Compact long focal distance axial reflection type optical system

Abstract

The invention relates to a compact long-focal-distance axial reflection type optical system, belongs to the technical field of optical remote sensing, and solves the problems that an existing off-axis reflection type optical system cannot have a long focal length and a wide view field at the same time, the system size and distortion are large, and the resolution is low. The system comprises a first reflecting mirror, an aperture diaphragm, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a fifth reflecting mirror, a sixth reflecting mirror and a detector which are sequentially arranged from an object space to an image space, wherein the centers of the reflecting mirrors are on the same plane and are not collinear; the first reflecting mirror, the third reflecting mirror and the fifth reflecting mirror are Zernike type free-form surfaces, the second reflecting mirror and the fourth reflecting mirror are even-order aspheric surfaces, and the focal power of the first reflecting mirror, the focal power of the second reflecting mirror, the focal power of the third reflecting mirror, the focal power of the fourth reflecting mirror and the focal power of the fifth reflecting mirror are sequentially arranged according to positive-negative-negative-positive-positive-positive. By adopting an off-axis five-reflection form, the high resolution and the long focal length are realized, the structure is compact, the distortion is small, and the field of view is larger.

Description

Technical Field

[0001] The invention relates to the field of optical remote sensing technology, in particular to a free-form surface imaging optical system in optical remote sensing technology, and more particularly to a compact long-focal-distance axial reflection optical system. 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 fixed, 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.

[0003] For traditional off-axis reflective optical systems, as the focal length and field of view increase, the envelope size will increase dramatically, posing a challenge to the compact design of high-resolution cameras. In addition, aberration correction is difficult and distortion is difficult 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 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 system with a wide rectangular field of view of 17°×2°, but did not use free-form surfaces and the system size was relatively large.

[0005] The Chinese patent with publication number CN114035309A discloses a wide-field and long-wavelength off-axis three-mirror optical system based on free-form surfaces. It is an off-axis reflective optical system. Its primary mirror uses a free-form surface, and the secondary and tertiary mirrors use even-order aspherical surfaces. However, the focal length is only 500 mm, and the imaging resolution is not high. In addition, the system uses a free-form surface with a lateral dimension close to a focal length. The system is relatively long, which is not conducive to the compact design of space remote sensing cameras.

[0006] Chinese patent publication number CN113031238A discloses a multi-mirror integrated large field of view long focal distance axial four-mirror optical system. Although its installation and adjustment difficulty is relatively low, its lateral dimension is close to the length of a focal length and the system volume is relatively large.

[0007] A Chinese patent with publication number CN102087407A discloses an ultra-large field of view off-axis total reflection optical system, in which the optical focal length is distributed in a negative-positive-negative-positive manner. 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] In order to solve the problems that the off-axis reflective optical system in the prior art cannot have both long focal length and wide field of view, and the system size and distortion are large and the resolution is low, the present invention provides a compact long focal distance off-axis reflective optical system, which is suitable for optical systems with a focal length of about 3m. Compared with the traditional off-axis reflective system with secondary imaging, the optical system of the present invention has a larger field of view, and the full field of view can even reach more than 5°. At the same time, since the back intercept of the optical system of the present invention is longer and the imaging distortion is extremely small, it is suitable for the free-form surface total reflection optical system of the multi-spectral detector, and has a smaller envelope size, and the system volume satisfies V≤0.017f 3 .

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

[0010] A compact long focal distance axial reflective optical system, comprising a first reflector, an aperture stop, a second reflector, a third reflector, a fourth reflector, a fifth reflector, a sixth reflector and a detector arranged in sequence from the object side to the image side, wherein the first reflector and the aperture stop are located at the same position, the centers of the reflectors are on the same plane and the centers of any two reflectors are not collinear;

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

[0012] Compared with the prior art, the compact long focal distance axial reflective optical system of the present invention adopts the form of off-axis five-mirror, and has a compact structure while achieving high resolution. Compared with the traditional off-axis reflective system of secondary imaging, it can achieve a larger field of view, and has a real exit pupil, and has a stronger ability to eliminate stray light. In addition, the imaging distortion of the optical system of the present invention is extremely small, and the structure is made more compact by folding the optical path through the sixth reflector. The free-form 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, and finally achieves excellent performance such as high resolution, wide field of view, low distortion and compactness. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] 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:

[0014] Figure 1 It is a schematic structural diagram of a compact long focal distance axis reflective optical system according to an embodiment of the present invention;

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

[0016] Figure 3 yes Figure 1 A Modulation Transfer Function (MTF) curve of a compact long focal distance axial reflective optical system shown;

[0017] Figure 4 yes Figure 1 FIG. 1 is a graph showing the F-Tan (theta) distortion of a compact long focal length axis reflective optical system.

[0018] Explanation of the reference numerals: 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 DESCRIPTION

[0019] 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.

[0020] The structure of the compact long focal distance axis reflective optical system of the present invention is as follows: Figure 1As shown. The optical system of the present invention adopts an off-axis reflective form, mainly including six reflectors, namely 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, and also including an aperture stop STOP and a detector. The first reflector M1, the third reflector M3, and the fifth reflector M5 are free-form surfaces, the second reflector M2 and the fourth reflector M4 are even-order aspheric surfaces, and the sixth reflector M6 is a plane reflector, which is used to fold the optical path, so that the optical system structure is more compact. The centers 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 are on the same plane, but not on the same straight line. The centers of any two reflectors are not collinear, that is, the centers of the six reflectors are on the same plane but not on the same line. The optical axes of the six reflectors do not coincide with the optical axis of the system, and the optical axis of the system is deflected once for each reflection.

[0021] The optical system is arranged in an orderly manner according to the xyz right-handed space coordinate system. Figure 2 As shown, the z-axis direction is defined as the optical axis direction and the y-axis is defined as Figure 2 In the plane, the x-axis is perpendicular to the yz coordinate plane, the yz coordinate plane is the meridian plane of the optical system, and the optical axis is deflected once when passing through each reflector system. The arrangement order of the reflectors and apertures is: the first reflector M1, the aperture aperture STOP (the positions of the first reflector M1 and the aperture aperture STOP coincide), the second reflector M2, the third reflector M3, the fourth reflector M4, the fifth reflector M5, the sixth reflector M6 and the detector image plane IMA.

[0022] The first reflector M1 is a free-form surface reflector with positive optical power, the second reflector M2 is a rectangular aspherical reflector with negative optical power, the third reflector M3 is a rectangular free-form surface reflector with negative optical power, the fourth reflector M4 is a rectangular aspherical reflector with positive optical power, the fifth reflector M5 is a rectangular free-form surface reflector with positive optical power, and the sixth reflector M6 is a rectangular plane reflector.

[0023] The reflection surfaces of the first reflector M1 and the second reflector M2 are arranged oppositely, the reflection surfaces of the second reflector M2 and the third reflector M3 are arranged oppositely, the reflection surfaces of the third reflector M3 and the fourth reflector M4 are arranged oppositely, the reflection surfaces of the fourth reflector M4 and the fifth reflector M5 are arranged oppositely, the reflection surfaces of the fifth reflector M5 and the sixth reflector M6 are arranged oppositely, and the sixth reflector M6 and the detector image plane IMA are arranged oppositely. In order to achieve a long focal length and make the structure of the optical system more compact, the optical system of the present invention adopts an off-axis five-reflection system structure, from an infinitely far target, it 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, then from the fourth reflector M4 to the fifth reflector M5, then from the fifth reflector M5 to the sixth reflector M6, and finally from the sixth reflector M6 to the detector image plane IMA, and finally an image is formed.

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

[0025] The present invention reasonably distributes the optical focal lengths of the five reflectors and adopts a positive-negative-negative-positive-positive optical focal length distribution arrangement scheme. Compared with the positive-negative-positive optical focal length distribution scheme of the traditional off-axis three-mirror optical system, the negative optical focal length of the second reflector is split into a second reflector M2 and a third reflector M3 to share, and the positive optical focal length of the third reflector is split into a fourth reflector M4 and a fifth reflector M5 to share. The advantage of the present invention is that higher resolution can be achieved while having a smaller envelope size.

[0026] The first reflector M1, the third reflector M3, and the fifth reflector M5 all adopt Zernike free-form surfaces, and their surface expressions are:

[0027]

[0028] This polynomial is divided into two parts. The first part is the quadratic aspheric basis term, and the second part is the Zernike polynomial. Among them, c is the curvature, k is the quadratic surface coefficient, A i are the Zernike polynomial coefficients, is the Zernike polynomial on the unit circle, ρ is the polar coordinate radius, is the angle of polar coordinates, N is the number of terms in the Zernike polynomial in the sequence, and x and y are the x- and y-coordinates of the surface shape, respectively. In short, the first term is the base contour of the entire free-form surface shape, and the other term represents the detailed expansion of the free-form surface.

[0029] Furthermore, the first reflector M1 , the third reflector M3 , and the fifth reflector M5 may adopt a 19-term Zernike fringe surface, ie, N=19, or may adopt a Zernike fringe surface with a greater number of terms, so as to further improve the resolution of the optical system.

[0030] The second reflector M2 and the fourth reflector M4 adopt an even-order aspheric design, and their surface expressions are:

[0031]

[0032] Among them, c is the curvature, k is the quadratic surface coefficient, x and y are the x-coordinate and y-coordinate of the surface respectively, α i is the aspheric coefficient, r 2i is a polynomial for an even-degree aspheric surface.

[0033] In order to further improve the resolution of the optical system, the surface shapes of the second reflector M2 and the fourth reflector M4 may be replaced with Zernike free-form surfaces.

[0034] Optionally, the detector uses a multi-spectral array detection element to achieve wide-band push-scan imaging.

[0035] Furthermore, the material 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 may be silicon carbide or microcrystalline material.

[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 It can be concluded that the MTF values ​​are all greater than 0.2 at the 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 of 3.2μm. In addition, Figure 4 is the distortion curve of the optical system, from Figure 4 It can be seen that the F-Tan (theta) distortion within the full field of view is no more than 0.2%, which is very small. The full field of view angle of the optical system of the present invention is greater than or equal to 5°.

[0037] To sum up, by implementing the technical solution of the present invention, the problems of low resolution and large imaging distortion of the off-axis reflective optical system are solved, so that the off-axis reflective optical system used in space remote sensing cameras has a compact structure and small distortion while having high resolution and a wide field of view, and at the same time has high resolution.

[0038] 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.

[0039] 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. A compact long focal distance axis reflective optical system, characterized in that: The invention comprises a first reflector, an aperture stop, a second reflector, a third reflector, a fourth reflector, a fifth reflector, a sixth reflector and a detector which are sequentially arranged from the object side to the image side, wherein the positions of the first reflector and the aperture stop coincide with each other, the centers of the reflectors are on the same plane and the centers of any two reflectors are not collinear; The first reflector is a free-form surface reflector with positive optical power, the second reflector is a rectangular aspheric reflector with negative optical power, the third reflector is a rectangular free-form surface reflector with negative optical power, the fourth reflector is a rectangular aspheric reflector with positive optical power, the fifth reflector is a rectangular free-form surface reflector with positive optical power, the sixth reflector is a rectangular plane reflector, and the first reflector, the third reflector and the fifth reflector are Zernike free-form surfaces, and the second reflector and the fourth reflector are even-order aspheric surfaces.

2. A compact long focal distance axis reflective optical system according to claim 1, characterized in that: The ratio of the center distance between the first reflector and the second reflector, the center distance between the second reflector and the third reflector, the center distance between the third reflector and the fourth reflector, the center distance between the fourth reflector and the fifth reflector, the center distance between the fifth reflector and the sixth reflector, and the center distance between the sixth reflector and the detector is 1:0.22:0.19:0.87:0.89:1.

26.

3. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The surface expressions of the first reflector, the third reflector and the fifth reflector are: Where c is the curvature, k is the quadratic surface coefficient, and A i are the Zernike polynomial coefficients, is the Zernike polynomial on the unit circle, ρ is the polar coordinate radius, is the angle of polar coordinates, N is the number of terms in the Zernike polynomial in the sequence, and x and y are the x- and y-coordinates of the surface respectively.

4. A compact long focal distance axis reflective optical system according to claim 3, characterized in that: The number of terms N in the Zernike polynomial is 19.

5. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The surface expressions of the second reflector and the fourth reflector are: Among them, c is the curvature, k is the quadratic surface coefficient, x and y are the x-coordinate and y-coordinate of the surface respectively, α i is the aspheric coefficient, r 2i is a polynomial for an even-degree aspheric surface.

6. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The detector is a multi-spectral array detection element.

7. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The first reflector, the second reflector, the third reflector, the fourth reflector, the fifth reflector and the sixth reflector are made of silicon carbide.

8. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The first reflector, the second reflector, the third reflector, the fourth reflector, the fifth reflector and the sixth reflector are made of microcrystalline material.

9. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The surface shapes of the second reflector and the fourth reflector are changed to Zernike free-form surfaces.

10. A compact long focal distance axis reflective optical system according to claim 1 or 2, characterized in that: The full field of view of the optical system is greater than or equal to 5°.

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