Compact long focal length reflective remote sensing imaging system based on relay folded optical path

Abstract

The application is suitable for the field of space optical remote sensing and reflective imaging technology, and provides a compact long-focus reflective remote sensing imaging system based on a Relay folding optical path, which comprises: a front-end main imaging unit adopting a coaxial two-mirror structure, including a primary mirror and a secondary mirror arranged in sequence along an optical axis, for receiving incident light and forming a convergent light beam; and a rear-end folding reflection unit arranged on an outgoing light path of the front-end main imaging unit, including at least four mirrors arranged in sequence along the light path, for folding the convergent light beam multiple times and guiding it to an image plane. The application adopts a light path form combining a front-end coaxial two-mirror main imaging structure and a rear-end multi-stage Relay folding reflection structure to perform full-aperture ground remote sensing imaging, and can realize high-resolution compact imaging under long-focus conditions. The application is suitable for application scenarios such as high-resolution ground remote sensing satellites, miniaturized space loads, near-space platforms and airborne long-focus imaging systems.

Description

Technical Field

[0001] This invention belongs to the field of space optical remote sensing and reflective imaging technology, and particularly relates to a compact long focal length reflective remote sensing imaging system based on a Relay folded optical path. Background Technology

[0002] Currently, high-resolution Earth remote sensing imaging systems mainly employ traditional coaxial two-mirror systems, three-mirror aberration-correcting systems (TMA), and folding reflector systems. Although these systems are widely used in the field of space remote sensing, they still have significant limitations in terms of long focal length, miniaturization, and high image quality in a coordinated design.

[0003] First, traditional coaxial two-mirror remote sensing systems widely adopt Cassegrain or RC coaxial two-mirror structures. For example, the US KH-11 optical reconnaissance satellite uses a large-aperture coaxial reflective imaging system with a primary mirror diameter of approximately 2.4m and a system focal length exceeding 20m, achieving sub-meter-level Earth resolution. This type of system has advantages such as mature structural design and high energy efficiency; however, due to its axially deployable optical path layout, the overall system length and mass are relatively large, placing high demands on the structural dimensions of the satellite platform.

[0004] Furthermore, traditional two-mirror systems have limited ability to correct off-axis aberrations, and are prone to significant coma, astigmatism, and field curvature under large field-of-view conditions. Therefore, they are generally only suitable for imaging tasks with smaller field of view. As focal lengths increase further, the system's structural size and the complexity of its optomechanical support increase rapidly, hindering applications on lightweight and miniaturized space platforms. These systems mainly suffer from the following problems:

[0005] (1) The system has a large axial dimension and insufficient structural compactness;

[0006] (2) Under large field of view conditions, off-axis aberrations are obvious and image quality uniformity is poor;

[0007] (3) Long focal length systems are heavy and require high platform load-bearing capacity.

[0008] Secondly, to enhance high-quality imaging capabilities with a large field of view, three-mirror aberration correction (TMA) systems are widely used in high-resolution remote sensing. For example, the OLI (Operational Land Imager) carried by NASA's Landsat-8 satellite employs a four-mirror aberration correction structure, achieving a field of view of 15° and realizing 15m panchromatic resolution and 30m multispectral resolution at an orbital altitude of 705km. Such systems offer unobstructed views, a large field of view, and strong aberration correction capabilities, providing significant advantages in medium-to-large field-of-view remote sensing imaging.

[0009] However, TMA and multi-mirror systems typically employ complex off-axis aspherical structures, with strong aberration coupling between multiple mirrors, demanding extremely high precision in surface alignment, assembly errors, and structural stability. As the number of mirrors increases, the system's degrees of freedom and optimization complexity rise rapidly, significantly increasing manufacturing, testing, and assembly costs. Furthermore, off-axis structures are highly sensitive to system thermal and mechanical stability, hindering miniaturization and low-cost engineering implementation. These systems primarily suffer from the following problems:

[0010] (1) The off-axis structure is complex and requires high precision in machining and assembly;

[0011] (2) The multi-reflection mirror image aberration coupling is severe, making system optimization difficult;

[0012] (3) The system has a high degree of freedom, but its manufacturing and maintenance costs are relatively high;

[0013] (4) The system structure is complex, which is not conducive to miniaturization and lightweight design.

[0014] Third, to balance long-focal-length imaging capabilities with system compactness, some space remote sensing systems employ a three-mirror Cassegrain (TMC) structure. TMC systems typically add a third mirror to a traditional two-mirror structure, introducing additional degrees of freedom to improve off-axis aberration and image quality. For example, the WorldView-3 high-resolution remote sensing satellite uses a three-mirror imaging structure, with a primary mirror aperture of approximately 1.1m and an orbital altitude of approximately 617km, achieving a panchromatic resolution of approximately 0.31m.

[0015] To achieve a large imaging swath and high image quality, such systems typically employ complex off-axis aspherical designs. While this results in high imaging performance, it significantly increases the complexity of the system structure. Furthermore, some high-resolution space cameras using a TMC (Transformer-Modifier-Controlled) structure can achieve a system MTF (Mean Transformer-Tolerant Flow) of 0.25–0.40 near the Nyquist frequency. However, due to the off-axis layout of multiple mirrors, the system is highly sensitive to mirror manufacturing and assembly errors, and strong aberration coupling exists between the multiple mirrors. These systems mainly suffer from the following problems:

[0016] (1) Off-axis multi-mirror structures are complex, and there is a strong aberration coupling relationship between multiple mirrors;

[0017] (2) There are many high-order aspherical surfaces, which require high precision in mirror processing, inspection and assembly.

[0018] (3) Although the system length has been compressed, the overall mechanical envelope size is still large under long focal length conditions;

[0019] (4) The structure has a high degree of freedom, but the system optimization is difficult and the engineering implementation is complex.

[0020] (5) Most of them use aspherical primary mirrors with large off-axis deviation, which results in higher costs for mirror processing and testing.

[0021] Fourth, to reduce the axial dimensions of long-focal-length systems, some remote sensing systems employ folded reflective optical path structures. These structures use additional folding mirrors to alter the beam propagation direction, thus compressing the optical path. For example, the improved versions of the US KH-11 optical reconnaissance system and some spaceborne long-focal-length remote sensing payloads utilize a multi-folding reflective layout to reduce the system's unfolded length and accommodate limited satellite compartment space. Some airborne long-focal-length remote sensing equipment also employs "Z"-shaped or "U"-shaped folded optical path structures to reduce the system's mechanical envelope size. These systems typically use plane mirrors to fold the optical path, which reduces system length, but the folding structure itself usually lacks aberration adjustment capabilities.

[0022] These systems typically use plane mirrors to fold the optical path, which reduces system length, but the folded structure itself usually lacks aberration adjustment capabilities. Under long focal lengths and large fields of view, multiple folds can easily introduce additional off-axis aberrations, image plane tilt, and stray light problems. Some traditional folded long focal length systems have an MTF of only about 0.2-0.3 at a spatial frequency of 30 lp / mm, with a significant decrease in edge field-of-view imaging quality, making it difficult to meet the requirements of high-resolution remote sensing imaging. Furthermore, traditional folded structures only geometrically compress the optical path and do not participate in system aberration optimization; therefore, with increasing folding times, the system's sensitivity to sub-mirror tilt errors and structural stability further increases. These systems mainly suffer from the following problems:

[0023] (1) Traditional folding structures mainly rely on plane mirrors and lack the ability to compensate for aberrations at the rear end;

[0024] (2) Multiple folding can easily introduce off-axis aberrations and image plane tilt, affecting the image quality of the entire field of view;

[0025] (3) The stray light path of the system is complex, and the stability and stray light control are difficult.

[0026] (4) The compression process can easily lead to a decrease in MTF, making it difficult to balance the requirements of high image quality and compactness.

[0027] In summary, existing high-resolution Earth remote sensing imaging systems still have significant shortcomings in terms of long focal length, miniaturization, and high image quality in a coordinated design. The main technical problems that need to be solved are as follows:

[0028] (1) Traditional long focal length systems generally adopt an axially deployable structure, which results in a large system size and weight, making it difficult to meet the requirements of miniaturized space load.

[0029] (2) Traditional folding structures usually only undertake the function of optical path deflection and lack the ability to coordinate and control the aberrations at the back end; (3) Although multi-mirror systems such as TMA and TMC have good aberration correction capabilities, they are complex in structure, have serious mirror coupling, and require high precision in processing and assembly.

[0030] (4) High-magnification secondary mirror structures tend to amplify off-axis aberrations and assembly errors, reducing system stability;

[0031] (5) Existing systems are difficult to simultaneously meet multiple performance requirements such as long focal length, high image quality, low distortion and miniaturization. Summary of the Invention

[0032] The purpose of this invention is to provide a compact long focal length reflective remote sensing imaging system based on a Relay folded optical path, aiming to solve the problems existing in the prior art.

[0033] This invention is implemented as follows: a compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path, comprising, along the direction of light propagation, the following components:

[0034] The front-end main imaging unit adopts a coaxial two-mirror structure, including a primary mirror and a secondary mirror arranged sequentially along the optical axis, which are used to receive incident light and form a convergent beam;

[0035] The rear-end folding reflection unit is disposed on the outgoing light path of the front-end main imaging unit, and includes at least four stages of mirrors arranged sequentially along the light path, for folding the convergent beam multiple times and guiding it to the image plane; wherein, at least one stage of mirror in the rear-end folding reflection unit is an aspherical mirror with optical power, for participating in system aberration compensation while folding the light path.

[0036] Furthermore, the primary reflector is a concave aspherical reflector; the secondary reflector is a convex aspherical reflector.

[0037] Furthermore, the conicity of the primary reflector is -1; the conicity of the secondary reflector... satisfy: .

[0038] Furthermore, the optical characteristics of the primary reflector satisfy: , The optical characteristics of the secondary mirror satisfy: , ;in, The focal length of the entire compact long-focal-length reflective remote sensing imaging system. , These are the focal lengths of the primary and secondary reflectors, respectively. , These are the radii of curvature of the primary and secondary mirrors, respectively.

[0039] Furthermore, the rear-end folding reflective unit comprises, in sequence along the optical path:

[0040] The first reflecting mirror is a plane reflecting mirror;

[0041] The second reflecting mirror is a weak aspherical concave reflecting mirror with optical power;

[0042] The third reflecting mirror is a convex reflecting mirror with optical power;

[0043] The fourth reflecting mirror is a concave reflecting mirror with optical power.

[0044] Furthermore, the conicity of the second reflecting mirror satisfy: The third and fourth reflecting mirrors are both spherical reflecting mirrors.

[0045] Furthermore, the following definitions are made: the air gap between the primary mirror and the secondary mirror is d1, the air gap between the secondary mirror and the first mirror is d2, the air gap between the first mirror and the second mirror is d3, the air gap between the second mirror and the third mirror is d4, the air gap between the third mirror and the fourth mirror is d5, and the air gap between the fourth mirror and the image plane is d6; then d1:d2:d3:d4:d5:d6=(1.4-1.6):(2.1-2.3):(0.8-1.0):(1.3-1.5):(1.5-1.7):(2.4-2.6).

[0046] Furthermore, the optical characteristics of the first reflector satisfy: , The optical characteristics of the second reflecting mirror satisfy: , The optical characteristics of the third reflecting mirror satisfy: , The optical characteristics of the fourth reflecting mirror satisfy: , ;in, The focal length of the entire compact long-focal-length reflective remote sensing imaging system. , , , These are the focal lengths of the first, second, third, and fourth reflecting mirrors, respectively. , , , These are the radii of curvature of the first, second, third, and fourth reflecting mirrors, respectively.

[0047] This invention provides a compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path. It employs a front-end coaxial two-mirror main imaging structure combined with a rear-end multi-stage relay folded reflection structure for full-aperture Earth remote sensing imaging. This enables high-resolution, compact imaging under long-focal-length conditions, achieving long-focal-length, high-image-quality, and low-distortion imaging within limited spatial dimensions. Specifically, the front-end main imaging unit uses a primary and secondary mirror to perform primary power allocation and primary aberration correction. The rear-end folded reflection unit uses a multi-stage folded mirror group to spatially reconstruct the converging beam (continuous folding and propagation state adjustment), participating in rear-end aberration compensation and image plane correction while compressing the optical path, thus significantly reducing the system's axial dimensions while ensuring imaging quality. This invention is applicable to high-resolution Earth remote sensing satellites, miniaturized space payloads, near-space platforms, and airborne long-focal-length imaging systems, and is particularly suitable for space remote sensing missions with high requirements for system compactness, lightweight design, and high image quality. As space remote sensing systems develop towards miniaturization, low cost, and high integration, this invention, through the coordinated design of a front-end coaxial dual-mirror main imaging structure and a rear-end multi-stage relay folding reflection structure, maintains good modulation transfer function performance and low geometric distortion while keeping the system structure compact. This can meet the comprehensive application requirements of high-resolution Earth remote sensing imaging systems for long focal length, miniaturization, and high image quality, thereby improving the space utilization and engineering feasibility of long focal length systems and having good engineering application value. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the structure of a compact long-focal-length reflective remote sensing imaging system based on a Relay folded optical path, provided in an embodiment of the present invention.

[0049] Figure 2 The optical path diagram of the compact long focal length reflective remote sensing imaging system provided in Example 1.

[0050] Figure 3 The MTF curve of the compact long focal length reflective remote sensing imaging system provided in Example 1 at 55.6 lp / mm.

[0051] In the figure: 1-Front-end main imaging unit; 2-Rear-end folding reflective unit; 3-Detector; 11-Main reflector; 12-Secondary reflector; 21-First reflector; 22-Second reflector; 23-Third reflector; 24-Fourth reflector. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0053] Existing long-focal-length remote sensing systems typically employ an axially unfolding structure, resulting in large system length and significant structural redundancy. Furthermore, traditional folding systems only possess optical path deflection capabilities, lacking the ability to coordinate back-end aberrations, making it difficult to balance system compactness with high-quality imaging requirements. This invention addresses these issues by proposing an optical path design method that combines a front-end coaxial two-mirror main imaging structure with a back-end relay folding reflection structure. This method maintains the system's long-focal-length imaging capabilities while achieving system structural compression and full-field-of-view aberration optimization.

[0054] Specifically, such as Figure 1 and Figure 2 As shown, in one embodiment of the present invention, a compact long focal length reflective remote sensing imaging system based on a relay folded optical path is provided, comprising, in sequence along the light propagation direction:

[0055] The front-end main imaging unit 1 adopts a coaxial two-mirror structure (Cassegrain structure), including a primary mirror 11 (aperture stop) and a secondary mirror 12 arranged sequentially along the optical axis, which are used to receive incident light and form a convergent beam.

[0056] The rear-end folding reflection unit 2 is disposed on the outgoing light path of the front-end main imaging unit 1, and includes at least four stages of reflectors arranged sequentially along the light path, for folding the convergent beam multiple times and guiding it to the image plane; wherein, at least one stage of the rear-end folding reflection unit is an aspherical reflector with optical power, for participating in system aberration compensation while folding the light path.

[0057] This compact long-focal-length reflective remote sensing imaging system can be used in conjunction with detector 3, achieving miniaturization and high integration of the long-focal-length system while ensuring high-resolution imaging performance.

[0058] Specifically, the primary reflector 11 is a concave aspherical reflector; the secondary reflector 12 is a convex aspherical reflector; wherein the conicity of the primary reflector 11 is -1; and the conicity of the secondary reflector 12 is... satisfy: The preferred value is -3.989. Both the primary reflector 11 and the secondary reflector 12 adopt an aspherical design, which can complete the main optical power distribution and primary aberration correction of the system.

[0059] In this embodiment of the invention, the primary reflector 11 adopts a conic coefficient. Compared to high-order hyperboloid primary mirrors in traditional RC systems, parabolic primary mirrors offer advantages such as simpler surface shape, lower manufacturing difficulty, and higher detection efficiency. They also provide good on-axis aberration correction capabilities with lower system complexity. Furthermore, The primary lens configuration helps reduce the system's dependence on high-magnification secondary lenses, thereby reducing the sensitivity to off-axis aberrations and the difficulty of system assembly and adjustment caused by excessive secondary lens magnification, and improving the engineering feasibility of long focal length systems.

[0060] like Figure 2 As shown, in a preferred embodiment of the present invention, the rear folding reflective unit 2 comprises, along the optical path, the following components in sequence:

[0061] The first reflecting mirror 21 is a plane reflecting mirror;

[0062] The second reflecting mirror 22 is a weak aspherical concave reflecting mirror with optical power;

[0063] The third reflecting mirror 23 is a convex reflecting mirror with optical power, specifically a spherical structure;

[0064] The fourth reflecting mirror 24 is a concave reflecting mirror with optical power, specifically a spherical structure.

[0065] In this embodiment of the invention, the rear-end folding reflection unit 2 consists of four mirrors, designed based on the Relay folding optical path concept. The Relay folding optical path concept refers to utilizing the relay imaging principle to retransmit and spatially reconstruct the convergent beam through multi-stage reflection units while maintaining the matching relationship between the preceding imaging information and the aperture. This achieves optical path folding, optical path compression, and aberration redistribution, resulting in a more compact system layout. Specifically, the first-stage mirror (i.e., the first mirror 21) adopts a planar reflection structure to change the propagation direction of the output beam from the front-end two-mirror system; the middle two-stage mirrors (i.e., the second mirror 22 and the third mirror 23) respectively adopt a combination structure of a weakly aspherical concave mirror and a convex mirror, readjusting the convergence state of the convergent beam to achieve aberration compensation and beam transmission in the Relay stage; the final-stage mirror (i.e., the fourth mirror 24) adopts a concave reflection structure to further correct the rear-end field curvature and complete the final image plane focusing. The rear-end folding reflective unit 2 not only undertakes the optical path folding function, but also participates in the system aberration balance, so that the system can achieve a compact structure while maintaining good full-field imaging capability.

[0066] The compact long-focal-length reflective remote sensing imaging system provided in this embodiment of the invention operates as follows: The system employs a long-focal-length reflective imaging structure combining coaxial dual mirrors and a rear-end relay folded reflection for full-aperture Earth remote sensing imaging. Incident parallel light from distant targets is first converged by the primary mirror 11 of the front-end main imaging unit 1, and after secondary reflection by the secondary mirror 12, forms a convergent beam. Since the front-end Cassegrain structure is responsible for establishing the system's main optical power, it can effectively correct spherical aberration and coma while achieving long-focal-length imaging, and forms a convergent beam after the secondary mirror 12. Subsequently, the convergent beam enters the rear-end folded reflection unit 2. Unlike TMC systems, which mainly rely on increasing the degrees of freedom of the mirrors to achieve aberration compensation and optical path compression, this embodiment of the invention does not further increase the degrees of freedom of the main optical system. Instead, it introduces the relay optical path design concept during the convergent beam propagation stage, using multi-stage mirrors to readjust the beam propagation direction, beam waist position, and propagation state, thereby achieving spatial reconstruction of the optical path propagation. As the cross-sectional size of the converging beam gradually decreases during propagation, the size of the mirrors in the rear-end folding reflection unit 2 can be significantly reduced. This allows for the remapping of a long propagation path within a limited space, compressing the original long focal length propagation distance, which was originally spread along the optical axis, into a limited mechanical envelope. Simultaneously, the weak aspherical mirrors in the rear-end folding reflection unit 2 participate in adjusting the propagation state of the converging beam, compensating for off-axis aberrations generated during multiple folds, and achieving simultaneous optimization of structural compression and aberration correction. Finally, the beam reconstructed by the rear-end folding reflection unit 2 reaches the image plane to form a clear image point, achieving an integrated design of long focal length, high resolution, and compact structure. The folded beam reaches the image plane to form a clear image point, enabling high-resolution imaging of ground targets.

[0067] Through coordinated optimization of the primary reflector 11, secondary reflector 12, and rear-end folding reflector unit 2, this embodiment of the invention achieves a full-field modulation transfer function (MTF) of approximately 0.25 or higher at a spatial frequency of 55 lp / mm, with good consistency in the meridional and sagittal curves of each field of view, indicating that the system has excellent full-field imaging capability and aberration control capability. The maximum distortion of the system is approximately 0.1009%, and the field curvature control is stable, meeting the geometric accuracy requirements of high-resolution Earth remote sensing imaging. In practical applications, combined with the system's 210mm aperture and 3500mm long focal length design, and matched with a 9μm-level pixel detector 3, a ground sampling distance of approximately 1.29m can be achieved, making it applicable to meter-level ground resolution remote sensing imaging tasks at orbital altitudes of 500km.

[0068] In a preferred embodiment of the present invention, the focal length of the compact long-focal-length reflective remote sensing imaging system can be set to 3499.96 mm, the total lens length is 365.305 mm, the spectral range is 0.450-0.800 μm, the system aperture is 210 mm, and the F-number is approximately 16.67. The field of view is a non-rotationally symmetric field of view, wherein the maximum field of view angle in the horizontal direction is ±0.450° (i.e., a full field of view angle of 0.9°), and the field of view angle in the vertical direction ranges from 0.800° to 1.100° (i.e., a full field of view angle of 0.3°). This asymmetric field of view design is suitable for strip-type Earth remote sensing imaging tasks, which is beneficial to improving the system's swath coverage capability and adaptability to remote sensing tasks.

[0069] like Figure 2 As shown, in a preferred embodiment of the present invention, the air gap between the primary reflector 11 and the secondary reflector 12 is defined as d1, the air gap between the secondary reflector 12 and the first reflector 21 is defined as d2, the air gap between the first reflector 21 and the second reflector 22 is defined as d3, the air gap between the second reflector 22 and the third reflector 23 is defined as d4, the air gap between the third reflector 23 and the fourth reflector 24 is defined as d5, and the air gap between the fourth reflector 24 and the image plane is defined as d6; then d1:d2:d3:d4:d5:d6=(1.4-1.6):(2.1-2.3):(0.8-1.0):(1.3-1.5):(1.5-1.7):(2.4-2.6).

[0070] Preferably, the air gap between the primary reflector 11 and the secondary reflector 12 is 146.456 mm, the air gap between the secondary reflector 12 and the first reflector 21 is 218.571 mm, the air gap between the first reflector 21 and the second reflector 22 is 91 mm, the air gap between the second reflector 22 and the third reflector 23 is 134.444 mm, the air gap between the third reflector 23 and the fourth reflector 24 is 152.316 mm, and the air gap between the fourth reflector 24 and the image plane is 247.062 mm.

[0071] It should be noted that the primary reflector 11, the secondary reflector 12, as well as the first reflector 21, the second reflector 22, the third reflector 23, and the fourth reflector 24 are all made of optical reflective materials.

[0072] In a preferred embodiment of the present invention, the design of each mirror in the compact long focal length reflective remote sensing imaging system provided by the present invention satisfies the following conditions:

[0073] (1) The optical characteristics of the primary reflector 11 satisfy:

[0074] , , ;

[0075] (2) The optical characteristics of the secondary mirror 12 satisfy:

[0076] , , ;

[0077] (3) The optical characteristics of the first reflecting mirror 21 satisfy:

[0078] , ;

[0079] (4) The optical characteristics of the second reflecting mirror 22 satisfy:

[0080] , , ;

[0081] (5) The optical characteristics of the third reflecting mirror 23 satisfy:

[0082] , , ;

[0083] (6) The optical characteristics of the fourth reflecting mirror 24 satisfy:

[0084] , , .

[0085] in, The focal length of the entire compact long-focal-length reflective remote sensing imaging system. , , , , , The focal lengths of the primary reflector 11, secondary reflector 12, first reflector 21, second reflector 22, third reflector 23 and fourth reflector 24 are respectively. , , , , , The radii of curvature of the primary reflector 11, secondary reflector 12, first reflector 21, second reflector 22, third reflector 23 and fourth reflector 24 are respectively. Refers to the conic coefficient of an aspherical surface.

[0086] The results of comparing the compact long focal length reflective remote sensing imaging system provided in this embodiment with existing traditional coaxial two-mirror systems, off-axis TMA systems, TMC systems, and traditional folding reflective systems are shown in Table 1.

[0087] Table 1 Comparison of Structural Performance of Each System

[0088]

[0089] As shown in Table 1, the compact long-focal-length reflective remote sensing imaging system provided by this invention has a more compact structure: traditional long-focal-length remote sensing systems mostly adopt an axially deployable structure. As the focal length increases, the overall length of the system increases rapidly, which is not conducive to the miniaturization of space payloads. This invention introduces a Relay folding reflection structure in the beam-converging stage in front of the image, enabling the long optical path to undergo multiple folds within a limited space, effectively compressing the axial dimensions of the system. Compared with traditional deployable two-reflector systems, while maintaining a long focal length of 3500mm or higher, the total system length is only about 365mm, significantly improving space utilization and making it more suitable for small satellites, airborne platforms, and applications in confined space.

[0090] Traditional folding reflector systems typically use only plane mirrors for optical path deflection. The folding structure itself lacks aberration adjustment capabilities, and repeated folding easily introduces off-axis aberrations and image plane tilt, leading to a decline in edge field-of-view imaging quality. This invention introduces a weak aspherical concave mirror and a subsequent concave mirror in the rear-end folding reflector unit 2. By readjusting the propagation state of the convergent beam, optical path compression is achieved while compensating for rear-end off-axis aberrations. System design results show that, near a spatial frequency of 50 lp / mm, the MTF (Mean Transmission Frequency) across the entire field of view remains above approximately 0.30, and the meridional and sagittal curves of each field of view exhibit good consistency. This indicates that the system has excellent full-field spatial frequency response capabilities, effectively ensuring the clear transmission of target edges, textures, and details.

[0091] Traditional coaxial two-mirror systems are prone to significant coma, astigmatism, and field curvature under large field-of-view conditions. While complex off-axis TMA systems can improve aberrations, they require high degrees of freedom and have complex structures. This invention employs a coaxial two-mirror structure with a front-end main imaging unit 1 handling the primary optical power and a rear-end folding reflection unit 2 participating in subsequent aberration adjustment. Through coordinated optimization of the front and rear optical structures, a comprehensive balance is achieved between spherical aberration, coma, field curvature, and distortion. The primary mirror 11 employs… The parabolic structure, compared to traditional high-order hyperboloid primary mirrors, offers better on-axis imaging characteristics and lower manufacturing complexity. It also reduces the system's dependence on high-magnification secondary mirrors, thereby decreasing off-axis aberration sensitivity and improving system stability. Field curvature and distortion analysis results show that the image plane shift in each field of view is small, with a maximum geometric distortion of approximately 0.1009%, which meets the geometric accuracy requirements of high-resolution Earth remote sensing imaging.

[0092] Compared to complex off-axis TMA or multi-mirror systems, this embodiment of the invention uses a coaxial two-mirror front-end main imaging unit 1 as the main imaging architecture. The overall structure is more regular, reducing the extreme requirements for processing and assembly precision in complex off-axis aspherical systems. Simultaneously, the system's main mirror 11 adopts... The parabolic shape is relatively simple, and the processing and testing technology is mature, which can effectively reduce manufacturing costs and engineering implementation difficulties. The rear-end folding reflection unit 2 adopts a combination of "plane mirror + weak aspherical mirror + spherical mirror", which reduces the number of high-order complex aspherical surfaces used while ensuring the back-end aberration optimization capability, which is conducive to improving the system's adaptability and structural stability.

[0093] This invention integrates the long-focal-length front-end main imaging unit 1 with the rear-end folded reflection unit 2 based on the Relay folded optical path concept, achieving system spatial reconstruction while ensuring imaging performance. This avoids the spatial redundancy problem caused by the large-size axially unfolding structure in traditional long-focal-length systems. Compared to traditional unfolding remote sensing systems, this invention enables high-resolution long-focal-length imaging in a smaller volume, improving system integration and platform adaptability.

[0094] Example 1: As Figure 2 As shown, this embodiment provides a compact long focal length reflective remote sensing imaging system including the aforementioned front-end main imaging unit 1 and rear-end folded reflective unit 2. By adopting the Cassegrain main optical structure combined with the rear-end convergent beam folded reflective optical path, long focal length compact imaging is achieved. The imaging optical path is finally focused on the linear array detector 3, realizing the acquisition of target information with a large swath width and high resolution.

[0095] The design parameters of this compact long-focal-length reflective remote sensing imaging system are as follows: focal length 3499.96mm; F-number 16.67; spatial pixel resolution 1.29m; mirror group configuration: 6 mirrors (primary mirror 11, secondary mirror 12, first mirror 21, second mirror 22, third mirror 23 and fourth mirror 24), achieving a spectral range of 0.450-0.800μm, a non-rotationally symmetric field of view, wherein the maximum field of view angle in the horizontal direction is ±0.450° (i.e., full field of view angle 0.9°), and the field of view angle range in the vertical direction is 0.800° to 1.100° (i.e., full field of view angle 0.3°); the aperture stop is located at the primary mirror 11.

[0096] The imaging quality of this compact long-focal-length reflective remote sensing imaging system is as follows: In this embodiment, detector 3 is a linear array monochrome CMOS detector with a pixel size of 9μm and a pixel scale of 8192×1. The parameters of detector 3 are compatible with the system's 3500mm long focal length and 0.9°×0.3° asymmetric field of view, while also considering current engineering feasibility, system sampling efficiency, and imaging resolution requirements. Under these pixel conditions, the system's Nyquist frequency is approximately 55.6 lp / mm. In this embodiment, detector 3 uses a pixel size of 9μm. Figure 3 As shown, within the 0.450-0.800μm operating band, the system's modulation transfer function (MTF) remains above 0.25 at the Nyquist frequency of approximately 55.6 lp / mm for a detector pixel size of 9μm. Furthermore, the MTF curves in the meridional and sagittal directions at each field of view exhibit good consistency, indicating that the system maintains good spatial frequency response and aberration control near the Nyquist frequency, effectively ensuring the clear transmission of target edges, textures, and details. System field curvature and distortion analysis results show that the field curvature variation is small at each field of view, with a maximum geometric distortion of approximately 0.1009%, indicating good geometric fidelity and meeting the application requirements of high-resolution Earth remote sensing imaging for geometric positioning accuracy and spatial resolution. Combining the system's 210mm aperture, 3500mm focal length, and 9μm pixel linear array detector parameters, the system can achieve a ground sampling distance (GSD) of approximately 1.29m at an orbital altitude of 500km.

[0097] ;

[0098] Meanwhile, due to the adoption of a rear-end relay-type multi-level folding reflection layout, compared with traditional axially deployable long focal length remote sensing systems, the axial dimension of the system can be significantly shortened, improving system integration and space platform adaptability.

[0099] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path, characterized in that, Along the direction of light propagation, the following are included in sequence: The front-end main imaging unit adopts a coaxial two-mirror structure, including a primary mirror and a secondary mirror arranged sequentially along the optical axis, which are used to receive incident light and form a convergent beam; The rear-end folding reflection unit is disposed on the outgoing light path of the front-end main imaging unit, and includes at least four stages of mirrors arranged sequentially along the light path, for folding the convergent beam multiple times and guiding it to the image plane; wherein, at least one stage of mirror in the rear-end folding reflection unit is an aspherical mirror with optical power, for participating in system aberration compensation while folding the light path.

2. The compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path according to claim 1, characterized in that, The primary reflector is a concave aspherical reflector; the secondary reflector is a convex aspherical reflector.

3. The compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path according to claim 2, characterized in that, The conicity of the primary reflector is -1; the conicity of the secondary reflector is... satisfy: .

4. The compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path according to claim 2 or 3, characterized in that, The optical characteristics of the primary reflector satisfy: , The optical characteristics of the secondary mirror satisfy: , ;in, The focal length of the entire compact long-focal-length reflective remote sensing imaging system. , These are the focal lengths of the primary and secondary reflectors, respectively. , These are the radii of curvature of the primary and secondary mirrors, respectively.

5. The compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path according to claim 1, characterized in that, The rear-end folding reflective unit comprises, along the optical path, the following: The first reflecting mirror is a plane reflecting mirror; The second reflecting mirror is a weak aspherical concave reflecting mirror with optical power; The third reflecting mirror is a convex reflecting mirror with optical power; The fourth reflecting mirror is a concave reflecting mirror with optical power.

6. The compact long-focal-length reflective remote sensing imaging system based on a relay folded optical path according to claim 5, characterized in that, The conicity of the second reflecting mirror satisfy: The third and fourth reflecting mirrors are both spherical reflecting mirrors.

7. The compact long-focal-length reflective remote sensing imaging system based on a Relay folded optical path according to claim 5 or 6, characterized in that, Definition: The air gap between the primary mirror and the secondary mirror is d1, the air gap between the secondary mirror and the first mirror is d2, the air gap between the first mirror and the second mirror is d3, the air gap between the second mirror and the third mirror is d4, the air gap between the third mirror and the fourth mirror is d5, and the air gap between the fourth mirror and the image plane is d6; then d1:d2:d3:d4:d5:d6=(1.4-1.6):(2.1-2.3):(0.8-1.0):(1.3-1.5):(1.5-1.7):(2.4-2.6).

8. The compact long-focal-length reflective remote sensing imaging system based on a Relay folded optical path according to claim 5 or 6, characterized in that, The optical characteristics of the first reflecting mirror satisfy: , The optical characteristics of the second reflecting mirror satisfy: , ; The optical characteristics of the third reflecting mirror satisfy: , The optical characteristics of the fourth reflecting mirror satisfy: , ;in, The focal length of the entire compact long-focal-length reflective remote sensing imaging system. , , , These are the focal lengths of the first, second, third, and fourth reflecting mirrors, respectively. , , , These are the radii of curvature of the first, second, third, and fourth reflecting mirrors, respectively.

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