A pre-pupil refrigeration type free-form off-axis four-mirror optical system

Abstract

The application belongs to the field of optical systems, and particularly relates to a pre-in-pupil refrigeration type free-form off-axis four-mirror optical system. The optical system comprises, from an object plane to a focal plane, a first mirror, a second mirror, a third mirror, a fourth mirror, a detector window and a detector cold shield which are sequentially arranged. The first mirror is arranged near the object plane. The second mirror is arranged on the reflected light path of the first mirror. The third mirror is arranged on the reflected light path of the second mirror. The fourth mirror is arranged on the reflected light path of the third mirror. The detector window and the detector cold shield are sequentially arranged on the reflected light path of the fourth mirror. The application can realize modular design of the reflective off-axis optical system, improve the integration and interchangeability of the photoelectric system, and has a larger relative aperture, thereby further improving the detection sensitivity of the optical system while keeping the optical path structure compact.

Description

TECHNICAL FIELD

[0001] The present application relates to a free-form surface off-axis optical system, in particular to a front entrance pupil design of a free-form surface off-axis four-mirror optical system. BACKGROUND

[0002] Off-axis reflective optical systems have been widely used due to their advantages of no chromatic aberration, good thermal resistance, simple structure, low self thermal radiation, etc., and have inherent advantages in high-performance infrared detection and imaging. The traditional off-axis reflective optical system with even aspheric surfaces has low design freedom, and it is difficult to meet the high requirements of photoelectric systems for large field of view, high resolution, high performance, miniaturization, etc. Free-form surface optical elements greatly increase the optimization variables of the optical system, which can achieve a larger imaging field of view and more excellent imaging performance, and have become a research hotspot of off-axis reflective optical systems.

[0003] Optical systems with front entrance pupil can independently complete imaging functions, and can also be combined with front-end afocal systems to achieve focal length or field of view expansion, which is of great significance for the modular design of photoelectric imaging systems. In addition, in high-performance infrared imaging and detection tasks, in order to improve the system detection sensitivity, a cooled infrared detector is usually used. The detector cold shield needs to be matched with the exit pupil of the optical system to achieve 100% cold shield efficiency. However, the modular design requires the optical system not only to have a front entrance pupil, i.e., the entrance pupil is matched with the front-end afocal system, but also the exit pupil is matched with the cold shield of the cooled detector, which poses great difficulties for optical design.

[0004] Currently, the disclosed free-form surface off-axis reflective optical systems are mostly used in non-cooled infrared imaging fields with low detection sensitivity. Such optical systems cannot match the cooled infrared detector, do not consider the modular design, and do not have a front entrance pupil. There are relatively few related documents on cooled off-axis reflective optical systems. The free-form surface off-axis reflective optical systems in the documents mostly only consider the matching of the exit pupil of the optical system with the cold shield of the detector, but do not consider the modular design requirements, and do not have a front entrance pupil. The related documents on cooled off-axis reflective optical systems with front entrance pupil are even more scarce. The optical systems in individual disclosed documents also have problems such as relatively small relative aperture, and the aperture is mostly F4, which makes the focal length of the optical system larger under the same detection capability, and the structure is not compact enough.

[0005] The article "Design of Free-form Surface Non-cooled Infrared Off-axis Three-mirror Optical System" published in the Journal of Infrared and Laser Engineering, Vol. 54, No. 2, February 2025, and Chinese patents with publication numbers CN103246053A and CN107290845A disclose various free-form surface off-axis reflective optical systems, but they cannot match the cooled infrared detector and do not have an external entrance pupil.

[0006] "Application of Freeform Surface in Cooling Type Off-axis Three-mirror Optical System", published in the journal of Infrared and Laser Engineering, Vol. 47, No. 9, September 2018; "Design of Cooling Type Freeform Surface Off-axis Reflective Optical System", published in the journal of Acta Optica Sinica, Vol. 39, No. 11, November 2019; "Freeform Surface Off-axis Four-mirror All-aluminum Optical System for Infrared Detection", published in the journal of Infrared and Laser Engineering, Vol. 52, No. 7, July 2023; and various cooling type freeform surface off-axis reflective optical systems disclosed in Chinese patents CN112213847B and CN112180576B, which can match cooling type infrared detectors but do not have an external entrance pupil. "Design Method of Three-conjugate Stray Light Elimination Optical System", published in the journal of Infrared and Millimeter Wave, Vol. 38, No. 1, February 2019, discloses an off-axis three-mirror stray light elimination optical system that satisfies the three-conjugate relationship, can match cooling type infrared detectors, but cannot realize the front entrance pupil, the cold shield efficiency cannot reach 100%, and the field of view and relative aperture are small, with an aperture of F4.

[0007] Chinese patent CN117492186A discloses an entrance pupil front infrared continuous zoom system, which can match cooling type infrared detectors and has a front entrance pupil, but it is a coaxial transmission type optical system, which has large self-thermal radiation and cannot meet the demand of high performance limit detection in infrared background. Chinese patent CN116880049A discloses a cooling type freeform surface off-axis three-mirror system with a front entrance pupil, which can also match cooling type infrared detectors, but its relative aperture is small, with an aperture of F4, and under the same detection capability, the smaller the relative aperture, the longer the optical focal length and the less compact the structure.

[0008] In summary, designing a cooling type infrared freeform surface off-axis reflective optical system with a large relative aperture, low distortion, large field of view, compact structure, and front entrance pupil is of great significance for realizing high sensitivity, high performance limit detection, and modular design of the optical system. SUMMARY

[0009] The purpose of the present application is to solve the technical problems that the existing freeform surface off-axis reflective optical system cannot realize the front entrance pupil, cannot match the cooling type infrared detector, or has a small relative aperture, large distortion, and insufficient compactness, and to provide an entrance pupil front cooling type freeform surface off-axis four-mirror optical system.

[0010] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0011] An entrance pupil front cooling type freeform surface off-axis four-mirror optical system, characterized in that:

[0012] The first mirror, the second mirror, the third mirror, the fourth mirror, the detector window and the detector cold shield are sequentially arranged from the object plane to the focal plane.

[0013] The first mirror is arranged near the object plane and is used for reflecting and converging the light rays incident in the field of view; the second mirror is arranged on the reflected light path of the first mirror and is used for twice reflecting the light rays reflected by the first mirror and forming an image plane between the second mirror and the third mirror; the third mirror is arranged on the reflected light path of the second mirror and is used for thrice reflecting the light rays reflected by the second mirror; the fourth mirror is arranged on the reflected light path of the third mirror and is used for reflecting the light rays reflected by the third mirror; the detector window and the detector cold shield are sequentially arranged on the reflected light path of the fourth mirror, and the light rays reflected by the fourth mirror sequentially pass through the detector window and the detector cold shield and are converged on the focal plane to form an image.

[0014] The reflecting surfaces of the first mirror, the second mirror and the third mirror are all XY polynomial free-form surfaces, and the reflecting surface of the fourth mirror is an even aspheric surface.

[0015] The first mirror, the second mirror and the fourth mirror all have positive focal powers, and the third mirror has a negative focal power.

[0016] The vertex curvature radii R1, R2, R3 and R4 of the first mirror, the second mirror, the third mirror and the fourth mirror respectively satisfy the following relationships:

[0017] 2≤│R1 / f│≤3;

[0018] 10≤│R2 / f│≤15;

[0019] 0.5≤│R3 / f│≤0.7;

[0020] 1.3≤│R4 / f│≤1.6;

[0021] Wherein, f is the total focal length of the optical system.

[0022] Further, a reference surface is arranged between the first mirror and the object plane, and a first three-dimensional rectangular coordinate system (X1, Y1, Z1) is defined with the center of the reference surface as the origin. x 1、 y 1、 z 1); a second three-dimensional rectangular coordinate system (X2, Y2, Z2) is defined with the vertex of the first mirror as the origin; x 2、 y 2、 z 2); and a third three-dimensional rectangular coordinate system (X3, Y3, Z3) is defined with the vertex of the second mirror as the origin. x 3、 y 3、 z3) ; a fourth three-dimensional rectangular coordinate system is defined with the vertex of the fourth mirror as the origin (X4, Y4, Z4) x 4、 y 4、 z 4) ; a fifth three-dimensional rectangular coordinate system is defined with the vertex of the fourth mirror as the origin (X5, Y5, Z5) x 5、 y 5、 z 5) ; a sixth three-dimensional rectangular coordinate system is defined with the center of the detector window as the origin (X6, Y6, Z6) 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;

[0023] The reference surface coincides with the entrance pupil position, and the distance between the entrance pupil and the Z direction of the first mirror is 35mm-45mm;

[0024] The 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 the clockwise direction is positive and the counterclockwise direction is negative;

[0025] The vertex coordinate position of the first mirror is (0, 185-195, 35-45);

[0026] The 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°, and the clockwise direction is positive and the counterclockwise direction is negative;

[0027] The vertex coordinate position of the second mirror is (0, -25--15, 5-15);

[0028] The 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°, and the clockwise direction is positive and the counterclockwise direction is negative;

[0029] The vertex coordinate position of the third mirror is (0, -55--45, 45-55);

[0030] The 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°, and the clockwise direction is positive and the counterclockwise direction is negative;

[0031] The vertex coordinate position of the fourth mirror is (0, -75--65, -60--50);

[0032] The 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°, and the clockwise direction is positive and the counterclockwise direction is negative;

[0033] The center coordinate position of the detector window is (0, -110--100, 25-35).

[0034] 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);

[0035] 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);

[0036] 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);

[0037] 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);

[0038] 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).

[0039] Furthermore, the equation of the XY polynomial free-form surface is:

[0040] ;

[0041] ;

[0042] ;

[0043] 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.

[0044] 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.

[0045] Furthermore, the first reflector, the second reflector, the third reflector and the fourth reflector are all made of aluminum alloy.

[0046] Further, the total focal length of the optical system f=75mm; the vertex curvature radius R1, R2, R3 and R4 of the first mirror, the second mirror, the third mirror and the fourth mirror are respectively:

[0047] |R1 / f|=2.67;

[0048] |R2 / f|=11.22;

[0049] |R3 / f|=0.65;

[0050] |R4 / f|=1.48.

[0051] The beneficial effects of the present application are:

[0052] 1. The optical system of the present application has a larger relative aperture on one hand, and improves the detection sensitivity of the optical system while maintaining a compact optical path structure; on the other hand, it has a larger field of view, a compact structure, a small optical system distortion, and does not produce ghost images and cold reflections, has low spontaneous radiation and excellent cold reflection suppression characteristics.

[0053] 2. The exit pupil of the optical system of the present application coincides with the detector cold shield, which can achieve 100% cold shield efficiency; and has a front entrance pupil, which can be combined with a front end afocal system or used alone, realizing the modular design of off-axis reflection type optical system, improving the integration and interchangeability of the optical system. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 is a structural schematic diagram of an embodiment of the present application;

[0055] Figure 2 is an optical path diagram of an embodiment of the present application;

[0056] Figure 3 is an optical transfer function diagram in an embodiment of the present application;

[0057] Figure 4 is a relative distortion diagram of the optical system in an embodiment of the present application;

[0058] The explanation of each number in the figure is as follows:

[0059] 1-reference surface, 2-first mirror, 3-second mirror, 4-third mirror, 5-fourth mirror, 6-detector window, 7-detector cold shield, 8-focal plane. DETAILED DESCRIPTION

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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) the fourth three-dimensional rectangular coordinate system is defined with the vertex of the fourth mirror 5 as the origin (X5, Y5, Z5) ; x 4、 y 4、 z 4) the fifth three-dimensional rectangular coordinate system is defined with the vertex of the fifth mirror 6 as the origin (X6, Y6, Z6) ; x 5、 y 5、 z 5) the sixth three-dimensional rectangular coordinate system is defined with the center of the detector window 7 as the origin (X7, Y7, Z7) ; x 6、 y 6、 z 6) all the three-dimensional rectangular coordinate systems are right-handed coordinate systems.

[0067] The second three-dimensional rectangular coordinate system has an angle a1 of 35°≤a1≤45° with the X axis of the first three-dimensional rectangular coordinate system, and is positive counterclockwise and negative clockwise.

[0068] The third three-dimensional rectangular coordinate system has an angle a2 of -30°≤a2≤-20° with the X axis of the first three-dimensional rectangular coordinate system, and is positive counterclockwise and negative clockwise.

[0069] The fourth three-dimensional rectangular coordinate system has an angle a3 of -75°≤a3≤-90° with the X axis of the first three-dimensional rectangular coordinate system, and is positive counterclockwise and negative clockwise.

[0070] The fifth three-dimensional rectangular coordinate system has an angle a4 of -5°≤a4≤-15° with the X axis of the first three-dimensional rectangular coordinate system, and is positive counterclockwise and negative clockwise.

[0071] The sixth three-dimensional rectangular coordinate system has an angle a5 of -20°≤a5≤-25° with the X axis of the first three-dimensional rectangular coordinate system, and is positive counterclockwise and negative clockwise.

[0072] f is the total focal length of the optical system, and the vertex radii R1, R2, R3 and R4 of the mirrors should satisfy the following relationships:

[0073] 2≤│R1 / f│≤3;

[0074] 10≤│R2 / f│≤15;

[0075] 0.5≤│R3 / f│≤0.7;

[0076] 1.3≤│R4 / f│≤1.6.

[0077] The XY polynomial free-form surface equation is:

[0078] ;

[0079] ;

[0080] .

[0081] wherein: is the sag of the curved surface; is the curvature of the curved surface; is the quadratic surface coefficient; is the coefficient of the mth term of the polynomial; is the coefficient of the nth term of the polynomial; and are the powers; m+n≤10.

[0082] In the embodiment of the present application, the relative aperture of the optical system is large, and the aperture is F2. In order to correct the aberration while maintaining reasonable processability, the number of free curved surfaces is reduced as much as possible, and the order is reduced. The highest order of the XY polynomial free curved surface of the first mirror 2 is 5, the highest order of the XY polynomial free curved surface of the second mirror 3 is 6, the highest order of the XY polynomial free curved surface of the third mirror 4 is 6, and the highest order of the even aspherical surface of the fourth mirror 5 is 8.

[0083] In the embodiment of the present application, the reflecting surfaces of the first mirror 2, the second mirror 3 and the third mirror 4 are respectively symmetrical about the YZ plane in the respective coordinate systems, that is, the XY polynomial only takes even terms of X. The reflecting surface of the fourth mirror 5 is a rotationally symmetrical surface.

[0084] The optical system parameters in the embodiment of the present application are as follows: the working waveband is 7.7 μm~11 μm, the total focal length f of the optical system is 75 mm, the aperture is F2, the number of pixels of the refrigeration type infrared detector is 640×512, the pixel size is 15 μm×15 μm, and the field of view angle is 7.4°×5.9°.

[0085] The rotation angle 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 mirror 2 is (0, 188.14, 40).

[0086] The rotation angle 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 mirror 3 is (0, -23.38, 10.93).

[0087] The rotation angle 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 mirror 4 is (0, -53.35, 50.49).

[0088] The rotation angle 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 mirror 5 is (0, -72.11, -57.57).

[0089] The sixth three-dimensional rectangular coordinate system has a rotation angle of -21.3° relative to the X axis of the first three-dimensional rectangular coordinate system, and the center coordinate of the detector window 6 is (0, -106.74, 30.83).

[0090] When the total focal length f of the optical system is 75 mm, the vertex radii R1, R2, R3 and R4 of the mirrors are specifically as follows:

[0091] │R1 / f│=2.67;

[0092] │R2 / f│=11.22;

[0093] │R3 / f│=0.65;

[0094] │R4 / f│=1.48.

[0095] The vertex coordinates and surface parameters of the mirrors in the embodiment of the application are shown in Table 1. The units of the vertex coordinates in Table 1 are millimeters (mm).

[0096] Table 1: Optical system parameters

[0097]

[0098] In the embodiment, the entrance pupil of the optical system is located between the object plane and the first mirror 2, and the Z-direction distance between the entrance pupil and the first mirror 2 is 40 mm. The optical system can be used for single imaging, or can be used in cooperation with a front no-focus system. When the optical system is used in cooperation with the front no-focus system, the exit pupil of the front no-focus system should coincide with the position of the entrance pupil of the optical system.

[0099] Figure 3 The optical transfer function curve of the embodiment is given. As can be seen from the figure, 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.

[0100] Figure 4 The relative distortion curve of the optical system of the embodiment is given. As can be seen from the figure, in most areas of the imaging field of view, the relative distortion is very small, and the maximum relative distortion of the optical system of the embodiment is only 3.78%.

[0101] The materials of the mirrors and the supports of the optical system of the embodiment can all be aluminum alloy materials, and the machining is performed by using a single-point turning machine tool. The machining has good machinability, and the machining surface precision can all meet the application requirements in the infrared waveband. In addition, since the materials of the mirrors and the supports are all aluminum alloy, the optical system has good temperature adaptability, can realize passive athermalization, and is not sensitive to the change of the ambient temperature.

Claims

1. An entry pupil front-mounted refrigeration type free-form off-axis four-mirror optical system, characterized in that: comprising a first mirror (2), a second mirror (3), a third mirror (4), a fourth mirror (5), a detector window (6) and a detector cold shield (7) arranged in order from the object plane to the focal plane (8): the first mirror (2) is arranged near the object plane, used for reflecting and converging the light incident in the field of view; the second mirror (3) is arranged on the reflected light path of the first mirror (2), used for reflecting the light reflected by the first mirror (2) for the second time, and forming an intermediate image plane between the second mirror (3) and the third mirror (4); the third mirror (4) is arranged on the reflected light path of the second mirror (3), used for reflecting the light reflected by the second mirror (3) for the third time; the fourth mirror (5) is arranged on the reflected light path of the third mirror (4), used for reflecting the light reflected by the third mirror (4) for the fourth time; the detector window (6) and the detector cold shield (7) are arranged in order on the reflected light path of the fourth mirror (5), and the light reflected by the fourth mirror (5) passes through the detector window (6) and the detector cold shield (7) in order, and converges on the focal plane (8) to form an image; the reflecting surface of the first mirror (2), the second mirror (3) and the third mirror (4) are all XY polynomial free-form surfaces, and the reflecting surface of the fourth mirror (5) is an even aspheric surface; the first mirror (2), the second mirror (3) and the fourth mirror (5) all have positive focal power, and the third mirror (4) has negative focal power; the vertex curvature radii R1, R2, R3 and R4 of the first mirror (2), the second mirror (3), the third mirror (4) and the fourth mirror (5) 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; wherein f is the total focal length of the optical system.

2. The entry pupil front-mounted refrigeration type free-form off-axis four-mirror optical system according to claim 1, characterized in that: a reference plane (1) is arranged between the first mirror (2) and the object plane; the reference plane (1) coincides with the entry pupil position, and the distance between the entry pupil and the first mirror (2) in the Z direction is 35mm-45mm; the angle a1 of the second three-dimensional rectangular coordinate system relative to the X axis of the first three-dimensional rectangular coordinate system is 35°≤a1≤45°, and the counterclockwise rotation is defined as positive and the clockwise rotation is defined as negative; the vertex coordinate position of the first mirror (2) is (0, 185-195, 35-45); the angle a2 of the third three-dimensional rectangular coordinate system relative to the X axis of the first three-dimensional rectangular coordinate system is -30°≤a2≤-20°; the vertex coordinate position of the second mirror (3) is (0, -25--15, 5-15); the angle a3 of the fourth three-dimensional rectangular coordinate system relative to the X axis of the first three-dimensional rectangular coordinate system is -75°≤a3≤-90°. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ A first three-dimensional rectangular coordinate system (X1, Y1, Z1) is defined with the center of the reference surface (1) as the origin x 1、 y 1、 z 1) A second three-dimensional rectangular coordinate system (X2, Y2, Z2) is defined with the vertex of the first mirror (2) as the origin x 2、 y 2、 z 2) A third three-dimensional rectangular coordinate system (X3, Y3, Z3) is defined with the vertex of the second mirror (3) as the origin x 3、 y 3、 z 3) A fourth three-dimensional rectangular coordinate system (X4, Y4, Z4) is defined with the vertex of the third mirror (4) as the origin x 4、 y 4、 z 4) A fifth three-dimensional rectangular coordinate system (X5, Y5, Z5) is defined with the vertex of the fourth mirror (5) as the origin x 5、 y 5、 z 5) A sixth three-dimensional rectangular coordinate system (X6, Y6, Z6) is defined with the center of the detector window (6) 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 vertex coordinate position of the third mirror (4) is (0, -55~ -45, 45~55); 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 mirror (5) is (0, -75~ -65, -60~ -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~ -100, 25~35).

3. The pre-pupil refrigeration type free-form off-axis four-mirror optical system according to claim 2, characterized in that: The Z-direction distance between the pre-pupil and the first mirror (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 mirror (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 mirror (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 mirror (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 mirror (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 pre-pupil refrigeration type free-form off-axis four-mirror optical system according to any one of claims 1-3, characterized in that: The equation of the XY polynomial free-form surface is: ； ； ； wherein: is the camber height of the curve; is the curvature of the curve; is the quadratic surface coefficient; is the polynomial coefficient of the mth term; and is the power; m+n < 10.

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

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

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

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