Refrigeration type free-form surface off-axis four-mirror relay optical system

Abstract

The present application relates to a kind of free curved surface off-axis four-reflection relay optical systems of refrigeration type, mainly solve the technical problems that existing relay optical systems exist large volume, small field of view, or cannot be realized with the 100% cold screen matching of refrigeration type detector. Including the first mirror, the second mirror, the third mirror, the fourth mirror, the detector cold window and the detector cold screen that are successively fixedly connected arrangement from object plane to focal plane;The first mirror is set in the position close to object plane side;Second mirror is set on the reflected light path of the first mirror;The third mirror is set on the reflected light path of the second mirror, and the light reflected by second mirror forms a primary imaging plane between second mirror and third mirror and then enters third mirror;The fourth mirror is set on the reflected light path of the third mirror;Detector cold window and detector cold screen are successively set on the reflected light path of the fourth mirror.

Description

Technical Field

[0001] This invention relates to a relay optical system, specifically a cooled freeform surface off-axis four-reflector relay optical system. Background Technology

[0002] A relay optical system is an optical system with a finite working object distance. In modular optical system design, by connecting a relay system in series with a front-end system, it is possible to achieve system size folding, image steering, entrance / exit pupil matching, focal length magnification, and field of view segmentation. In particular, when the relay system needs to be adapted to a cooled infrared detector with a cold stop, 100% cold stop matching must be achieved, which poses certain challenges to the design of this type of system.

[0003] Many of the cooled relay optical systems disclosed in the literature adopt a transmission optical structure. However, this type of structure is limited by the types of materials, making it difficult to correct chromatic aberration. On the other hand, the optical lenses are also difficult to operate in low-temperature environments such as 77K and 100K. Relay systems that adopt a reflective optical structure are mostly single-imaging structures and cannot achieve 100% matching with the cold aperture. Reflective relay systems that can be adapted to cooled detectors also have the problem of large size.

[0004] A relay system employing an off-axis three-mirror structure was disclosed in the article "Development of Multi-band Cold Optical Infrared Imaging Terminal" published in Volume 47, Issue 9 of the Chinese journal *Infrared and Laser Engineering* in 2018; an off-axis three-mirror Offner spectral splitting relay system was used in the article "Design and Analysis of Infrared Dual-band in Spectrometer Optical Systems" published in Volume 48, Issue 23 of the Chinese journal *Chinese Optics* in 2021; and a two-mirror relay system was reported in the article "Aberration Correction of Off-axis Simultaneous Polarization Super-resolution Imaging Optical System with Different Aperture" published in Volume 71, Issue 21 of the Chinese journal *Acta Physica Sinica* in 2022. All of the above relay systems are single-image structures and cannot achieve 100% cold-aperture matching with cooled detectors.

[0005] In summary, it is of great significance to design a total internal reflection relay optical system with a small size, a large field of view, and 100% compatibility with the cold stop of a cooled detector. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problems of existing relay optical systems, which are either limited by materials, making it difficult to achieve a wide operating band, or have large size, small field of view, or cannot achieve 100% cold stop matching with cooled detectors, and to provide a cooled freeform surface off-axis four-reflector relay optical system.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A cooled freeform surface off-axis four-reflector relay optical system, characterized by:

[0009] It includes a first reflecting mirror, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a detector cold window, and a detector cold aperture, which are fixedly connected in sequence from the object plane to the focal plane;

[0010] The first reflector is positioned close to the object surface to reflect and converge light within the field of view. The second reflector is positioned on the reflected light path of the first reflector to converge the light reflected by the first reflector a second time. The third reflector is positioned on the reflected light path of the second reflector, and the light reflected by the second reflector forms a primary imaging surface between the second and third reflectors before entering the third reflector. The fourth reflector is positioned on the reflected light path of the third reflector. The detector cold window and detector cold stop are sequentially positioned on the reflected light path of the fourth reflector, and the light reflected by the fourth reflector passes through the detector cold window and detector cold stop sequentially, converging at the focal plane for imaging.

[0011] The reflecting surfaces of the first, second, and third reflectors are all xy polynomial freeform surfaces;

[0012] The first, second, and fourth reflectors all have positive optical power, while the third reflector has negative optical power.

[0013] The magnification of the objective lens formed by the first and second reflecting mirrors is defined as m. obj If the magnification of the objective lens, composed of the first, second, third, and fourth reflecting mirrors, is m', then |m' / m obj |Satiates under the following conditions:

[0014] 0.5≤|m' / m obj |≤1.2.

[0015] Furthermore, defining the lateral magnification of the fourth reflecting mirror as m5, then m5 satisfies the following condition:

[0016] 0.65 < |m5| < 1.38.

[0017] Furthermore, the |m' / m obj The value of | is 0.7;

[0018] The value of m5 is 0.85.

[0019] Furthermore, the equation of the xy polynomial freeform surface is:

[0020]

[0021] Where: z is the surface elevation; c is the surface curvature; k is the quadratic surface coefficient; A j It is the coefficient of the j-th term in the polynomial. m and n are powers, m+n≤10; N is the number of terms, N≤66; x and y are x2 and y2 in the first mirror, x3 and y3 in the second mirror, and x4 and y4 in the third mirror.

[0022] Furthermore, a first three-dimensional rectangular coordinate system (x1, y1, z1) is defined with the center of the object surface as the origin, and this first three-dimensional rectangular coordinate system is a right-handed coordinate system; a second three-dimensional rectangular coordinate system (x2, y2, z2) is defined with the vertex of the first reflector as the origin, and this second three-dimensional rectangular coordinate system is a right-handed coordinate system; a third three-dimensional rectangular coordinate system (x3, y3, z3) is defined with the vertex of the second reflector as the origin, and this third three-dimensional rectangular coordinate system is a right-handed coordinate system; a fourth three-dimensional rectangular coordinate system (x4, y4, z4) is defined with the vertex of the third reflector as the origin, and this fourth three-dimensional rectangular coordinate system is a right-handed coordinate system; a fifth three-dimensional rectangular coordinate system (x5, y5, z5) is defined with the vertex of the fourth reflector as the origin, and this fifth three-dimensional rectangular coordinate system is a right-handed coordinate system; and a sixth three-dimensional rectangular coordinate system (x6, y6, z6) is defined with the center of the cold window of the detector as the origin, and this sixth three-dimensional rectangular coordinate system is a right-handed coordinate system.

[0023] The origin of the second three-dimensional rectangular coordinate system has coordinates (0, -2 to 8, 375 to 385) in the first three-dimensional rectangular coordinate system.

[0024] The origin of the third three-dimensional rectangular coordinate system has coordinates (0, -80 to -70, 104 to 114) in the first three-dimensional rectangular coordinate system.

[0025] The origin of the fourth three-dimensional rectangular coordinate system has coordinates (0, -194 to 184, 245 to 255) in the first three-dimensional rectangular coordinate system.

[0026] The origin of the fifth three-dimensional rectangular coordinate system has coordinates (0, -321 to -311, 172 to 182) in the first three-dimensional rectangular coordinate system.

[0027] The origin of the sixth three-dimensional rectangular coordinate system has coordinates (0, -212 to -202, 253 to 263) in the first three-dimensional rectangular coordinate system.

[0028] Furthermore, the coordinates of the origin of the second three-dimensional rectangular coordinate system in the first three-dimensional rectangular coordinate system are (0, -3.11, 380.9);

[0029] The origin of the third three-dimensional rectangular coordinate system has coordinates (0, -75.04, 109.7) in the first three-dimensional rectangular coordinate system.

[0030] The origin of the fourth three-dimensional rectangular coordinate system has coordinates (0, -189.9, 250.7) in the first three-dimensional rectangular coordinate system.

[0031] The origin of the fifth three-dimensional rectangular coordinate system has coordinates (0, -316.5, 177.6) in the first three-dimensional rectangular coordinate system.

[0032] The origin of the sixth three-dimensional rectangular coordinate system has coordinates (0, -207.9, 258.6) in the first three-dimensional rectangular coordinate system.

[0033] Furthermore, the rotation angle of the second three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 11°;

[0034] The rotation angle of the third three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is -8.6°;

[0035] The rotation angle of the fourth three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 15.5°;

[0036] The rotation angle of the fifth three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 54.2°;

[0037] The rotation angle of the fifth three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 15°.

[0038] Furthermore, a Leo stop and a field stop are sequentially arranged between the second and third reflecting mirrors along the optical path; and the Leo stop is located at the first image plane of the entrance pupil, while the field stop is located at the first image plane of the object plane.

[0039] Furthermore, a filter is also provided between the fourth reflector and the detector cold window.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] 1. The invented optical structure has no chromatic aberration, high transmittance, good thermal stability, and low internal radiation noise. Compared with the existing off-axis four-reflector relay optical system, it has advantages such as large field of view, low distortion, and compact size while achieving 100% cold aperture matching.

[0042] 2. The cooled freeform surface off-axis four-reflector relay optical system of the present invention is an object-side telecentric optical system. In the optical system, the positions of the first image plane of the entrance pupil and the first image plane of the object plane can be set with a Leo stop and a field stop to ensure the excellent stray light suppression capability of the entire system.

[0043] 3. The optical system described in this invention adopts a total reflection structure, which is free of chromatic aberration, enabling a single optical system to have both wide-band detection capabilities and to effectively achieve miniaturization, weight reduction and integration of detection methods. At the same time, it can also reduce the difficulty of optical debugging and realize functions such as multi-spectral imaging.

[0044] 4. The system described in this invention adopts an off-axis four-mirror relay optical structure. Through the reasonable spatial layout of each lens, the entire imaging optical path is folded in space in a U-shape, ensuring the compactness of the entire imaging system.

[0045] 5. The optical system described in this invention adopts a quasi-symmetrical optical structure, which ensures that the optical system has good distortion characteristics under various field-of-view conditions.

[0046] 6. The present invention adopts an off-axis four-reflector relay optical structure. Each optical element is eccentric and tilted relative to the optical axis, which will not cause cold reflection self-imaging of the detector and has excellent cold reflection suppression characteristics.

[0047] 7. The optical system of the system described in this invention has only four lenses in total, exhibiting good tolerance characteristics. The optical materials used for each mirror are all commonly used mirror substrate materials, possessing good availability and processability. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the optical structure of an embodiment of the present invention (the Rio aperture and field aperture are not shown in the figure);

[0049] Figure 2 This is a schematic diagram of the optical structure and coordinate system according to an embodiment of the present invention.

[0050] In the diagram: 1-object plane, 2-first reflecting mirror, 3-second reflecting mirror, 4-third reflecting mirror, 5-fourth reflecting mirror, 6-detector cold window, 7-detector cold stop, 8-focal plane, 9-Leo stop, 10-field stop. Detailed Implementation

[0051] To make the objectives, advantages, and features of this invention clearer, the following detailed description of a cooled freeform surface off-axis four-reflector relay optical system proposed by this invention, in conjunction with the accompanying drawings and specific embodiments, will be provided. The advantages and features of this invention will become clearer according to the following specific embodiments. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise proportions, only used to conveniently and clearly assist in illustrating the objectives of the embodiments of this invention; furthermore, the structures shown in the drawings are often part of the actual structure.

[0052] Figure 1 and 2This is a preferred embodiment of the entrance pupil pre-cooled freeform surface off-axis four-mirror relay optical system of the present invention. A first mirror, a second mirror, a third mirror, a fourth mirror, a detector cold window 6, and a detector cold stop 7 are sequentially fixedly arranged from the object plane 1 to the focal plane 8. The first mirror, second mirror, third mirror, and fourth mirror are all monolithic mirrors, namely the first mirror 2, the second mirror 3, the third mirror 4, and the fourth mirror 5, respectively, and all four mirrors employ freeform surface shapes.

[0053] The center of the object plane 1 of the optical system coincides with the origin of the coordinate system defining the vertex positions of the first reflecting mirror 2, the second reflecting mirror 3, the third reflecting mirror 4, and the fourth reflecting mirror 5.

[0054] In this embodiment, the first reflector 2 is positioned near the object surface 1 in the cooled freeform surface off-axis four-reflector relay optical system. It reflects and converges the light emitted from the object surface 1 within the field of view, and further converges it through the second reflector 3. After forming a primary imaging surface between the second reflector 3 and the third reflector 4, the light diverges and is incident on the surface of the third reflector 4. The third reflector 4 diverges and reflects the light reflected by the second reflector 3 to the fourth reflector 5. The fourth reflector 5 reflects the light entering through the third reflector 4, and after passing through the detector cold window 6 and the detector cold stop 7, it finally converges onto the focal plane 8 of the detector to complete the imaging.

[0055] The cooled freeform surface off-axis four-reflector relay optical system of this embodiment is an object-side telecentric optical system. The optical system has a primary image plane of entrance pupil and a primary image plane of object plane 1, and can be sequentially set with Leo stop 9 and field stop 10 along the optical path direction.

[0056] If we take Figure 2 A first three-dimensional rectangular coordinate system (x1, y1, z1) is defined with the center of the object surface 1 as the origin, and this first three-dimensional rectangular coordinate system is a right-handed coordinate system. A second three-dimensional rectangular coordinate system (x2, y2, z2) is defined with the vertex of the first reflecting mirror 2 as the origin, and this second three-dimensional rectangular coordinate system is a right-handed coordinate system. A third three-dimensional rectangular coordinate system (x3, y3, z3) is defined with the vertex of the second reflecting mirror 3 as the origin, and this third three-dimensional rectangular coordinate system is a right-handed coordinate system. A fourth three-dimensional rectangular coordinate system (x4, y3, z3) is defined with the vertex of the third reflecting mirror 4 as the origin. 4The fourth three-dimensional rectangular coordinate system (x5, y5, z5) is defined with the vertex of the fourth reflector 5 as the origin, and the fifth three-dimensional rectangular coordinate system (x6, y6, z6) is defined with the center of the cold window 6 of the detector as the origin, and the sixth three-dimensional rectangular coordinate system (x6, y6, z6) is defined with the center of the cold window 6 of the detector as the origin, and the sixth three-dimensional rectangular coordinate system (x6, y6, z6) is defined with the center of the cold window 6 of the detector as the origin, and the sixth three-dimensional rectangular coordinate system (x6, y6, z6) is defined with the center of the cold window 6 of the detector as the origin. Specifically, the origin of the second three-dimensional rectangular coordinate system has coordinates (0, -2 to 8, 375 to 385) in the first three-dimensional rectangular coordinate system; the origin of the third three-dimensional rectangular coordinate system has coordinates (0, -80 to -70, 104 to 114) in the first three-dimensional rectangular coordinate system; the origin of the fourth three-dimensional rectangular coordinate system has coordinates (0, -194 to 184, 245 to 255) in the first three-dimensional rectangular coordinate system; the origin of the fifth three-dimensional rectangular coordinate system has coordinates (0, -321 to -311, 172 to 182) in the first three-dimensional rectangular coordinate system; and the origin of the sixth three-dimensional rectangular coordinate system has coordinates (0, -212 to -202, 253 to 263) in the first three-dimensional rectangular coordinate system.

[0057] The coordinates of each reflector vertex and the coordinate direction in space can be given in the (x1, y1, z1) coordinate system.

[0058] The general expression for an xy polynomial freeform surface is:

[0059]

[0060] Where z is the surface elevation, c is the surface curvature, k is the quadratic surface coefficient, and A j It is the coefficient of the j-th term in the polynomial. m and n are powers, and generally m+n≤10; N is the number of terms, and its value is generally no greater than 66.

[0061] In the first reflector 2, x and y are taken as x2 and y2 respectively; in the second reflector 3, x and y are taken as x3 and y3 respectively; in the third reflector 4, x and y are taken as x4 and y4 respectively; and in the fourth reflector 5, x and y are taken as x5 and y5 respectively.

[0062] Specifically, in this embodiment, the first reflector 2, the second reflector 3, the third reflector 4, and the fourth reflector 5 are respectively sixth-order polynomial freeform surfaces with respect to (x2,y2), (x3,y3), (x4,y4), and (x5,y5). Furthermore, in this embodiment, the cooled freeform surface off-axis four-reflector relay optical system is symmetric about the yz plane, retaining only the even-order terms of x, further improving the manufacturability of the entire optical system. At this time, the expression for the xy polynomial freeform surface is:

[0063]

[0064] In this embodiment, the xy polynomial parameters of the reflecting surfaces of the first reflecting mirror 2, the second reflecting mirror 3, the third reflecting mirror 4, and the fourth reflecting mirror 5 are detailed in Table 1.

[0065] At this point, if the detector cold window 6, the detector cold aperture 7, and the detector focal plane 8 are used to form a detector assembly, the first reflector 2, the second reflector 3, and the fourth reflector 5 all have positive optical power, the third reflector 4 has negative optical power, and they are sequentially fixed to the detector assembly to form a complete imaging system.

[0066] Furthermore, let the magnification of the objective lens formed by the first reflecting mirror 2 and the second reflecting mirror 3 be m. obj When the magnification of a cooled freeform surface off-axis four-reflector relay optical system is m', then |m' / m obj |Satiates under the following conditions:

[0067] 0.5≤|m' / m obj |≤1.2;(1)

[0068] Condition (1) is a formula that limits the magnification of the objective lens formed by the first reflecting mirror 2 and the second reflecting mirror 3 in the optical system. By satisfying condition (1), the miniaturization of the overall shape envelope of the optical system can be guaranteed. If condition (1) is lower than its lower limit, the third reflecting mirror 4 and the fourth reflecting mirror 5 cannot properly correct the field curvature and distortion caused by the large field of view, which complicates the subsequent system and becomes a problem. On the other hand, if condition (1) is higher than its upper limit, it is beneficial for the aberration correction of the objective lens formed by the first reflecting mirror 2 and the second reflecting mirror 3 in the optical system, but the lateral size of the optical system increases, making it difficult to miniaturize the entire optical system.

[0069] Furthermore, let the lateral magnification of the fourth reflecting mirror 5 be m5, and let m5 satisfy the following condition:

[0070] 0.65 < |m5| < 1.38; (2)

[0071] Condition (2) is a formula that limits the magnification range of the fourth reflecting mirror 5. By satisfying condition (2), the miniaturization of the longitudinal envelope of the entire optical system can be guaranteed, and the field curvature and astigmatism present in the primary image formed by the previous optical system can be well corrected. If condition (2) is lower than its lower limit, the imaging optical path becomes longer, and the miniaturization of the optical system becomes difficult. On the other hand, if condition (2) is higher than its upper limit, it is beneficial to the longitudinal miniaturization of the optical system, but the correction of field curvature and astigmatism becomes difficult, and the formed image is distorted.

[0072] As described above, the off-axis four-reflector relay optical system of this embodiment, by simultaneously satisfying the above conditions, achieves 100% cold stop matching, miniaturization, large field of view, low distortion, and good correction of various aberrations generated by light throughout the entire field of view, thus obtaining excellent optical performance.

[0073] The following describes various numerical data related to the zoom optical system involved in the embodiments.

[0074] For an optical system, F / # is 1.5. F# is the aperture number, which is the reciprocal of the ratio of the entrance pupil diameter to the focal length, i.e., F = f / D.

[0075] Compatible detector array: 640×512;

[0076] Adapted detector pixel size: 25μm×25μm;

[0077] Operating spectral range: 7.7 μm to 10.5 μm;

[0078] Object size: 62.4 mm;

[0079] Relative distortion across the entire field of view: ≤5.0%;

[0080] Heat dissipation temperature range: -45℃~+70℃;

[0081] Table 1 shows the position of each mirror vertex relative to the (x1, y1, z1) coordinate system and the surface shape parameters of the reflecting surface.

[0082]

[0083]

[0084] Note: NR is the normalized radius, NR = 0 indicates that the surface equation is not normalized; α is the rotation angle about the positive direction of the ox axis of the first three-dimensional rectangular coordinate system, with counterclockwise being positive. All dimensions in the table are in millimeters, and angles are in degrees.

[0085] Table 2 Parameter Table for Each Condition in the Example

[0086]

[0087] The cooled off-axis four-mirror relay optical system provided in this embodiment has no chromatic aberration, high transmittance, good thermal stability, and low radiation noise. Compared with the relay optical systems reported in the prior art, it has advantages such as a large field of view, low distortion, compact size, and spatial foldability, and is suitable for various applications such as low temperature optical remote sensing and infrared monitoring and measurement.

[0088] In addition, in this embodiment, a replaceable filter can be added in front of the detector's cold window 6. When the optical system needs to operate in different spectral bands, the filter for the corresponding spectral band is switched in, and then the optical image of the corresponding spectral band can be obtained.

[0089] This specification uses XY polynomials to characterize the surface shape of the designed optical system, but this should not be construed as a limitation on the surface shape described in this invention. For those skilled in the art, high-precision conversion between surfaces with different equations can be easily achieved through fitting and transforming different surface shapes. Furthermore, conversions between surface shapes not yet included in optical design software can also be achieved using surface fitting tools.

[0090] The substrate material and support structure of each reflector in this invention can be made of ordinary aluminum alloy, which has excellent machinability.

[0091] This invention uses a mirror substrate material whose coefficient of linear expansion matches that of the optomechanical structure material. Under typical operating conditions, such as the full temperature range of -45℃ to +70℃, it can achieve passive optical thermal aberration and compensate for defocus caused by temperature changes in the optomechanical structure or mirror substrate material.

[0092] This invention uses an optomechanical structural material with the same coefficient of linear expansion as the substrate material of the reflector, which will not defocus over an extremely wide temperature range, ensuring excellent imaging. In particular, it can be applied to application scenarios with temperatures of 77K or lower, meeting the needs of various low-temperature optics applications, such as various optoelectronic aiming pods and turrets, low-temperature optical remote sensing, infrared alarm monitoring, and other applications.

[0093] The above description of the present invention using illustrative examples is intended to be exemplary and does not limit the scope of protection of the present invention. Therefore, it will be apparent to those skilled in the art that feature substitutions or modifications can be made to the described present invention without departing from the scope of the claims.

Claims

1. A cooled freeform surface off-axis four-reflector relay optical system, characterized in that: It includes a first reflecting mirror, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a detector cold window, and a detector cold aperture, which are fixedly connected in sequence from the object plane to the focal plane; The first reflector is positioned close to the object surface to reflect and converge light within the field of view; the second reflector is positioned on the reflected light path of the first reflector to converge the light reflected by the first reflector a second time; the third reflector is positioned on the reflected light path of the second reflector, and the light reflected by the second reflector forms a primary imaging surface between itself and the third reflector before entering the third reflector; the fourth reflector is positioned on the reflected light path of the third reflector; the detector cold window and detector cold stop are sequentially positioned on the reflected light path of the fourth reflector, and the light reflected by the fourth reflector passes sequentially through the detector cold window and detector cold stop, converging to the focal plane for imaging; The reflecting surfaces of the first, second, third, and fourth reflectors are all xy polynomial freeform surfaces; The first, second, and fourth reflectors all have positive optical power, while the third reflector has negative optical power. The magnification of the objective lens formed by the first and second reflecting mirrors is defined as m. obj If the objective lens, composed of the first, second, third, and fourth reflecting mirrors, has a magnification of m', then |m' / m obj |Satiates under the following conditions: 0.5≤|m’ / m obj |≤1.2; If the lateral magnification of the fourth reflecting mirror is defined as m5, then m5 satisfies the following condition: 0.65<|m5|<1.38； A first three-dimensional rectangular coordinate system (x1, y1, z1) is defined with the center of the object surface as the origin, and this first three-dimensional rectangular coordinate system is a right-handed coordinate system. A second three-dimensional rectangular coordinate system (x2, y2, z2) is defined with the vertex of the first reflector as the origin, and this second three-dimensional rectangular coordinate system is a right-handed coordinate system. A third three-dimensional rectangular coordinate system (x3, y3, z3) is defined with the vertex of the second reflector as the origin, and this third three-dimensional rectangular coordinate system is a right-handed coordinate system. A fourth three-dimensional rectangular coordinate system (x4, y4, z4) is defined with the vertex of the third reflector as the origin, and this fourth three-dimensional rectangular coordinate system is a right-handed coordinate system. A fifth three-dimensional rectangular coordinate system (x5, y5, z5) is defined with the vertex of the fourth reflector as the origin, and this fifth three-dimensional rectangular coordinate system is a right-handed coordinate system. A sixth three-dimensional rectangular coordinate system (x6, y6, z6) is defined with the center of the detector's cold window as the origin, and this sixth three-dimensional rectangular coordinate system is a right-handed coordinate system. The origin of the second three-dimensional rectangular coordinate system has coordinates (0, -2 to 8, 375 to 385) in the first three-dimensional rectangular coordinate system. The origin of the third three-dimensional rectangular coordinate system has coordinates (0, -80 to -70, 104 to 114) in the first three-dimensional rectangular coordinate system. The origin of the fourth three-dimensional rectangular coordinate system has coordinates (0, -194 to 184, 245 to 255) in the first three-dimensional rectangular coordinate system. The origin of the fifth three-dimensional rectangular coordinate system has coordinates (0, -321 to -311, 172 to 182) in the first three-dimensional rectangular coordinate system. The origin of the sixth three-dimensional rectangular coordinate system has coordinates (0, -212 to -202, 253 to 263) in the first three-dimensional rectangular coordinate system.

2. The cooled freeform surface off-axis four-reflector relay optical system according to claim 1, characterized in that: The |m' / m obj The value of | is 0.7; The value of m5 is 0.

85.

3. A cooled freeform surface off-axis four-reflector relay optical system according to claim 1 or 2, characterized in that: The equation of the xy polynomial freeform surface is: ； Where: z is the surface elevation; c is the surface curvature; k is the quadratic surface coefficient; A j It is the coefficient of the j-th term in the polynomial. m and n are powers, m+n≤10; N is the number of terms, N≤66; x and y are x2 and y2 in the first mirror, x3 and y3 in the second mirror, and x4 and y4 in the third mirror.

4. The cooled freeform surface off-axis four-reflector relay optical system according to claim 3, characterized in that: The origin of the second three-dimensional rectangular coordinate system has coordinates (0, -3.11, 380.9) in the first three-dimensional rectangular coordinate system. The origin of the third three-dimensional rectangular coordinate system has coordinates (0, -75.04, 109.7) in the first three-dimensional rectangular coordinate system. The origin of the fourth three-dimensional rectangular coordinate system has coordinates (0, -189.9, 250.7) in the first three-dimensional rectangular coordinate system. The origin of the fifth three-dimensional rectangular coordinate system has coordinates (0, -316.5, 177.6) in the first three-dimensional rectangular coordinate system. The origin of the sixth three-dimensional rectangular coordinate system has coordinates (0, -207.9, 258.6) in the first three-dimensional rectangular coordinate system.

5. A cooled freeform surface off-axis four-reflector relay optical system according to claim 4, characterized in that: The rotation angle of the second three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 11°; The rotation angle of the third three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is -8.6°; The rotation angle of the fourth three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 15.5°; The rotation angle of the fifth three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 54.2°; The rotation angle of the fifth three-dimensional rectangular coordinate system relative to the ox axis of the first three-dimensional rectangular coordinate system is 15°.

6. A cooled freeform surface off-axis four-reflector relay optical system according to claim 1 or 2, characterized in that: A Leo stop and a field stop are sequentially arranged between the second and third reflectors along the optical path; the Leo stop is located at the first image plane of the system entrance pupil, and the field stop is located at the first image plane of the object plane.

7. A cooled freeform surface off-axis four-reflector relay optical system according to claim 6, characterized in that: A filter is also provided between the fourth reflector and the detector's cold window.

Images