An off-axis three-mirror optical system compensator and its design method

Abstract

The present invention discloses an off-axis three-mirror optical system compensator and a design method thereof, the core feature of which is to insert an optical plate of a certain thickness as a compensator in front of the interferometer focus of the off-axis three-mirror self-collimation interference optical path, which is used to compensate for the aberration of rear optical path components such as windows and filters. Under the constraints of the off-axis three-mirror design optical path, the self-collimation interference optical path model with the compensator is optimized by establishing multiple optical path configurations, thereby obtaining the optimal optical plate thickness. The material of the optical plate compensator is selected and optically coated according to the working band of the interferometer. The present invention is simple and clear, and can greatly reduce the deviation of the off-axis three-mirror optical system adjustment test, improve the efficiency and accuracy of the off-axis three-mirror self-collimation interference test, and is particularly suitable for the adjustment test of off-axis three-mirror optical systems such as medium-wave infrared or long-wave infrared off-axis three-mirror optical systems with flat components such as windows and filters in the rear optical path.

Description

Technical Field

[0001] The invention belongs to the field of optical design or optical detection, and relates to a compensator structure and a design method for assembling and testing an off-axis three-mirror optical system. Background Art

[0002] As a fully reflective optical system, the off-axis three-mirror optical system offers the advantages of a wide operating band and zero chromatic aberration, while avoiding the disadvantage of central light blocking and achieving a clear aperture on the order of 1 to 2 meters. The three mirrors offer sufficient design variables and are lightweight, enabling a field of view of approximately 20° while maintaining resolution and image quality, leading to widespread application.

[0003] For example, the Mars remote sensing satellite Tianwen-1 developed by my country uses an off-axis three-mirror optical system for its high-resolution remote sensing camera, with a focal length of 4640mm, a clear aperture of 387mm, and a field of view angle of 2°.

[0004] The alignment and testing of off-axis three-mirror optical systems typically utilizes autocollimation interferometry. The alignment and testing optical path primarily consists of the off-axis three-mirror system, a plane mirror, and an interferometer. In the autocollimation interferometer optical path, the interferometer focus is located on the focal plane of the off-axis three-mirror system. The optical wavefront emitted by the interferometer is transformed into a plane wave by the off-axis three-mirror optical system, collimated by the plane mirror, and then converged at the optical system's focal point after passing through the optical system again.

[0005] The detection wavefront emitted by the interferometer carries information about the wavefront aberration of the off-axis three-mirror optical system being measured and returns to the interferometer. The interferometer then measures the wavefront aberration of the system in the corresponding field of view under the current state. Based on the results of the autocollimation interferometry measurement, a misalignment solution model can be established through computer-assisted alignment and sensitivity analysis methods. This allows for fine adjustment of the mirrors of the off-axis optical system being measured to achieve the desired optical path state.

[0006] A large-field-of-view, off-axis, three-mirror optical system, with windows, filters, dichroic filters, and other planar optical components connected to it, enables high-resolution imaging, multi-channel imaging, and multispectral imaging. While optical flat plates are afocal components and do not affect the optical power of the optical system, parallel flat plates are not ideal optical components and cannot form perfect images.

[0007] In particular, when an optical plate is placed in a converging optical path, its impact on overall aberrations is even more significant, especially in optical paths with large relative apertures and wide fields of view. Therefore, during the optical design phase of an off-axis three-mirror system, the effect of the parallel plate thickness on aberrations must be considered. During this phase, the optical components of the off-axis three-mirror system, such as windows, filters, color separators, and beam splitters, must be considered to form a complete imaging optical path.

[0008] After the off-axis three-mirror optical system is processed, only when the three single mirrors and the system optical path have been fully tested and meet the predetermined indicators can the next step of work be carried out and the overall integration and testing of the optoelectronic remote sensing system be participated in.

[0009] However, in the self-collimating interferometry measurement optical path of the off-axis three-mirror optical system, if the position and posture of the three mirrors are not constrained, the optical path integrated by the three mirrors will only move in the direction of minimizing the wave aberration and fall into the local minimum, making it difficult to meet the image quality requirements of the entire field of view and deviating from the predetermined optical path state.

[0010] If the alignment test of an off-axis three-mirror optical system fails to consider the effects of optical plates such as windows and filters in the rear optical path, the wavefront design residuals of the optical system composed of the three mirrors will increase. When aligning the optical path using autocollimation interferometers, the residual aberrations of the off-axis three-mirror system and the wavefront errors caused by mirror position errors are mixed, making them difficult to distinguish and separate, which greatly complicates the alignment of the optical system.

[0011] In the off-axis three-mirror optical system in the visible light band, the windows and filters in the subsequent optical path can usually pass through the working band of the interferometer. Therefore, in the off-axis three-mirror self-alignment test and adjustment optical path in the visible light band, an optical plate made of the same glass material and the same thickness is used as a compensator, which naturally makes it possible to achieve the consistency between the state of the off-axis three-mirror self-alignment test optical path and the designed optical path.

[0012] However, the optical path of off-axis three-mirror optical systems operating in the short-wave near-infrared, medium-wave infrared, and long-wave infrared bands usually uses optical materials such as single-crystal silicon (Silicon), single-crystal germanium (Germanium), and zinc selenide (Zinc Selenide). Most of these materials cannot pass through the operating band of the laser interferometer, especially after being coated for the operating band, making it even more difficult to pass the operating wavelength of the laser interferometer.

[0013] Optical materials in the infrared range have a high refractive index, and even a small thickness cannot be ignored. Therefore, the impact of these windows on aberration correction must be considered during the design of the aforementioned optical system. Therefore, when assembling and testing off-axis three-mirror optical systems in the mid- and long-wave infrared range, the effects of optical components such as infrared windows and filters must be considered to achieve an optical path state that is close to or consistent with the actual system requirements.

[0014] Drawing on the concepts of compensator design and zero compensation inspection for aspheric surface detection, it is necessary to design an optical compensator for the off-axis three-mirror self-collimation interference optical path, especially for the installation and adjustment test of medium-wave infrared or long-wave infrared off-axis three-mirror optical systems and off-axis three-mirror optical systems with flat components such as windows and color separators in the rear optical path.

[0015] That is, in the autocollimation interferometry optical path, the optical compensator is used to replace the optical flat plate element of the rear optical path, and at the same time compensate for the residual aberration of the off-axis three-mirror autocollimation optical path, so as to solve the installation and test problems of optical systems with window plates and filters in the rear optical path, thereby obtaining the off-axis three-mirror measurement and installation optical path closest to the original system optical path.

[0016] Two existing research results exist. In 2021, Li Zhaoyang et al. reported in the Acta Optica Sinica a compensator structure for an off-axis three-mirror optical system. This structure employs a set of lenses installed in the optical path behind the off-axis three-mirror system to serve as a compensator for system adjustment and testing. However, simulation analysis revealed that this approach has design deficiencies. It fails to consider the constraints of the original optical path state. During the design of the pre-installed off-axis three-mirror optical system compensator, there is no correlation with the original system, making it difficult to obtain a consistent optical path state.

[0017] Li Ming and colleagues at the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, used a computational holographic compensator to compensate for the intermediate real image plane of a Rug-type off-axis three-mirror optical system. This compensator is used for alignment of the primary and secondary mirrors in off-axis three-mirror optical systems. The design and fabrication of CGH compensators are challenging, requiring high machining precision. The proposed method offers advantages in both simplicity and implementation.

[0018] Different from the existing technology and the methods reported in the literature, the present invention uses the designed off-axis three-mirror optical system as a constraint to optimize the design of the compensator of the off-axis three-mirror self-collimation optical path, which not only compensates for the aberration of the self-collimation interference optical path, but also replaces the optical elements of the rear optical path, thereby obtaining the best compensator design. Only by performing the self-collimation adjustment test of the off-axis three-mirror optical system according to this optimal compensator can the optical state be close to the design value.

[0019] That is, compared with the existing methods, the compensator structure and design method proposed in the present invention perform self-collimation interference adjustment and testing on the off-axis three-mirror optical system, minimize the impact on the optical path state, and obtain an optical path state closest to the design value. Summary of the Invention

[0020] According to the application background of off-axis three-mirror optical system alignment test, a compensator structure for off-axis three-mirror optical system alignment test was proposed.

[0021] The main content of the present invention is: adding an optical flat plate of a certain thickness as a compensator for the off-axis three-mirror optical system after the interferometer focus of the self-collimation interference measurement light path.

[0022] The light emitted by the interferometer diverges through the interferometer focus 1 and passes through the compensator, and then passes through the three-mirror 3, the secondary mirror 4, the primary mirror 5, and the incident self-collimating plane mirror 6 in sequence; then the light returns from the plane mirror 6 along the original path, passes through the primary mirror 5, the secondary mirror 4, the three-mirror 3 in sequence, passes through the compensator again, and finally converges to the interferometer focus 1, forming an off-axis three-mirror self-collimating interference optical path with a compensator.

[0023] The number of the optical flat plates 2 is one or more, and the material is selected according to the working band of the interferometer and optically coated.

[0024] The steps of the method for calculating the thickness of the optical plate 2 are as follows:

[0025] 1) In the optical design software, create Configuration I. The optical path model is the original off-axis three-mirror optical system, including the off-axis three-mirror optical system and the window, filter, and dichroic optical elements. All parameters remain unchanged. That is, Configuration I has no design variables and serves as a constraint.

[0026] 2) Establish Configuration II. The optical path model is an off-axis three-mirror optical system and a compensator. The thickness of the compensator, the distance from the compensator to the interferometer focus 1, and the distance from the compensator to the vertex of the three mirrors 3 are the design variables of Configuration II.

[0027] 3) Establish Configuration III. The optical path model is an off-axis three-mirror optical system and a compensator. The thickness of the compensator is the same as that of Configuration II. At the same time, the two spacing parameters of the reflectors, the distance from the compensator to the interferometer focus 1, and the distance from the compensator to the vertex of the three mirrors 3 are used as design variables for Configuration III.

[0028] 4) Establish Configuration IV, the original off-axis three-mirror optical system, including the original components such as windows, filters, and color separations. The two spacing parameters of the reflectors are the same as the two corresponding parameters of Configuration III, and only the back intercept is used as the design variable of Configuration IV;

[0029] 5) By minimizing the wavefront aberration, the design variables of configurations I, II, III, and IV are optimized simultaneously to obtain the optimal optical compensator thickness and reflection position parameters.

[0030] The technical effects of the present invention are as follows:

[0031] (a) The compensator is a small-diameter optical plate with a small clear aperture, which can be manufactured in conventional sizes of approximately 25 mm to 50 mm. The compensator is assembled as a module using existing optical fixtures and can be independently assembled and then integrated with the autocollimation interferometer optical path. The simple compensation structure facilitates the assembly, adjustment, and testing of the off-axis three-mirror system.

[0032] (b) The system compensator is close to the interferometer focus and is insensitive to thickness and angle errors, as well as installation distance errors. As long as the thickness error of the compensator is properly controlled, the needs of system assembly and testing can be met. Therefore, the compensator of the present invention is extremely easy to process and test.

[0033] (c) The compensation effect is significant and is particularly suitable for the assembly and adjustment test of off-axis three-mirror optical systems with medium-wave infrared or long-wave infrared, as well as off-axis three-mirror optical systems with flat components such as windows and color separators in the rear optical path, thus overcoming the shortcomings of existing methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of the self-collimation interferometry principle of an off-axis three-mirror optical system with a compensator.

[0035] Figure 2 This is a diagram of a long-wave infrared off-axis three-mirror optical system with windows and filters.

[0036] Figure 3 It is a long-wave infrared off-axis three-mirror optical system equipped with a compensator. DETAILED DESCRIPTION

[0037] The present invention provides a compensator structure for the assembly and adjustment testing of an off-axis three-mirror optical system. The core feature of the compensator structure is that an optical plate 1 of appropriate thickness is placed in front of the focus of the interferometer in the self-collimating interference optical path composed of the interferometer and the off-axis three-mirror optical system to be tested, thereby achieving the equivalent replacement of the parallel plate optical element in the optical path behind the off-axis three-mirror and the aberration compensation function.

[0038] The thickness design of optical plate compensator is a key step in compensator design.

[0039] The specific implementation steps for the thickness design of the optical plate compensator are as follows:

[0040] Step (a) In the optical design software, a configuration I is established. The optical path model is the original off-axis three-mirror optical system, including the off-axis three-mirror optical system and optical components such as windows, filters, and dichroic filters. All parameters remain unchanged, that is, configuration I has no design variables.

[0041] Step (b) establishing Configuration II, where the optical path model is an off-axis three-mirror optical system and a compensator, wherein the thickness of the compensator, the distance from compensator 2 to the interferometer focus 1, and the distance from the compensator to the vertex of the three mirrors 3 are used as design variables for Configuration II;

[0042] Step (c) establishing Configuration III, where the optical path model is an off-axis three-mirror optical system and a compensator, wherein the thickness of the compensator is the same as the corresponding parameters of Configuration II, and the two spacing parameters of the reflectors, the distance from the compensator to the interferometer focus 1, and the distance from the compensator to the vertex of the three mirrors 3 are used as design variables for Configuration III;

[0043] Step (d) establishing Configuration IV, an off-axis three-mirror optical system, including components such as windows, filters, and color separation plates, wherein the two spacing parameters of the reflectors are the same as the two corresponding parameters of Configuration III, and only the back intercept is used as a design variable for Configuration IV;

[0044] In step (e), the design variables of configurations I, II, III, and IV are optimized simultaneously by minimizing the wavefront aberration, thereby obtaining the optimal optical compensator thickness and reflection position parameters.

[0045] According to the above steps, the optimal compensator thickness, clear aperture and other parameters are optimized. According to the working band of the interferometer, the appropriate glass material and coating treatment are selected, and it is installed at the appropriate position of the interferometer focus in the off-axis three-mirror self-collimation optical path.

[0046] The key step in the thickness design of the optical flat compensator of the present invention is to utilize the multiple configuration function of the optical design software to establish a reference and constraint relationship between the designed optical system and the off-axis three-mirror optical system for installation and testing. Under the constraints of the designed optical system, the compensator parameters of the installation and testing optical path are optimized to achieve the optimal design of the compensator.

[0047] The present invention is further described below with reference to the accompanying drawings and examples. The parameters of the optical system design are as follows:

[0048] Effective aperture: 200.0mm;

[0049] Field of view: 12.0°

[0050] Focal length: 400.0mm;

[0051] Working band: 8.0~10.0μm.

[0052] The window is made of single crystal germanium with a thickness of 8.0 mm, and the filter is made of single mirror germanium with a thickness of 1.0 mm. The initial design parameters of the optical system are as follows:

[0053]

[0054]

[0055] Based on the above initial optical path parameters, after proper optimization and in combination with appropriate constraints, the required optical system is obtained.

[0056] According to the final optical design model, the optical compensator model is established, which includes four configuration states.

[0057] Configuration (I) Original optical design, no optimized variables.

[0058] Configuration (II) Three mirrors and a compensator. The optical compensator is made of quartz. The distance from the three mirrors to the compensator, the distance from the compensator to the interferometer, and the thickness of the compensator are three parameters set as optimization variables.

[0059] Configuration (III) consists of three mirrors and a compensator, where the thickness of the compensator is equal to the corresponding parameters of configuration (II), and the two spacing parameters of the three mirrors, the distance from the compensator to the interferometer focus, and the distance from the compensator to the vertex of the three mirrors are set as optimization variables.

[0060] Configuration (IV) is the original optical design, where the two spacing parameters of the three mirrors are equal to the corresponding parameters of configuration (III), and the back intercept is set as the optimization variable. The other parameters are the same as those of configuration (I) and remain unchanged.

[0061] By simultaneously optimizing the design variables for Configurations I, II, III, and IV to minimize wavefront aberration, the optimal optical plate compensator thickness was determined to be 3.79 mm. Using this compensator to adjust the autocollimation optical path, the spacing between the three mirrors in the off-axis optical system varied by only 0.25 mm and 0.09 mm.

[0062] The three mirrors were adjusted using a 3.79mm quartz optical plate compensator, resulting in an optical system that was consistent with the design. Substituting this off-axis mirror configuration into the original optical path, imaging quality parameters such as speckle and optical transfer function remained consistent with the original system's optical path.

[0063] Without using a compensator to adjust the self-collimating optical path, the distance between the three reflectors of the off-axis optical system changes by 2.77mm and 1.37mm, and the corresponding back intercept change is 34.88mm.

[0064] The above examples have been verified by actual assembly and adjustment tests, proving the accuracy and feasibility of the compensator structure and design method of the present invention. The compensator can be fixed in front of the interferometer focus using shelf products such as lens clamps and two-dimensional linear slides.

[0065] In summary, the present invention provides a structure and design method for an off-axis three-mirror optical system compensator. On the basis of the off-axis three-mirror self-collimating interferometer measurement optical path, an optical compensator is added to the interferometer focus, which can not only replace the optical path difference of optical components such as the window filter in the rear optical path, but also compensate for the aberration of the optical components in the rear optical path, so that the assembly and adjustment test of the off-axis three-mirror optical system can be carried out smoothly and the optical path state closest to the predetermined one can be obtained.

[0066] The compensator design method of the present invention can be further extended to design, assemble, adjust and test compensators for self-collimating optical paths of off-axis three-mirror optical systems with lens groups in the rear optical path, and has wider applicability.

[0067] The above description is only a common implementation case of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An off-axis three-mirror optical system compensator, characterized in that: An optical plate (2) of a certain thickness is inserted in front of the interferometer focus of the self-collimation interference light path of the off-axis three-mirror optical system as a compensator; The light emitted by the interferometer diverges through the interferometer focus (1) and passes through the compensator, and then passes through the three mirrors (3), the secondary mirror (4), the primary mirror (5), and the incident self-collimating plane mirror (6); then the light returns from the plane mirror (6) along the original path, passes through the primary mirror (5), the secondary mirror (4), the three mirrors (3), and passes through the compensator again, and finally converges to the interferometer focus (1), forming an off-axis three-mirror self-collimating interference light path with a compensator; The steps of the method for calculating the thickness of the optical plate (2) are as follows: 1) In the optical design software, create Configuration I. The optical path model is the original off-axis three-mirror optical system, including the off-axis three-mirror optical system and the window, filter, and dichroic optical elements. All parameters remain unchanged. That is, Configuration I has no design variables and serves as a constraint. 2) Establish Configuration II. The optical path model is an off-axis three-mirror optical system and a compensator. The thickness of the compensator, the distance from the compensator to the interferometer focus (1), and the distance from the compensator to the vertex of the three mirrors (3) are the three parameters used as design variables for Configuration II. 3) Establish configuration III. The optical path model is an off-axis three-mirror optical system and a compensator. The thickness of the compensator is the same as the corresponding parameters of configuration II. At the same time, the two spacing parameters of the reflectors, the distance from the compensator to the interferometer focus (1), and the distance from the compensator to the vertex of the three mirrors (3) are used as design variables of configuration III. 4) Establish Configuration IV, the original off-axis three-mirror optical system, including the original components such as windows, filters, and color separations. The two spacing parameters of the reflectors are the same as the two corresponding parameters of Configuration III, and only the back intercept is used as the design variable of Configuration IV; 5) By minimizing the wavefront aberration, the design variables of configurations I, II, III, and IV are optimized simultaneously to obtain the optimal optical compensator thickness and reflection position parameters.

2. The off-axis three-mirror optical system compensator according to claim 1, wherein: The number of the optical flat plates (2) is one or more, and the material is selected according to the working band of the interferometer and optically coated.

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