aspherical surface design, and their surface shape expressions are as follows:

[0031]

[0032] Where c is the curvature, k is the quadratic surface coefficient, x and y are the x and y coordinates of the surface shape, respectively, and α i r is the aspherical coefficient. 2i It is a polynomial of even degree aspherical surface.

[0033] To further improve the resolution of the optical system, the surface shapes of the second mirror M2 and the fourth mirror M4 can be replaced with Zernike freeform surfaces.

[0034] Optionally, the detector employs a multispectral array detector element to achieve wide-swath pushbroom imaging.

[0035] Furthermore, the materials of the first reflector M1, the second reflector M2, the third reflector M3, the fourth reflector M4, the fifth reflector M5, and the sixth reflector M6 can be silicon carbide or microcrystalline materials.

[0036] Assuming the detector pixel size is 3.2 μm, the Nyquist frequency of the optical system can be calculated to be 156 lp / mm. Figure 3 The results show that the MTF value is greater than 0.2 at maximum field of view, indicating that the optical system has high imaging quality and meets the imaging requirements of a detector with a single pixel size of 3.2 μm. Furthermore, Figure 4 The distortion curve of the optical system is shown from... Figure 4 As can be seen, the F-Tan(theta) distortion across the entire field of view is no greater than 0.2%, which is extremely small. The optical system of this invention has a field of view greater than or equal to 5°.

[0037] In summary, by implementing the technical solution of this invention, the problems of low resolution and large imaging distortion in off-axis reflective optical systems are solved. This enables off-axis reflective optical systems used in space remote sensing cameras to achieve both high resolution and wide field of view, while also being compact and having low distortion.

[0038] 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.

[0039] 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 long focal distance axicon optical system characterized by, It includes a first reflecting mirror, an aperture stop, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a fifth reflecting mirror, a sixth reflecting mirror, and a detector arranged sequentially from the object side to the image side. The positions of the first reflecting mirror and the aperture stop coincide, the centers of each reflecting mirror are on the same plane, and the centers of any two reflecting mirrors are not collinear. The first reflector is a freeform surface reflector with positive optical power, the second reflector is a rectangular aspherical reflector with negative optical power, the third reflector is a rectangular freeform surface reflector with negative optical power, the fourth reflector is a rectangular aspherical reflector with positive optical power, the fifth reflector is a rectangular freeform surface reflector with positive optical power, and the sixth reflector is a rectangular plane reflector. Furthermore, the first, third, and fifth reflectors are Zernike freeform surfaces, and the second and fourth reflectors are even-order aspherical surfaces. The ratio of the center distances between the first and second reflectors, the second and third reflectors, the third and fourth reflectors, the fourth and fifth reflectors, the fifth and sixth reflectors, and the sixth reflector and the detector is 1:0.22:0.19:0.87:0.89:1.

26. The envelope volume V of the optical system satisfies ,in The focal length of the optical system is given, and the total field of view of the optical system is greater than or equal to 5°.

2. The compact long-focal-distance axisymmetric optical system according to claim 1, characterized in that, The surface shape expressions of the first reflector, the third reflector, and the fifth reflector are as follows: in, For curvature, For quadratic surface coefficients, These are the coefficients of the Zernike polynomial. For the Zernike polynomial on the unit circle, The radius is in polar coordinates. Angles in polar coordinates Let be the number of terms in the Zernike polynomials in the sequence. and These are the x-coordinate and y-coordinate of the surface, respectively.

3. A compact long focal length axicon optical system according to claim 2, wherein The number of terms of the Zernike polynomials is 19.

4. A compact long focal length axicon optical system according to claim 1, wherein The surface shape expressions of the second and fourth reflecting mirrors are as follows: in, For curvature, For quadratic surface coefficients, and These are the x and y coordinates of the surface, respectively. The aspheric coefficient, It is a polynomial of even degree aspherical surface.

5. A compact long-focal-distance axisymmetric optical system according to claim 1, characterized in that, The detector is a multispectral array detector element.

6. A compact long-focal-distance axis-reflective optical system according to claim 1, characterized in that, The first, second, third, fourth, fifth, and sixth reflectors are made of silicon carbide.

7. A compact long focal length axicon optical system according to claim 1, wherein The first, second, third, fourth, fifth, and sixth reflectors are made of microcrystalline material.

8. A compact long focal length axicon optical system according to claim 1, wherein The surface shapes of the second and fourth reflectors are replaced with Zernike freeform surfaces.

Images