Entrance pupil front refrigeration type free-form surface off-axis four-mirror optical system

Abstract

The invention belongs to the field of optical systems, and particularly relates to an entrance pupil front refrigeration type free-form surface off-axis four-mirror optical system. The optical system comprises a first reflecting mirror, a second reflecting mirror, a third reflecting mirror, a fourth reflecting mirror, a detector window and a detector cold diaphragm which are sequentially arranged from an object plane to a focal plane, wherein the first reflecting mirror is arranged at a position close to the object plane; the second reflecting mirror is arranged on a reflecting light path of the first reflecting mirror; the third reflecting mirror is arranged on a reflecting light path of the second reflecting mirror; the fourth reflecting mirror is arranged on a reflecting light path of the third reflecting mirror; and a detector window and a detector cold diaphragm are sequentially arranged on a reflection light path of the fourth reflector. According to the invention, the modular design of the reflective off-axis optical system can be realized, and the integration level and interchangeability of the photoelectric system are improved; and the relative aperture is larger, so that the detection sensitivity of the optical system is further improved while the optical path structure is kept compact.

Description

Technical Field

[0001] The present invention relates to a free-form surface off-axis optical system, in particular to an entrance pupil pre-cooled free-form surface off-axis four-mirror optical system. Background Art

[0002] Off-axis reflective optical systems have been widely used due to their lack of chromatic aberration, excellent heat resistance, simple structure, and low inherent thermal radiation. They offer inherent advantages, particularly in high-performance infrared detection and imaging. Traditional off-axis reflective optical systems using even-order aspheric surfaces have limited design freedom and struggle to meet the stringent requirements of optoelectronic systems for large fields of view, high resolution, high performance, and miniaturization. Freeform optical elements significantly increase the optimization variables of optical systems, enabling larger imaging fields and superior imaging performance, and have become a research hotspot for off-axis reflective optical systems.

[0003] Optical systems with a pre-placed entrance pupil can independently perform imaging functions and can also be combined with a front-end afocal system to achieve an expansion of focal length or field of view, which is of great significance for the modular design of optoelectronic imaging systems. In addition, in high-performance infrared imaging and detection tasks, to improve the detection sensitivity of the system, cooled infrared detectors are often used. The detector cold gate needs to match the optical system exit pupil to achieve 100% cold gate efficiency. However, modular design requires that the optical system not only have a pre-placed entrance pupil, that is, the entrance pupil must not only match the front-end afocal system, but also the exit pupil must match the cooled detector cold gate, which poses a great challenge to optical design.

[0004] Currently, the free-form surface off-axis reflective optical systems that have been disclosed are mostly used in the field of uncooled infrared imaging, and have low detection sensitivity. Such optical systems cannot match cooled infrared detectors, do not consider modular design, and do not have a front entrance pupil. There are relatively few published literatures related to cooled off-axis reflective optical systems. Most of the free-form surface off-axis reflective optical systems in the literature only consider the matching of the optical system exit pupil and the detector cold stop, but do not consider the requirements of modular design. They do not have a front entrance pupil. There are even fewer literatures related to cooled off-axis reflective optical systems with a front entrance pupil. The optical systems in some published literature also have problems such as relatively small aperture, and the aperture is mostly F4, which makes the focal length of the optical system larger and the structure less compact under the same detection capability.

[0005] "Design of Free-Form Surface Uncooled Infrared Off-Axis Three-Mirror Optical System" published in the journal "Infrared and Laser Engineering" Volume 54, Issue 2 in February 2025, and Chinese patents with publication numbers CN103246053A and CN107290845A, disclose a variety of free-form surface off-axis reflective optical systems, but none of them can be matched with cooled infrared detectors and do not have external entrance pupils.

[0006] "Application of Free-Form Surfaces in Cooled Off-Axis Three-Mirror Optical Systems" published in the 9th issue of Volume 47 of the journal "Infrared and Laser Engineering" in September 2018; "Design of Cooled Free-Form Surface Off-Axis Reflective Optical System" published in the 11th issue of Volume 39 of the journal "Acta Optica Sinica" in November 2019; "Free-Form Surface Off-Axis Four-Mirror All-Aluminum Optical-Mechanical Infrared Detection System" published in the 7th issue of Volume 52 of the journal "Infrared and Laser Engineering" in July 2023; various cooled free-form surface off-axis reflective optical systems disclosed in Chinese patents with publication numbers CN112213847B and CN112180576B, which can match cooled infrared detectors but do not have external entrance pupils. The article "Research on the Design Method of Triple Conjugate Stray Light Cancellation Optical System" published in the Journal of Infrared and Millimeter Waves, Volume 38, Issue 1, in February 2019, discloses an off-axis three-reflection stray light cancellation optical system that satisfies the triple conjugation relationship. It can be matched with a cooled infrared detector, but it cannot achieve entrance pupil pre-positioning, and the cold stop efficiency cannot reach 100%. In addition, the field of view and relative aperture are small, with an aperture of only F4.

[0007] Chinese patent publication number CN117492186A discloses a pre-entrance pupil infrared continuous zoom system. While it can be used with a cooled infrared detector and also features a pre-entrance pupil, it is a coaxial transmissive optical system. This type of transmissive optical system inherently radiates significant heat and cannot meet the requirements for high-performance extreme detection in infrared backgrounds. Chinese patent publication number CN116880049A discloses a pre-entrance pupil cooled free-form surface off-axis three-mirror system. This system also features a pre-entrance pupil and can also be used with a cooled infrared detector, but its relative aperture is small, at only F4. For equivalent detection capabilities, a smaller relative aperture results in a longer optical focal length and a less compact structure.

[0008] In summary, designing a cooled infrared free-form surface off-axis reflective optical system with large relative aperture, low distortion, large field of view, compact structure, and a front entrance pupil is of great significance for achieving high sensitivity, high performance extreme detection, and modular design of the optical system. Summary of the Invention

[0009] The purpose of the present invention is to solve the technical problems that the existing free-form surface off-axis reflective optical system cannot achieve entrance pupil pre-positioning and cannot match the cooled infrared detector; or the relative aperture is small, the distortion is large, and the volume is not compact enough, and to provide a free-form surface off-axis four-mirror optical system with entrance pupil pre-positioning and cooling.

[0010] To achieve the above object, the technical solution adopted by the present invention is: A free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling, which is special in that: It includes a first reflector, a second reflector, a third reflector, a fourth reflector, a detector window and a detector cold stop, which are arranged in sequence from the object plane to the focal plane: The first reflector is arranged near the object plane and is used to reflect and converge the light incident within the field of view; the second reflector is arranged on the reflection light path of the first reflector and is used to reflect the light reflected by the first reflector for a second time, and form a primary image plane between the second reflector and the third reflector; the third reflector is arranged on the reflection light path of the second reflector and is used to reflect the light reflected by the second reflector for a third time; the fourth reflector is arranged on the reflection light path of the third reflector and is used to reflect the light reflected by the third reflector; the detector window and the detector cold stop are arranged in sequence on the reflection light path of the fourth reflector, and the light reflected by the fourth reflector passes through the detector window and the detector cold stop in sequence and converges to the focal plane to form an image; The reflecting surfaces of the first reflecting mirror, the second reflecting mirror and the third reflecting mirror are all XY polynomial free-form surfaces, and the reflecting surface of the fourth reflecting mirror is an even-order aspheric surface; The first reflector, the second reflector and the fourth reflector all have positive optical power, and the third reflector has negative optical power; The vertex curvature radii R1, R2, R3 and R4 of the first reflector, the second reflector, the third reflector and the fourth reflector respectively satisfy the following relationships: 2≤│R1 / f│≤3; 10≤│R2 / f│≤15; 0.5≤│R3 / f│≤0.7; 1.3≤│R4 / f│≤1.6; Where f is the total focal length of the optical system.

[0011] Furthermore, a reference surface is provided between the first reflector and the object surface, and a first three-dimensional rectangular coordinate system ( x 1. y 1. z 1); define a second three-dimensional rectangular coordinate system with the vertex of the first reflector as the origin ( x 2. y 2. z 2); define the third three-dimensional rectangular coordinate system with the vertex of the second reflector as the origin ( x 3. y 3. z 3); define a fourth three-dimensional rectangular coordinate system with the vertex of the third reflector as the origin ( x 4. y 4. z 4); define a fifth three-dimensional rectangular coordinate system with the vertex of the fourth reflector as the origin ( x 5. y 5.z 5); define the sixth three-dimensional rectangular coordinate system with the center of the detector window as the origin ( x 6. y 6. z 6); The first three-dimensional rectangular coordinate system to the sixth three-dimensional rectangular coordinate system are all right-handed coordinate systems; The reference surface coincides with the entrance pupil, and the Z-direction distance between the entrance pupil and the first reflector is 35 mm to 45 mm; The rotation angle α1 of the second three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is 35°≤α1≤45°, where counterclockwise around the X-axis is positive and clockwise around the X-axis is negative; The vertex coordinate position of the first reflector is (0, 185-195, 35-45); The rotation angle α2 of the third three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -30°≤α2≤-20°, where counterclockwise around the X-axis is positive and clockwise around the X-axis is negative; The vertex coordinate position of the second reflector is (0, -25 to -15, 5 to 15); The rotation angle α3 of the fourth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -75°≤α3≤-90°, where counterclockwise around the X-axis is positive and clockwise around the X-axis is negative; The vertex coordinate position of the third reflector is (0, -55 to -45, 45 to 55); The rotation angle α4 of the fifth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -5°≤α4≤-15°, where counterclockwise around the X-axis is positive and clockwise around the X-axis is negative; The vertex coordinate position of the fourth reflector is (0, -75 to -65, -60 to -50); The rotation angle α5 of the sixth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -20°≤α5≤-25°, where counterclockwise around the X-axis is positive and clockwise around the X-axis is negative; The center coordinate position of the detector window is (0, -110~-100, 25~35).

[0012] Furthermore, the Z-axis distance between the entrance pupil and the first reflector is 40 mm; the rotation angle α1 of the second three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is 39.14°, and the vertex coordinate position of the first reflector is (0, 188.14, 40); The rotation angle α2 of the third three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -23.77°, and the vertex coordinate position of the second reflector is (0, -23.38, 10.93); The rotation angle α3 of the fourth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -83.72°, and the vertex coordinate position of the third reflector is (0, -53.35, 50.49); The rotation angle α4 of the fifth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -10.65°, and the vertex coordinate position of the fourth reflector is (0, -72.11, -57.57); The rotation angle α5 of the sixth three-dimensional rectangular coordinate system relative to the X axis of the first three-dimensional rectangular coordinate system is -21.3°, and the center coordinate position of the detector window is (0, -106.74, 30.83).

[0013] Furthermore, the equation of the XY polynomial free-form surface is: ; ; ; in: is the surface sag; is the surface curvature; is the quadratic surface coefficient; For the polynomial Term coefficient; and is a power; m+n≤10.

[0014] Furthermore, the reflecting surfaces of the first reflecting mirror, the second reflecting mirror and the third reflecting mirror are symmetrical about the YZ plane in their respective coordinate systems, and the reflecting surface of the fourth reflecting mirror is a rotationally symmetrical surface.

[0015] Furthermore, the first reflector, the second reflector, the third reflector and the fourth reflector are all made of aluminum alloy.

[0016] Furthermore, the total focal length of the optical system is f=75 mm; the vertex curvature radii R1, R2, R3, and R4 of the first reflector, the second reflector, the third reflector, and the fourth reflector are respectively: │R1 / f│=2.67; │R2 / f│=11.22; │R3 / f│=0.65; │R4 / f│=1.48.

[0017] The beneficial effects of the present invention are: 1. On the one hand, the optical system of the present invention has a larger relative aperture, which further improves the detection sensitivity of the optical system while maintaining a compact optical path structure; on the other hand, it not only has the characteristics of a large field of view, a compact structure, and small optical system distortion, but also does not produce ghost images and cold reflections, has low spontaneous radiation and excellent cold reflection suppression characteristics.

[0018] 2. The optical system of the present invention has an exit pupil that coincides with the detector cold stop, enabling 100% cold stop efficiency. It also features a front-end entrance pupil, enabling it to be used in combination with a front-end afocal system or alone, thus achieving a modular design for an off-axis reflective optical system and improving the integration and interchangeability of the optical system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a structural diagram of an embodiment of the present invention; Figure 2 is a light path diagram of an embodiment of the present invention; Figure 3 is an optical transfer function diagram in an embodiment of the present invention; Figure 4 is a relative distortion diagram of the optical system in an embodiment of the present invention; The descriptions of the numbers in the figure are as follows: 1-reference plane, 2-first reflector, 3-second reflector, 4-third reflector, 5-fourth reflector, 6-detector window, 7-detector cold stop, 8-focal plane. DETAILED DESCRIPTION

[0020] To make the objectives, advantages, and features of the present invention more clear, the following is a further detailed description of the free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling proposed by the present invention, in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent based on the following specific embodiments.

[0021] See also Figure 1 and Figure 2 In this embodiment, a free-form surface off-axis four-mirror optical system with a pre-entrance pupil cooling system includes a first reflector 2, a second reflector 3, a third reflector 4, a fourth reflector 5, a detector window 6 and a detector cold stop 7, which are arranged in sequence from the object plane to the focal plane 8.

[0022] The first reflector 2 is set at a position close to the object surface, and can reflect and converge the incident light and reflect the light to the surface of the second reflector 3. The second reflector 3 is set on the reflection light path of the first reflector 2, and can reflect the light reflected by the first reflector 2 for a second time, and form a primary image surface between the second reflector 3 and the third reflector 4. The third reflector 4 is set on the reflection light path of the second reflector 3, and can reflect the light reflected by the second reflector 3 for a third time. The reflective surface shapes of the first reflector 2, the second reflector 3 and the third reflector 4 are all The fourth reflector 5 is arranged on the reflection light path of the third reflector 4 and is used to reflect the divergent light after being reflected by the third reflector 4. The reflective surface of the fourth reflector 5 is an even-order aspheric surface.

[0023] The first reflecting mirror 2 , the second reflecting mirror 3 and the fourth reflecting mirror 5 all have positive optical power, and the third reflecting mirror 4 has negative optical power.

[0024] The detector window 6 and the detector cold stop 7 are sequentially arranged on the reflection light path of the fourth reflector 5. The light reflected by the fourth reflector 5 passes through the detector window 6 and the detector cold stop 7 in sequence and converges onto the focal plane 8 for imaging.

[0025] The detector cold stop 7 is the aperture stop of the optical system, and is also the exit pupil of the optical system; the image of the aperture stop formed by the various reflective surfaces of the front optical system is the entrance pupil, which is located between the first reflector 2 and the object plane.

[0026] The entrance pupil position of the optical system is taken as reference plane 1, and the center of reference plane 1 is taken as the origin to define the first three-dimensional rectangular coordinate system ( x 1. y 1. z 1); define a second three-dimensional rectangular coordinate system with the vertex of the first reflector 2 as the origin ( x 2. y 2. z 2); define a third three-dimensional rectangular coordinate system with the vertex of the second reflector 3 as the origin ( x 3. y 3. z 3); define a fourth three-dimensional rectangular coordinate system with the vertex of the third reflector 4 ( x 4. y 4. z 4); define a fifth three-dimensional rectangular coordinate system with the vertex of the fourth reflector 5 ( x 5. y 5. z 5); define a sixth three-dimensional rectangular coordinate system with the center of the detector window 6 as the origin ( x 6. y 6. z 6); All three-dimensional rectangular coordinate systems are right-handed coordinate systems.

[0027] The rotation angle α1 of the second three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is 35°≤α1≤45°, and counterclockwise around the X-axis is positive and clockwise around the X-axis is negative.

[0028] The rotation angle α2 of the third three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system about the X axis is -30°≤α2≤-20°, where counterclockwise around the X axis is positive and clockwise around the X axis is negative.

[0029] The fourth three-dimensional rectangular coordinate system has an X-axis rotation angle α3 relative to the first three-dimensional rectangular coordinate system of -75°≤α3≤-90°, where counterclockwise around the X-axis is positive and clockwise around the X-axis is negative.

[0030] The rotation angle α4 of the fifth three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system along the X axis is -5°≤α4≤-15°, where counterclockwise rotation around the X axis is positive and clockwise rotation around the X axis is negative.

[0031] The rotation angle α5 of the sixth three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system along the X axis is -20°≤α5≤-25°, where counterclockwise rotation around the X axis is positive and clockwise rotation around the X axis is negative.

[0032] f is the total focal length of the optical system, and the vertex curvature radii R1, R2, R3, and R4 of each reflector should satisfy the following relationship: 2≤│R1 / f│≤3; 10≤│R2 / f│≤15; 0.5≤│R3 / f│≤0.7; 1.3≤│R4 / f│≤1.6.

[0033] The XY polynomial free-form surface equation is: ; ; .

[0034] in: is the surface sag; is the surface curvature; is the quadratic surface coefficient; For the polynomial Term coefficient; and is a power; m+n≤10.

[0035] In the embodiment of the present invention, the relative aperture of the optical system is large, and the aperture is F2. In order to correct aberrations while maintaining reasonable machinability, the number of free-form surfaces is reduced as much as possible and the order is lowered. The highest order of the XY polynomial free-form surface of the first reflector 2 is set to 5th, the highest order of the XY polynomial free-form surface of the second reflector 3 is set to 6th, the highest order of the XY polynomial free-form surface of the third reflector 4 is set to 6th, and the highest order of the even-order aspheric surface of the fourth reflector 5 is set to 8th.

[0036] In the embodiment of the present invention, the reflecting surfaces of the first reflector 2, the second reflector 3 and the third reflector 4 are symmetrical about the YZ plane in their respective coordinate systems, that is, the XY polynomial only takes even-order terms of X. The reflecting surface of the fourth reflector 5 is a rotationally symmetric surface.

[0037] The optical system parameters in the embodiment of the present invention are as follows: the operating band is 7.7 μm to 11 μm, the total focal length f of the optical system is 75 mm, the aperture is F2, the number of pixels of the cooled infrared detector is 640×512, the pixel size is 15 μm×15 μm, and the field of view angle is 7.4°×5.9°.

[0038] The X-axis rotation angle α1 of the second three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system is 39.14°, and the vertex coordinate position of the first reflector 2 is (0, 188.14, 40).

[0039] The X-axis rotation angle α2 of the third three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system is -23.77°, and the vertex coordinate position of the second reflector 3 is (0, -23.38, 10.93).

[0040] The X-axis rotation angle α3 of the fourth three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system is -83.72°, and the vertex coordinate position of the third reflector 4 is (0, -53.35, 50.49).

[0041] The X-axis rotation angle α4 of the fifth three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system is −10.65°, and the vertex coordinate position of the fourth reflector 5 is (0, −72.11, −57.57).

[0042] The X-axis rotation angle α5 of the sixth three-dimensional rectangular coordinate system relative to the first three-dimensional rectangular coordinate system is −21.3°, and the center coordinate position of the detector window 6 is (0, −106.74, 30.83).

[0043] When the total focal length f of the optical system is 75 mm, the vertex curvature radii R1, R2, R3 and R4 of each reflector are specifically: │R1 / f│=2.67; │R2 / f│=11.22; │R3 / f│=0.65; │R4 / f│=1.48.

[0044] The vertex coordinates and surface parameters of each reflector in the embodiment of the present invention are shown in Table 1. The units of the vertex coordinates in Table 1 are all millimeters (mm).

[0045] Table 1 Optical system parameters

[0046] In this embodiment, the entrance pupil of the optical system is located between the object plane and the first reflector 2, and the Z-direction spacing between the entrance pupil and the first reflector 2 is 40 mm. The optical system can be used for imaging alone or in conjunction with a front afocal system. When used in conjunction with a front afocal system, the exit pupil of the front afocal system should coincide with the entrance pupil position of the optical system of this embodiment.

[0047] Figure 3 The optical transfer function curve of this embodiment is given. It can be seen from the figure that the imaging quality of the optical system is excellent and the optical transfer function MTF of each field of view is close to the diffraction limit.

[0048] Figure 4 The relative distortion curve of the optical system of this embodiment is given. It can be seen from the figure that the relative distortion is very small in most areas of the imaging field of view, and the maximum relative distortion of the optical system of the embodiment is only 3.78%.

[0049] Each reflector and support material in this embodiment's optical system is made of aluminum alloy, processed using a single-point lathe. This provides excellent machinability, and the surface accuracy achieved meets the requirements for infrared applications. Furthermore, since both the reflector and support materials are made of aluminum alloy, the optical system exhibits excellent temperature adaptability, achieving passive athermalization and being insensitive to ambient temperature changes.

Claims

1. A free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling, characterized by: The invention comprises a first reflector (2), a second reflector (3), a third reflector (4), a fourth reflector (5), a detector window (6) and a detector cold stop (7) which are arranged in sequence from the object plane to the focal plane (8): The first reflector (2) is arranged at a position close to the object surface, and is used to reflect and converge the light incident within the field of view; the second reflector (3) is arranged on the reflection light path of the first reflector (2), and is used to reflect the light reflected by the first reflector (2) for a second time, and form a primary image plane between the second reflector (3) and the third reflector (4); the third reflector (4) is arranged on the reflection light path of the second reflector (3), and is used to reflect the light reflected by the second reflector (3) for a third time; the fourth reflector (5) is arranged on the reflection light path of the third reflector (4), and is used to reflect the light reflected by the third reflector (4) for a fourth time; the detector window (6) and the detector cold stop (7) are arranged in sequence on the reflection light path of the fourth reflector (5), and the light reflected by the fourth reflector (5) passes through the detector window (6) and the detector cold stop (7) in sequence and converges to the focal plane (8) to form an image; The reflecting surfaces of the first reflecting mirror (2), the second reflecting mirror (3) and the third reflecting mirror (4) are all XY polynomial free-form surfaces, and the reflecting surface of the fourth reflecting mirror (5) is an even-order aspheric surface; The first reflector (2), the second reflector (3) and the fourth reflector (5) all have positive optical power, and the third reflector (4) has negative optical power; The vertex curvature radii R1, R2, R3 and R4 of the first reflector (2), the second reflector (3), the third reflector (4) and the fourth reflector (5) respectively satisfy the following relationship: 2≤│R1 / f│≤3; 10≤│R2 / f│≤15; 0.5≤│R3 / f│≤0.7; 1.3≤│R4 / f│≤1.6; Where f is the total focal length of the optical system.

2. The free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling according to claim 1, characterized in that: A reference surface (1) is provided between the first reflector (2) and the object surface; A first three-dimensional rectangular coordinate system ( x 1. y 1. z 1); define a second three-dimensional rectangular coordinate system ( x 2. y 2. z 2); define a third three-dimensional rectangular coordinate system ( x 3. y 3. z 3); define a fourth three-dimensional rectangular coordinate system ( x 4. y 4. z 4); define a fifth three-dimensional rectangular coordinate system ( x 5. y 5. z 5); define a sixth three-dimensional rectangular coordinate system ( x 6. y 6. z 6); The first three-dimensional rectangular coordinate system to the sixth three-dimensional rectangular coordinate system are all right-handed coordinate systems; The reference surface (1) coincides with the entrance pupil position, and the Z-direction spacing between the entrance pupil and the first reflector (2) is 35 mm to 45 mm; The rotation angle α1 of the second three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is 35°≤α1≤45°, where counterclockwise around the X-axis is defined as positive and clockwise as negative; The vertex coordinate position of the first reflector (2) is (0, 185-195, 35-45); The rotation angle α2 of the third three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -30°≤α2≤-20°; The vertex coordinate position of the second reflector (3) is (0, -25 to -15, 5 to 15); The rotation angle α3 of the fourth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -75°≤α3≤-90°; The vertex coordinate position of the third reflector (4) is (0, -55 to -45, 45 to 55); The rotation angle α4 of the fifth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -5°≤α4≤-15°; The vertex coordinate position of the fourth reflector (5) is (0, -75 to -65, -60 to -50); The rotation angle α5 of the sixth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -20°≤α5≤-25°; The center coordinate position of the detector window (6) is (0, -110 to -100, 25 to 35).

3. The free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling according to claim 2, characterized in that: The Z-direction distance between the entrance pupil and the first reflector (2) is 40 mm; The rotation angle α1 of the second three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is 39.14°, and the vertex coordinate position of the first reflector (2) is (0, 188.14, 40); The rotation angle α2 of the third three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -23.77°, and the vertex coordinate position of the second reflector (3) is (0, -23.38, 10.93); The rotation angle α3 of the fourth three-dimensional rectangular coordinate system relative to the X-axis of the first three-dimensional rectangular coordinate system is -83.72°, and the vertex coordinate position of the third reflector (4) is (0, -53.35, 50.49); The rotation angle α4 of the fifth three-dimensional rectangular coordinate system relative to the X axis of the first three-dimensional rectangular coordinate system is -10.65°, and the vertex coordinate position of the fourth reflector (5) is (0, -72.11, -57.57); The rotation angle α5 of the sixth three-dimensional rectangular coordinate system relative to the X axis of the first three-dimensional rectangular coordinate system is -21.3°, and the center coordinate position of the detector window (6) is (0, -106.74, 30.83).

4. The free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling according to any one of claims 1 to 3, characterized in that: The equation of the XY polynomial free-form surface is: ； ； ； in: is the surface sag; is the surface curvature; is the quadratic surface coefficient; For the polynomial Term coefficient; and is a power; m+n≤10.

5. The free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling according to claim 4, characterized in that: The reflection surfaces of the first reflection mirror (2), the second reflection mirror (3) and the third reflection mirror (4) are symmetrical about the YZ plane in their respective coordinate systems, and the reflection surface of the fourth reflection mirror (5) is a rotationally symmetrical surface.

6. The free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling according to claim 5, characterized in that: The first reflector (2), the second reflector (3), the third reflector (4) and the fourth reflector (5) are all made of aluminum alloy material.

7. The free-form surface off-axis four-mirror optical system with pre-entrance pupil cooling according to claim 1, characterized in that: The total focal length of the optical system is f=75mm; The vertex curvature radii R1, R2, R3 and R4 of the first reflector (2), the second reflector (3), the third reflector (4) and the fourth reflector (5) are respectively: │R1 / f│=2.67; │R2 / f│=11.22; │R3 / f│=0.65; │R4 / f│=1.48。

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