A refrigeration type reflective long-focus lens suitable for different object distances and a focusing method

Abstract

The present application relates to the technical field of optical lens, in particular to a refrigeration type reflective long-focus lens suitable for different object distances and a focusing method, comprising: off-axis beam-reducing primary mirror, off-axis beam-reducing secondary mirror, off-axis three-mirror primary mirror, off-axis three-mirror secondary mirror, off-axis three-mirror tertiary mirror and diaphragm arranged in sequence from object plane to image plane along the direction of light incidence; the off-axis beam-reducing primary mirror and the off-axis beam-reducing secondary mirror are both concave mirrors with positive focal power, and the focal points of the two mirrors coincide, forming an afocal beam-reducing system; the position of the off-axis beam-reducing secondary mirror can be moved to achieve focusing for different object distances; the off-axis three-mirror primary mirror is a concave mirror with positive focal power, the off-axis three-mirror secondary mirror is a convex mirror with negative focal power, and the off-axis three-mirror tertiary mirror is a concave mirror with positive focal power, which together form a refrigeration type off-axis three-mirror lens for eliminating induced aberration caused by off-axis field aberration of the front beam-reducing system.

Description

Technical Field

[0001] This invention relates to the field of optical lens technology, specifically to a cooled reflective telephoto lens suitable for different object distances and a focusing method thereof. Background Technology

[0002] In long-range target detection and high-resolution imaging applications, optical systems often need to possess both long focal length and large aperture characteristics. Long focal length ensures magnification for distant targets, while a large aperture ensures sufficient energy harvesting capability; together, they determine the system's angular resolution and detection sensitivity. However, traditional long focal length refractive lenses face two inherent challenges in design: first, on-axis chromatic aberration, especially second-order spectral aberration, is difficult to effectively correct using conventional material combinations, limiting image quality; second, the system's axial dimensions and volume increase dramatically with focal length, severely constraining platform mounting and engineering implementation.

[0003] To overcome the aforementioned bottlenecks, total internal reflection optical structures are often employed in engineering. In a reflective system, the primary mirror bears the majority of the optical power. Since the reflection process does not introduce dispersion, it is naturally free of chromatic aberration, offering significant advantages in ultra-telephoto lens designs. Furthermore, by folding the optical path, the overall system length can be greatly shortened, facilitating a compact layout. Therefore, total internal reflection configurations have become the preferred solution for ultra-telephoto lenses.

[0004] For cooled detectors operating in the mid-infrared band, to ensure optimal detection sensitivity, the optical system must achieve 100% cold stop efficiency. This means the aperture stop of the optical system must precisely coincide with the cold stop of the cooling mechanism. This results in cooled reflective telephoto lenses typically only producing clear images of objects at infinity or a specific distance. To address this, existing technologies typically introduce a transmission-type compensation lens group into the reflective optical path to adjust the optical power distribution, achieving clear focus on targets at different distances and ensuring the aperture stop and cold stop are aligned. However, the transmission material exhibits inherent chromatic aberration in the mid-infrared band, which undermines the original reflective system's advantage of a wide-spectrum, aberration-free design, narrowing the operating wavelength range. Furthermore, adding a transmission group changes the system from total internal reflection to a hybrid catadioptric-reflective structure, rendering its spectral adaptability unsuitable for ultra-wideband applications.

[0005] In conclusion, how to maintain the efficiency of the cold aperture of a cooled reflective telephoto lens while simultaneously achieving wide-spectrum imaging capabilities and clear focusing at multiple distances has become a pressing technical challenge in the design of this type of telephoto lens. Summary of the Invention

[0006] 1. The problem to be solved To address the problem that existing cooled reflective telephoto lenses cannot simultaneously achieve both wide-spectrum imaging capabilities and clear focusing at multiple distances, this invention provides a cooled reflective telephoto lens and focusing method suitable for different object distances, which possesses both wide-spectrum imaging capabilities and the ability to achieve clear focusing at multiple distances.

[0007] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: The first aspect of this invention provides a cooled reflective telephoto lens suitable for different object distances, comprising: An off-axis primary mirror, an off-axis secondary mirror, an off-axis primary mirror, an off-axis secondary mirror, an off-axis secondary mirror, and an aperture stop are arranged sequentially from the object plane to the image plane along the incident direction of light. Both the off-axis primary beam-shrinking mirror and the off-axis secondary beam-shrinking mirror are concave mirrors with positive optical power, and the focal points of the two mirrors coincide, forming an afocal beam-shrinking system; the position of the off-axis secondary beam-shrinking mirror can be moved to achieve focusing for different object distances; The off-axis three-mirror primary mirror is a concave mirror with positive optical power, the off-axis three-mirror secondary mirror is a convex mirror with negative optical power, and the off-axis three-mirror is a concave mirror with positive optical power. Together, they form a cooled off-axis three-mirror lens, which is used to eliminate induced aberrations caused by off-axis field-of-view aberrations of the front-mounted beam-shrinking system.

[0008] As a preferred embodiment of the present invention, the beam-shrinking ratio of the off-axis beam-shrinking system is M=f1 / f2=4, where f1 is the focal length of the off-axis beam-shrinking primary mirror and f2 is the focal length of the off-axis beam-shrinking secondary mirror.

[0009] As a preferred embodiment of the present invention, the focal length of the off-axis beam-shrinking primary mirror is 400mm and the air gap thickness is -500mm. The off-axis beam-shrinking secondary mirror has a focal length of 100mm and an air gap thickness of 300mm.

[0010] As a preferred embodiment of the present invention, the total focal length of the cooled reflex telephoto lens is f = M × f3, where f3 is the overall focal length of the cooled off-axis three-lens reflex lens.

[0011] As a preferred embodiment of the present invention, the total focal length of the cooled reflex telephoto lens is f=700mm, and the overall focal length of the cooled off-axis three-lens lens is f3=175mm.

[0012] As a preferred embodiment of the present invention, the focal length of the off-axis three-mirror primary mirror is 70mm and the air gap thickness is -55mm; The off-axis three-mirror has a focal length of -29mm and an air gap thickness of 184mm. The focal length of the off-axis three-lens mirror is 82.5mm, and the air gap thickness is -124mm.

[0013] As a preferred embodiment of the present invention, the operating F-number of the cooled reflective telephoto lens is 4, and the image plane diagonal of the cooled reflective telephoto lens is 20mm.

[0014] As a preferred embodiment of the present invention, the object distance focusing range of the cooled reflex telephoto lens is 100m to ∞.

[0015] As a preferred embodiment of the present invention, the operating wavelength of the cooled reflective telephoto lens is 400~9500nm.

[0016] A second aspect of the present invention provides a focusing method using the above-described cooled reflective telephoto lens suitable for different object distances; When the object distance decreases, the off-axis beam-shrinking secondary mirror moves in the opposite direction along the optical axis and downward in the direction perpendicular to the optical axis; As the object distance increases, the off-axis beam-shrinking secondary mirror moves in the positive direction along the optical axis and upward in the direction perpendicular to the optical axis.

[0017] As a preferred embodiment of the present invention, the displacement of the off-axis beam-shrinking secondary mirror along the optical axis is defined as... x, the displacement in the direction perpendicular to the optical axis is Δy; When the object distance changes, the amount of movement Δy of the off-axis beam-shrinking secondary mirror in the direction perpendicular to the optical axis is the amount of movement along the optical axis. 0.59-0.62 times x; Total movement of the off-axis beam-shrinking secondary mirror ; Total movement The distance L between the object and the object satisfies the following relationship: .

[0018] 3. Beneficial effects Compared with the prior art, the beneficial effects of the present invention are as follows: This invention comprises, along the incident light direction from the object plane to the image plane, an off-axis beam-shrinking primary mirror, an off-axis beam-shrinking secondary mirror, an off-axis three-mirror primary mirror, an off-axis three-mirror secondary mirror, an off-axis three-mirror system, and an aperture stop. The off-axis beam-shrinking primary mirror and the off-axis beam-shrinking secondary mirror are both concave mirrors with positive optical power, and their focal points coincide, forming a focusless beam-shrinking system. The position of the off-axis beam-shrinking secondary mirror can be moved; by adjusting its position, a cooled reflective telephoto lens can achieve clear imaging of objects at different object distances. The off-axis three-mirror primary mirror is a concave mirror with positive optical power, the off-axis three-mirror secondary mirror is a convex mirror with negative optical power, and the off-axis three-mirror system is a concave mirror with positive optical power. Together, they form a cooled off-axis three-mirror lens used to eliminate induced aberrations caused by off-axis field-of-view aberrations in the front beam-shrinking system. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the optical path of the cooled reflective telephoto lens in this invention; Figure 2 This is a schematic diagram of the off-axis beam-shrinking secondary mirror movement curve after fitting when the object distance decreases in this invention. Figure 3 This is a schematic diagram of the fitting curve between the total movement and the object distance in this invention; Figure 4 This is a schematic diagram of the shortwave imaging quality at an object distance of 100m in this invention; Figure 5 This is a schematic diagram of the shortwave imaging quality at an object distance of 1000m in this invention; Figure 6 This is a schematic diagram of the shortwave imaging quality at an object distance of 10000m in this invention; Figure 7 This refers to the imaging quality in the 400-900nm band when the object distance is infinitely far in this invention; Figure 8 This refers to the imaging quality in the 900-1700nm band when the object distance is infinitely far in this invention; Figure 9 This refers to the imaging quality in the 1700~2600nm band when the object distance is infinite in this invention; Figure 10 This refers to the imaging quality in the 3000~5000nm band when the object distance is infinitely far in this invention; Figure 11 This refers to the imaging quality in the 7700~9500nm band when the object distance is infinitely far in this invention.

[0020] Explanation of the labels in the diagram: 1. Off-axis primary mirror; 2. Off-axis secondary mirror; 3. Off-axis primary mirror; 4. Off-axis secondary mirror; 5. Off-axis secondary mirror; 6. Aperture stop. Detailed Implementation

[0021] To further understand the content of this invention, a detailed description of the invention will be provided in conjunction with the accompanying drawings and embodiments.

[0022] Example 1 like Figure 1 As shown, this embodiment provides a cooled reflex telephoto lens suitable for different object distances, including an off-axis beamforming system, a cooled off-axis three-lens reflex lens, and an aperture 6 arranged sequentially from the object plane to the image plane along the light incident direction.

[0023] Off-axis beam shrinking systems preprocess the light beam without altering its divergence or convergence, precisely compressing the beam diameter. For example, a four-fold off-axis beam shrinking system can precisely compress the beam diameter to one-quarter of its original size. An off-axis beam shrinking system includes an off-axis primary mirror 1 and an off-axis secondary mirror 2, arranged sequentially from the object plane to the image plane along the incident direction of the light. Both the primary mirror 1 and the secondary mirror 2 are concave mirrors with positive optical power, and their focal points coincide, forming a focusless beam shrinking system.

[0024] The position of the off-axis beam-shrinking secondary mirror 2 can be moved. By adjusting the position of the off-axis beam-shrinking secondary mirror 2, the cooled reflective telephoto lens can achieve clear imaging of objects at different object distances, such as... Figures 4-6 As shown, at different object distances, the lens modulation at a spatial frequency of 34 lp / mm is greater than 0.4, which is greater than the general requirement of 0.2 that the human eye can distinguish. Meanwhile, the pixel size of the cooled detectors is relatively large (mostly 15 micrometers). The spatial resolution of the lens fully meets the spatial resolution requirements of the sensor. Therefore, this cooled reflective telephoto lens produces good imaging at different object distances, meeting the system's requirements. The moving mechanism for driving the off-axis beam-shrinking secondary mirror 2 can be a piezoelectric ceramic mechanism or a servo motor with a lead screw drive, or other moving mechanisms that can move the off-axis beam-shrinking secondary mirror 2 to achieve focusing.

[0025] The specific principle of adjusting the off-axis beam-shrinking secondary mirror 2 is as follows: In the initial defocal state (i.e., when the object distance is infinite), the image-side focal point of the off-axis primary mirror 1 and the object-side focal point of the off-axis secondary mirror 2 coincide (confocal), with a distance d = f1 + f2, where f1 is the focal length of the off-axis primary mirror 1 and f2 is the focal length of the off-axis secondary mirror 2. The system emits parallel light to an object at infinity.

[0026] When an object moves to a finite distance, the incident light is no longer parallel but becomes a beam with some divergence (or convergence). At this point, the real image formed by the off-axis primary mirror 1 no longer falls at the original focal point but moves back and forth along the optical axis. This image, as the object of the off-axis secondary mirror 2, has changed position. If the off-axis secondary mirror 2 remains stationary, its imaging of this new object distance will inevitably result in an image point that deviates from the originally designed detector plane and cannot be focused. After the off-axis secondary mirror 2 moves along the optical axis and perpendicular to the optical axis, the distance from the image point of the off-axis primary mirror 1 to the secondary mirror changes accordingly. This is equivalent to adjusting the "object distance" of the off-axis secondary mirror 2. By selecting an appropriate amount of movement, the image distance of the off-axis secondary mirror 2 can be made exactly equal to the distance from the detector to the off-axis secondary mirror 2, thereby obtaining a clear image point on the detector.

[0027] A cooled off-axis three-mirror lens is used to eliminate induced aberrations caused by off-axis field-of-view aberrations in a front-mounted beam-shrinking system. It comprises an off-axis primary mirror 3, an off-axis secondary mirror 4, and an off-axis three-mirror 5, arranged sequentially from the object plane to the image plane along the direction of light incidence. The primary mirror 3 is a concave mirror with positive optical power, the secondary mirror 4 is a convex mirror with negative optical power, and the three-mirror 5 is a concave mirror with positive optical power.

[0028] In one embodiment, the beam-shrinking ratio of the off-axis beam-shrinking system is M = f1 / f2 = 4, where f1 is the focal length of the off-axis beam-shrinking primary mirror 1 and f2 is the focal length of the off-axis beam-shrinking secondary mirror 2. Specifically, the focal length of the off-axis beam-shrinking primary mirror 1 is 400mm, and the focal length of the off-axis beam-shrinking secondary mirror 2 is 100mm.

[0029] The total focal length of the cooled reflex telephoto lens is f = M × f3, where f3 is the overall focal length of the cooled off-axis three-lens reflex lens. Specifically, the total focal length of the cooled reflex telephoto lens is f = 700mm, and the overall focal length of the cooled off-axis three-lens reflex lens is f3 = 175mm. The focal length of the off-axis three-lens primary mirror 3 is 70mm, the focal length of the off-axis three-lens secondary mirror 4 is -29mm, and the focal length of the off-axis three-lens reflex mirror 5 is 82.5mm.

[0030] In this embodiment, the radius of curvature and air gap thickness of each lens are shown in the table below: Table 1. Radius of curvature and air gap thickness of each lens

[0031] That is, the air gap thickness of the off-axis primary mirror 1 is -500mm; the air gap thickness of the off-axis secondary mirror 2 is 300mm; the air gap thickness of the off-axis primary mirror 3 is -55mm; the air gap thickness of the off-axis secondary mirror 4 is 184mm; and the air gap thickness of the off-axis secondary mirror 5 is -124mm.

[0032] In one embodiment, the cooled reflective telephoto lens operates in the wavelength range of 400~9500nm, has an image plane diagonal of 20mm, an operating F-number of 4, and an entrance pupil diameter of 175mm at a total focal length of 700mm. It is capable of high-resolution imaging from extremely long distances with relatively strong light transmission across an extremely wide spectral range from visible light to long-wave infrared. Figures 7-9 As shown, in the 400~900nm, 900~1700nm, and 1700~2600nm bands, the lens has a modulation index greater than 0.4 at a spatial frequency of 34LP / mm, which is greater than the general requirement of 0.2 that the human eye can distinguish. Meanwhile, the pixel size of the cooled detectors is relatively large (mostly 15 micrometers), and the spatial resolution of the lens fully meets the spatial resolution requirements of the sensor. For example... Figures 10-11 As shown, in the ranges of 3000~5000nm and 7700~9500nm, the modulation transfer function of the lens is close to the diffraction limit. At this point, the imaging quality of the lens is not constrained by geometric optics, but by physical limits. Figures 7-11 This demonstrates that the cooled reflective telephoto lens provides good imaging quality for various small wavebands in the 400~9500nm range at infinity, meeting the system's usage requirements.

[0033] In one embodiment, the object distance focusing range of the cooled reflective telephoto lens is 100m to ∞, for example, it can be 100m, 500m, 1000m, 5000m, 10000m, etc. Figures 4-6 As shown, the cooled reflective telephoto lens produces good images at different object distances, meeting the system's usage requirements.

[0034] In one embodiment, the polynomial expression for the freeform surface of a cooled off-axis three-lens reflex lens is as follows: .

[0035] The polynomial expression consists of two parts, representing the basic conic surface basis and the polynomial perturbation term.

[0036] The basic conical surface base (the first half of the expression) is responsible for providing the basic geometry of the lens surface.

[0037] .

[0038] in: r: Radial coordinate of the lens surface .

[0039] c: Vertex curvature, which is the reciprocal of the radius at the vertex (c=1 / R).

[0040] k: Conic Constant, which determines the shape of the base surface: k=0: spherical surface; k<-1: hyperboloid; k=-1: paraboloid; -1<k<0: ellipsoid (prolate ellipsoid); k>0: oblate ellipsoid.

[0041] The polynomial perturbation term (the latter part of the expression) is an additional sag superimposed on the base surface, which is used to fine-tune the local shape of the surface.

[0042] .

[0043] Where: Ai: the coefficient of the i-th polynomial term.

[0044] Ei(x,y): the i-th polynomial basis.

[0045] Ei(x,y) is usually a power series term in the form of x m y n (such as x, y, x 2 , xy, y 2 , x 3 , etc.).

[0046] Table 2. Base Expression Coefficient Table of Free-form Surface of Cooled Off-axis Three-mirror Lens

[0047] Table 3. Additional Polynomial Coefficient Table of Free-form Surface of Cooled Off-axis Three-mirror Lens

[0048] Second Embodiment On the basis of the above embodiments, this embodiment provides a focusing method, which uses the aforementioned cooled reflective telephoto lens applicable to different object distances. When the object distance is infinity, the off-axis beam shrinking system is in an initial afocal state, the image-side focal point of the off-axis beam shrinking primary mirror 1 coincides with the object-side focal point of the off-axis beam shrinking secondary mirror 2, which is the initial state (the movement amounts along the optical axis direction and the direction perpendicular to the optical axis are both 0). It is specified that the direction in which the off-axis beam shrinking secondary mirror 2 moves away from the off-axis beam shrinking primary mirror 1 along the central axis of the primary and secondary mirrors is the negative direction. When the object distance decreases, the off-axis beam shrinking secondary mirror 2 moves in the reverse direction along the optical axis, and the off-axis beam shrinking secondary mirror 2 moves downward in the direction perpendicular to the optical axis. When the object distance increases, the off-axis beam shrinking secondary mirror 2 moves in the forward direction along the optical axis, and the off-axis beam shrinking secondary mirror 2 moves upward in the direction perpendicular to the optical axis.

[0049] The movement amounts corresponding to different object distances are shown in the following table: Table 4. Off-axis beam-shrinking secondary mirror 2 movement corresponding to different object distances

[0050] In order to accurately control the spatial motion trajectory of the off-axis beam-shrinking secondary mirror 2 during the focusing process, a fitting analysis was performed based on the displacement data at different object distances listed in Table 4. The results show that the secondary mirror strictly follows the following spatial geometry and linear linkage rules during the focusing motion: The displacement of the off-axis beam-shrinking secondary mirror 2 along the optical axis is defined as follows: x (unit: mm), and the displacement perpendicular to the optical axis is Δy (unit: mm). As the object distance gradually decreases from infinity, the off-axis beam-shrinking secondary mirror 2 moves in the opposite direction along the optical axis (i.e., towards the off-axis beam-shrinking primary mirror 1), and simultaneously moves downwards along the direction perpendicular to the optical axis. Both x and Δy are negative in value.

[0051] Fitting verification showed that, across the entire travel range from 100m to infinity, the displacement Δy perpendicular to the optical axis and the displacement along the optical axis... x maintains a constant linear proportional relationship, and the fitted off-axis beam-shrinking secondary mirror 2 movement curve is as follows: Figure 2 As shown, and satisfies the following formula: .

[0052] That is, the amount of movement perpendicular to the optical axis is approximately three-fifths of the amount of movement along the optical axis. Considering actual manufacturing and assembly errors and detector assembly tolerances, in engineering implementation, this proportionality coefficient can be allowed to fluctuate within the range of 0.59 to 0.62, all of which can ensure that the system has good image quality.

[0053] Meanwhile, since the actual trajectory of the off-axis beam-shrinking secondary mirror 2 is a linear translation in three-dimensional space, its total composite movement Δs (i.e., the "total movement" in Table 4) is not... The algebraic addition of x and Δy is not followed by the Euclidean distance (Pythagorean theorem) principle in spatial vector composition, and its precise calculation formula is: .

[0054] The above proportional relationship Substituting into the above equation, a simplified engineering approximation of the relationship between the total displacement and the axial displacement can be derived: .

[0055] Furthermore, based on the data in Table 4, an inverse proportional function is fitted to the axial displacement Δx and the object distance L (L is in meters), yielding the following empirical relationship: .

[0056] Correspondingly, the displacement in the vertical direction satisfies: .

[0057] Correspondingly, the fitting curve of total movement versus object distance is as follows: Figure 3 It satisfies the following relationship: .

[0058] The above inverse relationship shows that the focusing compensation displacement is directly proportional to the reciprocal of the object distance. This is consistent with the paraxial theory of thin lenses in geometric optics, which states that the defocusing amount varies with the object distance. That is, the closer the object distance, the greater the compensation stroke required by the secondary lens. When the object distance approaches infinity, the displacement approaches zero, and the system returns to the initial defocused state (Δx=Δy=0).

[0059] By controlling the above-mentioned linkage motion trajectory, this invention eliminates the need for any transmission-type focusing group. It only requires driving the off-axis beam-shrinking secondary mirror 2 to translate along a specific two-dimensional oblique trajectory. This achieves continuous, clear imaging across the entire object distance range from 100m to infinity, while ensuring 100% cold aperture efficiency and without introducing additional chromatic aberration. The motion fitting curve of the off-axis beam-shrinking secondary mirror 2 is shown below. Figure 2 As shown, after fitting, the angle between the secondary mirror focusing direction and the optical axis is approximately 30.92°. The specific displacement at each object distance is shown in Table 4.

[0060] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A cooled reflective telephoto lens suitable for different object distances, characterized in that, include: Off-axis primary mirror (1), off-axis secondary mirror (2), off-axis primary mirror (3), off-axis secondary mirror (4), off-axis secondary mirror (5), and aperture (6) are arranged sequentially from the object plane to the image plane along the incident direction of light. The off-axis primary beam-shrinking mirror (1) and the off-axis secondary beam-shrinking mirror (2) are both concave mirrors with positive optical power, and the focal points of the two mirrors coincide, forming an afocal beam-shrinking system; the position of the off-axis secondary beam-shrinking mirror (2) can be moved to achieve focusing for different object distances; The off-axis three-mirror primary mirror (3) is a concave mirror with positive optical power, the off-axis three-mirror secondary mirror (4) is a convex mirror with negative optical power, and the off-axis three-mirror (5) is a concave mirror with positive optical power. Together they form a cooled off-axis three-mirror lens, which is used to eliminate induced aberrations caused by off-axis field-of-view aberrations of the front beam-shrinking system. The beam-shrinking ratio of the off-axis beam-shrinking system is M=f1 / f2=4, where f1 is the focal length of the off-axis beam-shrinking primary mirror (1) and f2 is the focal length of the off-axis beam-shrinking secondary mirror (2). The total focal length of the cooled reflex telephoto lens is f = M × f3, where f3 is the overall focal length of the cooled off-axis three-lens reflex lens.

2. The cooled reflective telephoto lens suitable for different object distances according to claim 1, characterized in that: The focal length of the off-axis beam-shrinking primary mirror (1) is 400mm and the air gap thickness is -500mm; The off-axis beam-shrinking secondary mirror (2) has a focal length of 100 mm and an air gap thickness of 300 mm.

3. The cooled reflective telephoto lens suitable for different object distances according to claim 1, characterized in that: The total focal length of the cooled reflex telephoto lens is f=700mm, and the overall focal length of the cooled off-axis three-lens lens is f3=175mm.

4. The cooled reflective telephoto lens suitable for different object distances according to claim 3, characterized in that: The focal length of the off-axis three-mirror primary mirror (3) is 70mm, and the air gap thickness is -55mm; The focal length of the off-axis three-reflector (4) is -29mm, and the air gap thickness is 184mm; The focal length of the off-axis three-mirror system (5) is 82.5 mm, and the air gap thickness is -124 mm.

5. The cooled reflective telephoto lens suitable for different object distances according to claim 1, characterized in that: The operating F-number of the cooled reflective telephoto lens is 4, and the image plane diagonal of the cooled reflective telephoto lens is 20mm.

6. The cooled reflective telephoto lens suitable for different object distances according to claim 5, characterized in that: The object distance focusing range of the cooled reflex telephoto lens is 100m~∞; The operating wavelength of the cooled reflective telephoto lens is 400~9500nm.

7. A focusing method, characterized in that: Use the cooled reflective telephoto lens suitable for different object distances as described in any one of claims 1-6; When the object distance decreases, the off-axis beam-shrinking secondary mirror (2) moves in the opposite direction along the optical axis and moves downward in the direction perpendicular to the optical axis; As the object distance increases, the off-axis beam-shrinking secondary mirror (2) moves in the positive direction along the optical axis and upward in the direction perpendicular to the optical axis.

8. The focusing method according to claim 7, characterized in that: The displacement of the off-axis beam-shrinking secondary mirror (2) along the optical axis is defined as follows: x, the displacement in the direction perpendicular to the optical axis is Δy; When the object distance changes, the amount of movement Δy of the off-axis beam-shrinking secondary mirror (2) in the direction perpendicular to the optical axis is the amount of movement along the optical axis. 0.59-0.62 times x; Total movement of the off-axis beam-shrinking secondary mirror (2) ; Total movement The distance L between the object and the object satisfies the following relationship: 。

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