Precise assembly method of off-axis three-mirror optical system

Abstract

The invention relates to an assembling method of an optical system, in particular to a precise assembling method of an off-axis three-mirror optical system, and solves the problems that an existing off-axis three-mirror optical system has non-rotational symmetry and optical axes of all components are not on the same axis, so that asymmetric wave aberration exists in the off-axis three-mirror optical system; and high-precision assembly cannot be realized. The precise assembly method of the off-axis three-mirror optical system comprises the following steps: 1) roughly positioning the primary mirror assembly, the secondary mirror assembly and the three-mirror assembly respectively until the angular deviation of each mirror group is less than or equal to 10 ''and the eccentric tolerance is less than or equal to 0.05 mm; the method comprises the steps of (1) analyzing the wave aberration of the off-axis three-mirror optical system and screening the freedom degree according to the wave aberration to carry out image quality correction, (3) calculating the misalignment amount M of a secondary mirror and a third mirror, and (4) precisely adjusting the angles of the secondary mirror and the third mirror according to the misalignment amount M to enable the full-view-field wave aberration of the off-axis three-mirror optical system to be smaller than a threshold value, and completing precise assembly of the off-axis three-mirror optical system.

Description

Technical Field

[0001] The invention relates to an assembly method of an optical system, in particular to a precision assembly method of an off-axis three-mirror optical system. Background Art

[0002] Reflective optical systems are widely used in modern space-based and air-based large-aperture telescopes. Coaxial reflective optical systems generally adopt the form of primary and secondary mirrors and lenses. In coaxial reflective optical systems, the secondary mirror structure causes the system to be blocked at the center, limiting the limited range of the image plane, thereby restricting the field of view of the coaxial reflective optical system and causing light energy loss. Secondly, the center blockage will also cause the modulation transfer function of the coaxial reflective optical system to decrease in the medium and low frequency parts, affecting its final imaging quality. The coaxial reflective optical system has a small applicable band range. In order to solve the problems brought by the coaxial reflective optical system, optical designers construct off-axis reflective optical systems by means of pupil off-axis, field of view off-axis, and tilt or eccentricity of the reflector elements. This off-axis reflective optical system has the advantages of a reflective optical system and can meet the imaging requirements of a large field of view and full band, while avoiding edge scattering effects and center blockage, with higher energy concentration and dynamic range, and can meet the needs of high-resolution extreme detection.

[0003] The off-axis three-mirror optical system is a typical off-axis reflective optical system. It has been widely used in many fields due to its advantages such as no obstruction, no ghost image, and large field of view. However, due to its non-rotational symmetry, there are asymmetric wave aberrations in the off-axis three-mirror optical system, and the optical axes of the components are not on the same axis. It is impossible to use the traditional coaxial optical system assembly method to achieve high-precision assembly, which brings great challenges to the assembly of the off-axis three-mirror optical system. Summary of the invention

[0004] The purpose of the present invention is to solve the technical problem that the existing off-axis three-mirror optical system has non-rotational symmetry and the optical axes of each component are not on the same axis, resulting in asymmetric wave aberration in the off-axis three-mirror optical system and inability to achieve high-precision assembly, and to provide a precision assembly method for an off-axis three-mirror optical system.

[0005] In order to achieve the above object, the present invention adopts the following technical solution:

[0006] A precision assembly method for an off-axis three-mirror optical system, the off-axis three-mirror optical system comprising a primary mirror assembly, a secondary mirror assembly and a three-mirror assembly; the special feature of the method is that the method comprises the following steps:

[0007] 1] Taking the primary mirror assembly as the reference, roughly position the primary mirror assembly, secondary mirror assembly and triple mirror assembly respectively until the angle deviation of each mirror assembly is less than or equal to 10″ and the eccentricity tolerance is less than or equal to 0.05mm;

[0008] 2] Use Zemax optical design software to analyze the wavefront aberration of the off-axis three-mirror optical system, and adjust the rotational freedom of the secondary mirror assembly and the three-mirror assembly on the x-axis and y-axis according to the wavefront aberration to perform image quality correction;

[0009] 3] The full-field Zernike wave aberration coefficient matrix ΔZ and the sensitivity matrix A are analyzed using Zemax optical design software, and the misalignment M of the secondary mirror (7) and the tertiary mirror (12) is calculated:

[0010] M=(A T A+ρI) -1 A T ΔZ

[0011] Among them, A T is the transposed matrix of the sensitivity matrix A; ρ is the damping coefficient; I represents the unit matrix;

[0012] 4] According to the misalignment amount M, the angles of the secondary mirror and the tertiary mirror are precisely adjusted so that the wave aberration of the off-axis three-mirror optical system in the entire field of view is less than the threshold value, thus completing the precise assembly of the off-axis three-mirror optical system.

[0013] Further, step 1] is specifically as follows:

[0014] 1.1) Roughly position the primary mirror assembly;

[0015] 1.2) Transfer the reference of the back reference surface A of the primary mirror in the primary mirror assembly to the third mirror and the secondary mirror;

[0016] 1.3) Roughly position the secondary mirror assembly;

[0017] 1.4) Roughly position the three-mirror assembly.

[0018] Further, step 1.1) is specifically as follows:

[0019] 1.1.1. Preliminary positioning of the main mirror assembly: reserve the main mirror assembly installation hole on the base plate, and the main mirror assembly is fixedly connected to the base plate installation hole through the threaded hole of the main mirror base;

[0020] 1.1.2, set the reference surface;

[0021] The upper surface of the bottom plate in the off-axis three-mirror optical system is used as the bottom plate reference surface C;

[0022] The protruding side wall surface of the bottom plate close to the primary mirror and parallel to the primary mirror is used as the bottom plate reference surface D;

[0023] The protruding side wall surface of the bottom plate away from the primary mirror and parallel to the primary mirror is used as the bottom plate reference surface E;

[0024] The side wall surface of the bottom plate close to the primary mirror and perpendicular to the primary mirror is used as the bottom plate reference surface B;

[0025] The side wall surface of the bottom plate away from the primary mirror and perpendicular to the primary mirror is used as the bottom plate reference surface F;

[0026] 1.1.3. Adjust the angle of the primary mirror;

[0027] The first plane reflector is set at the base plate reference plane D, and the first theodolite and the first plane reflector are used for self-alignment to derive the reference direction; then, the angle between the back reference plane A of the primary mirror and the base plate reference plane D is adjusted according to the reference direction, so that the back reference plane A of the primary mirror is perpendicular to the optical axis of the first theodolite, and thus parallel to the base plate reference plane D;

[0028] 1.1.4. Adjust the eccentricity of the primary mirror;

[0029] The outer cylindrical surface of the primary mirror is measured by using a three-coordinate measuring arm to fit the primary mirror axis. Taking the base plate reference surfaces B, C, and D as references, the primary mirror eccentricity is adjusted according to the measured distance x between the primary mirror axis and the base plate reference surface C, the horizontal distance y between the primary mirror axis and the base plate reference surface B, and the horizontal distance z between the primary mirror back reference surface A and the base plate reference surface D;

[0030] 1.1.5. Determine whether the angle deviation between the primary mirror back reference plane A and the base plate reference plane D is less than or equal to 10″, and whether the primary mirror eccentricity tolerance is less than or equal to 0.05 mm. If so, proceed to step 1.2), otherwise return to step 1.1.3.

[0031] Further, step 1.2) is specifically as follows:

[0032] 1.2.1. Perform benchmark transfer on the three mirrors;

[0033] The first theodolite is used to align with the back reference surface A of the primary mirror to indicate the reference direction of the back reference surface A of the primary mirror; the first theodolite is rotated 90° counterclockwise and then sighted with the second theodolite to transmit the reference direction of the back reference surface A of the primary mirror to the second theodolite; the second theodolite is then rotated 90° counterclockwise and its optical axis is used to indicate the reference direction of the back reference surface A of the primary mirror;

[0034] 1.2.2. Perform benchmark transfer on the secondary mirror;

[0035] After the first theodolite is rotated 90° clockwise, it is aligned with the first plane reflector at the base plate reference surface D to calibrate the reference direction. Then, the first theodolite is translated to a position corresponding to the three mirrors. After the first theodolite is rotated 90° clockwise, it is aligned with the second plane reflector located on the optical path of the first theodolite to ensure that the reference direction remains unchanged. Then, the first theodolite is rotated 90° counterclockwise to align with the third theodolite near the base plate reference surface E, and the back reference surface A of the primary mirror is transferred to the third theodolite.

[0036] Further, step 1.3) is specifically as follows:

[0037] 1.3.1. Preliminary positioning of the secondary mirror assembly: reserve a mounting hole for the secondary mirror assembly on the base plate, and fix the secondary mirror assembly to the mounting hole of the base plate through the threaded hole of the secondary mirror support seat;

[0038] 1.3.2. Adjust the angle of the secondary mirror;

[0039] According to the theoretical difference M of the angle between the secondary mirror back reference plane and the primary mirror back reference plane A, the third theodolite is rotated counterclockwise by 0.38° to determine the secondary mirror back reference direction, and the angle between the secondary mirror back reference plane and the primary mirror back reference plane A is adjusted according to the secondary mirror back reference direction and self-aligned with the third theodolite;

[0040] 1.3.3. Adjust the eccentricity of the secondary mirror;

[0041] Use the three-coordinate measuring arm to measure the outer cylindrical surface of the secondary mirror and fit the secondary mirror axis. Take the three surfaces of the base plate reference planes B, C, and E as references, and adjust the secondary mirror eccentricity according to the measured horizontal distance y1 between the secondary mirror axis and the reference plane B, the distance x1 between the secondary mirror axis and the base plate reference plane C, and the horizontal distance z1 between the secondary mirror back reference plane and the base plate reference plane E;

[0042] 1.3.4. Determine whether the angle deviation between the secondary mirror back reference plane and the primary mirror back reference plane A is less than or equal to 10", and whether the secondary mirror eccentricity tolerance is less than or equal to 0.05mm. If so, execute step 1.4), otherwise return to step 1.3.2.

[0043] Further, step 1.4) is specifically as follows:

[0044] 1.4.1. Preliminary positioning of the three-mirror assembly: reserve the installation holes of the three-mirror assembly on the base plate, and fix the three-mirror assembly to the installation holes of the base plate through the threaded holes of the three-mirror support seat;

[0045] 1.4.2. Adjust the angles of the three mirrors;

[0046] According to the theoretical difference N between the angles of the three-mirror back reference plane and the primary mirror back reference plane A, the second theodolite is rotated 0.2° clockwise to determine the three-mirror back reference direction. According to the three-mirror back reference direction, the angles of the three-mirror back reference plane and the primary mirror back reference plane A are adjusted and self-aligned with the second theodolite.

[0047] 1.4.3. Adjust the eccentricity of the three mirrors;

[0048] Use the three-coordinate measuring arm to measure the outer cylindrical surface of the three mirrors and fit the three mirror axes. Take the base plate reference planes C, D, and F as references, and adjust the eccentricity of the three mirrors according to the measured horizontal distance x2 from the three mirror axes to the reference plane C, the horizontal distance z2 from the back reference plane of the three mirrors to the base plate reference plane D, and the horizontal distance y2 from the three mirror axes to the base plate reference plane F;

[0049] 1.4.4. Determine whether the angle deviation between the back reference plane of the three mirrors and the back reference plane A of the main mirror is less than or equal to 10", and whether the eccentricity tolerance of the three mirrors is less than or equal to 0.05mm. If so, proceed to step 2], otherwise return to step 1.4.2.

[0050] Further, step 2] is specifically as follows:

[0051] 2.1) Build a wavefront aberration detection system to obtain the wavefront aberration of the off-axis three-mirror optical system;

[0052] 2.2) The wavefront aberration of the off-axis three-mirror optical system is analyzed by Zemax optical design software, and the tx1 / ty1 rotational freedom of the secondary mirror and the tx2 / ty2 rotational freedom of the three mirrors can be obtained by screening, where tx1 represents the rotation of the secondary mirror along the x1 axis, ty1 represents the rotation of the secondary mirror around the y1 axis, tx2 represents the rotation of the three mirrors around the x2 axis, and ty2 represents the rotation of the three mirrors around the y2 axis;

[0053] 2.3) Adjust the rotation of the secondary mirror tx1 / ty1 and the tertiary mirror tx2 / ty2 to achieve image quality correction.

[0054] Further, step 2.1) is specifically as follows:

[0055] 2.1.1. Use the first theodolite and the primary mirror back reference surface A for self-alignment to derive the reference direction;

[0056] 2.1.2. Keep the reference direction and translate the first theodolite to the side of the primary mirror away from the three mirrors, and rotate the first theodolite 9° clockwise to indicate the normal direction of the third plane reflector set on the E side of the base plate reference surface;

[0057] 2.1.3. Adjust the third plane reflector and align it with the first theodolite to complete the positioning of the third plane reflector;

[0058] 2.1.4. A 4D laser interferometer is set at a position corresponding to the three mirrors, and the 4D laser interferometer is used to emit laser to the three mirrors and detect the wave aberration of the off-axis three-mirror optical system.

[0059] Furthermore, in step 3, the misalignment M of the secondary mirror and the tertiary mirror is calculated using the adaptive damped least squares method; the size of the sensitivity matrix A is 25×12; and the size of the full-field Zernike wave aberration coefficient matrix ΔZ is 25×1.

[0060] Further, in step 4], the threshold is 0.05λ@632.8nm.

[0061] Beneficial effects of the present invention:

[0062] (1) The off-axis three-mirror optical system assembled by the precision assembly method of the off-axis three-mirror optical system of the present invention has a focal length of 1800 mm, an off-axis field of view of 9°, and a field of view size of ±1°. Within the full field of view, the wavefront error RMS of the off-axis three-mirror optical system is better than 0.05λ@632.8nm, the diffuse spot is smaller than the Airy disk size, reaching the diffraction limit, and the imaging quality is excellent.

[0063] (2) The tolerance analysis of the off-axis three-mirror optical system was carried out. According to the results of the tolerance analysis, with the main mirror as the reference, the angle deviation of the remaining mirror groups can be controlled within 10″, and the eccentricity tolerance can be controlled within 0.05mm to ensure that the imaging quality meets the requirements.

[0064] (3) Based on the requirements of angular deviation and eccentricity tolerance, the present invention adopts a three-coordinate measuring arm and multiple autocollimation theodolites to roughly locate the optical components (i.e., the primary mirror assembly, the secondary mirror assembly and the three-mirror assembly), determine the spatial position of each optical component in three dimensions, and realize precise measurement and eccentricity adjustment with an accuracy of up to 0.02 mm.

[0065] (4) The present invention aims at the fact that it is impossible to explore the quantitative relationship between the wave aberration and the degree of freedom of the misaligned system through artificial experience in the off-axis three-mirror optical system, analyzes the influence characteristics of the 6 degrees of freedom of each mirror group on the wave aberration of the off-axis three-mirror optical system, and screens out the appropriate degrees of freedom to perform misalignment correction of the system wave aberration. In the off-axis three-mirror optical system, the mirror groups to be adjusted are the secondary mirror and the tertiary mirror, with a total of 12 degrees of freedom. Since it is costly and difficult to adjust all the degrees of freedom of all mirror groups, the degrees of freedom sensitive to the wave aberration are determined based on the sensitivity matrix and the two-dimensional adjustment mechanism of the secondary mirror and the two-dimensional adjustment mechanism of the tertiary mirror are adjusted to avoid blind adjustment. The insensitive degrees of freedom can be guaranteed by structural limiting and coarse adjustment.

[0066] (5) The adaptive damped least square method adopted in the present invention selects a suitable damping coefficient through an algorithm to accurately solve the misalignment to achieve an excellent image quality correction effect. It can avoid over-adjustment while ensuring the accuracy and reliability of the correction result.

[0067] (6) The method of the present invention can be applied to the precise assembly of off-axis multi-reflection optical systems, providing an efficient and reliable method for the precise adjustment of complex optical systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1is a schematic diagram of the three-dimensional structure of an off-axis three-mirror optical system in an embodiment of the present invention;

[0069] Figure 2 yes Figure 1 A top view of

[0070] FIG3( a ) is a schematic diagram showing the rough positioning of a primary mirror assembly in an embodiment of a precision assembly method for an off-axis three-mirror optical system of the present invention;

[0071] FIG3( b ) is a bottom view of FIG3( a );

[0072] Figure 4 It is a schematic diagram of the reference transfer from the primary mirror to the third mirror in an embodiment of a precision assembly method of an off-axis three-mirror optical system of the present invention;

[0073] Figure 5 It is a schematic diagram of the primary mirror to secondary mirror reference transfer in an embodiment of a precision assembly method of an off-axis three-mirror optical system of the present invention;

[0074] FIG6( a ) is a schematic diagram showing the rough positioning of a secondary mirror assembly in an embodiment of a precision assembly method for an off-axis three-mirror optical system of the present invention;

[0075] FIG6( b ) is a bottom view of FIG6( a );

[0076] FIG. 7( a ) is a schematic diagram showing the rough positioning of three mirror components in an embodiment of a precision assembly method for an off-axis three-mirror optical system of the present invention;

[0077] FIG7( b ) is a bottom view of FIG7( a );

[0078] Figure 8 It is a schematic diagram of image quality detection in an embodiment of a precision assembly method of an off-axis three-mirror optical system of the present invention;

[0079] Fig. 9 This is a full-field wave aberration result diagram of an embodiment of a precision assembly method for an off-axis three-mirror optical system of the present invention.

[0080] Description of reference numerals:

[0081] 1. Primary mirror base; 2. Primary mirror rib plate; 3. Primary mirror support seat; 4. Primary mirror seat; 5. Primary mirror; 6. Secondary mirror aperture; 7. Secondary mirror; 8. Secondary mirror base; 9. Secondary mirror two-dimensional adjustment mechanism; 10. Secondary mirror two-dimensional adapter frame; 11. Secondary mirror support seat; 12. Third mirror; 13. Third mirror base; 14. Three-mirror two-dimensional adapter frame; 15. Three-mirror two-dimensional adjustment mechanism; 16. Three-mirror support seat; 17. Bottom plate; 18. First plane reflector; 19. Second plane reflector; 20. Third plane reflector; 21. 4D laser interferometer; 22. First theodolite; 23. Second theodolite; 24. Third theodolite. DETAILED DESCRIPTION

[0082] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0083] like Figure 1 , Figure 2 As shown, an off-axis three-mirror optical system includes a base plate 17, a primary mirror assembly integrated on the base plate 17, a secondary mirror assembly, and a three-mirror assembly; the primary mirror assembly includes a primary mirror base 1, a primary mirror rib plate 2, a primary mirror support seat 3, a primary mirror seat 4 and a primary mirror 5; the secondary mirror assembly includes a secondary mirror aperture 6, a secondary mirror 7, a secondary mirror base 8, a secondary mirror two-dimensional adjustment mechanism 9, a secondary mirror two-dimensional adapter frame 10 and a secondary mirror support seat 11; the three-mirror assembly includes a three-mirror 12, a three-mirror support seat 16, a three-mirror two-dimensional adapter frame 14, a three-mirror two-dimensional adjustment mechanism 15 and a three-mirror support seat 16; wherein the primary mirror assembly is connected to the base plate 17 through the primary mirror base 1; the secondary mirror assembly is connected to the base plate 17 through the secondary mirror two-dimensional adapter frame 10, the secondary mirror two-dimensional adjustment mechanism 9 and the secondary mirror support seat 11; similarly, the three-mirror assembly is connected to the base plate 17 through the three-mirror two-dimensional adapter frame 14, the three-mirror two-dimensional adjustment mechanism 15 and the three-mirror support seat 16; the base plate 17 provides a bottom mounting reference for all structures.

[0084] The back reference surfaces of the primary mirror 5, the secondary mirror 7 and the third mirror 12 are polished, and the verticality deviation between the polished surface and the optical axis of the mirror group is better than 30", which can be used as the reference direction of the mirror group. After the parallel light enters the off-axis three-mirror optical system, it passes through the primary mirror 5, the secondary mirror 7 and the third mirror 12 in sequence, and finally forms an ideal image at the rear end focus. The off-axis three-mirror optical system installed by the method of the present invention has a focal length of 1800mm, an off-axis field of view of 9°, and a field of view size of ±1°. Within the full field of view, the wavefront error RMS of the off-axis three-mirror optical system is better than 0.05λ@632.8nm, the diffuse spot is smaller than the Airy disk size, reaching the diffraction limit, and the imaging quality is excellent.

[0085] After tolerance analysis, taking the primary mirror assembly as the reference, the angle deviation between the secondary mirror 7 and the tertiary mirror 12 is 10″ and the eccentricity tolerance is 0.05mm. The mechanical positioning accuracy of the existing threaded connection cannot meet this requirement. The precision assembly and image quality adjustment of the off-axis three-mirror optical system involves the degrees of freedom of multiple components such as the secondary mirror 7 and the tertiary mirror 12, and it is difficult to accurately identify and adjust their degrees of freedom and their sizes through experience, making assembly more difficult.

[0086] The precision assembly method of the off-axis three-mirror optical system proposed in the present invention mainly includes three key steps: precise positioning of the primary mirror 5, adjustment of the secondary mirror 7 and the three-mirror 12, solution of the misalignment based on the sensitivity pair matrix, and optimization of the image quality. In the coarse adjustment stage, a multi-instrument network (i.e., a theodolite and a plane reflector) is used to measure and control the angle deviation of the mirror group within 10′, and the eccentricity tolerance is controlled within 0.05 mm to eliminate large misalignment deviations. In the fine adjustment stage, the misalignment of the secondary mirror 7 and the three-mirror 12 is accurately calculated through the sensitivity matrix algorithm, and the misalignment is accurately implemented using the secondary mirror two-dimensional adjustment mechanism 9 and the three-mirror two-dimensional adjustment mechanism 15, and a 4D laser interferometer 21 is used to ensure that the imaging quality of the off-axis three-mirror optical system meets the requirements.

[0087] The present invention firstly performs tolerance analysis on the mirror assembly. According to the tolerance analysis results, with the primary mirror 5 as the reference, the angle deviation of the remaining mirror assemblies should be controlled within 10″, and the eccentricity tolerance should be controlled within 0.05mm to ensure that the imaging quality meets the requirements. Based on the angle deviation and eccentricity tolerance requirements, network measurement is used to roughly position the optical components (i.e., the primary mirror assembly, the secondary mirror assembly, and the three-mirror assembly). The instruments used for the network measurement include a three-coordinate measuring arm and a plurality of autocollimating theodolites. The three-coordinate measuring arm has high precision, flexibility, and portability. By measuring the position of the probe in space, the three-dimensional coordinates of each point on the surface of the optical assembly are determined, and the relationship between the spatial points, lines, and surfaces is obtained, so as to fit the mirror axis and the outer cylindrical surface, determine the spatial position of each optical assembly in three dimensions, and realize precise measurement and eccentricity adjustment with an accuracy of up to 0.02mm. The autocollimating theodolite adopts the principle of optical autocollimation imaging, and can be self-aligned with the reserved reference surface of each mirror assembly to accurately control the spatial angle deviation of each mirror assembly within 10″. In this way, high-precision positioning of the optical assembly can be achieved. However, due to the existence of processing deviations, it still cannot fully meet the requirements of optical imaging. Ultimately, the primary mirror assembly, secondary mirror assembly, and three-mirror assembly need to be precisely adjusted based on the wave aberration measurement results. This process only eliminates large deviations.

[0088] Since the quantitative relationship between the wave aberration and the degree of freedom of the misaligned system cannot be explored through artificial experience in the off-axis three-mirror optical system, the influence characteristics of the 6 degrees of freedom of each mirror group on the wave aberration of the off-axis three-mirror optical system are analyzed, and the appropriate degrees of freedom are screened for misalignment correction of the system wave aberration. In particular, in the off-axis three-mirror optical system, the mirror groups to be adjusted are the secondary mirror 7 and the tertiary mirror 12, with a total of 12 degrees of freedom. Adjusting all the degrees of freedom of all mirror groups is costly and difficult to achieve. Therefore, the degrees of freedom sensitive to wave aberration are determined based on the sensitivity matrix and the adjustment mechanism is designed to avoid blind adjustment. Insensitive degrees of freedom can be guaranteed by structural limiting and coarse adjustment. Through quantitative analysis of the sensitivity matrix, the rotational degrees of freedom of the secondary mirror assembly and the tertiary mirror assembly on the x\y axis are most sensitive to the wave aberration of the off-axis three-mirror optical system, so these four degrees of freedom are adjusted as the misalignment amount of image quality correction.

[0089] The present invention is based on a precision assembly method of an off-axis three-mirror optical system, and the specific steps are as follows:

[0090] 1. Roughly position the main mirror assembly. Reserve the installation hole for the main mirror assembly on the base plate 17. The main mirror assembly is fixedly connected to the installation hole of the base plate 17 through the threaded hole of the main mirror base 1, which is used for the preliminary positioning of the main mirror assembly. Take the back of the main mirror 5 as the back reference plane A of the main mirror 5, take the base plate 17 as the reference and the upper surface of the base plate 17 in the off-axis three-mirror optical system as the base plate reference plane C; the protruding side wall surface of the base plate 17 close to the main mirror 5 and parallel to the main mirror 5 is taken as the base plate reference plane D; the protruding side wall surface of the base plate 17 away from the main mirror 5 and parallel to the main mirror 5 is taken as the base plate reference plane E; the side wall surface of the base plate 17 close to the main mirror 5 and perpendicular to the main mirror 5 is taken as the base plate reference plane B; the side wall surface of the base plate 17 away from the main mirror 5 and perpendicular to the main mirror 5 is taken as the base plate reference plane F; adjust the main mirror 5 to the angle deviation of each mirror group is less than or equal to 10", and the eccentricity tolerance is less than or equal to 0.05mm.

[0091] As shown in Fig. 3(a), the angle of the primary mirror 5 is adjusted. The first plane reflector 18 is used to be close to the base plate reference surface D, and the first theodolite 22 is used to align with the first plane reflector 18 to derive the reference direction. The back of the primary mirror 5 is polished, and the angle between the back reference surface A of the primary mirror 5 and the base plate reference surface D is adjusted according to the reference direction through the self-alignment imaging principle of the first theodolite 22 to ensure that the back reference surface A of the primary mirror 5 is aligned with the first theodolite 22. At this time, the reference direction of the back reference surface A of the primary mirror 5 is perpendicular to the optical axis of the first theodolite 22, and thus parallel to the base plate reference surface D, completing the rough positioning of the angle of the primary mirror 5.

[0092] As shown in Fig. 3(b), the eccentricity of the primary mirror 5 is adjusted. The outer cylindrical surface of the primary mirror 5 is measured by the three-coordinate measuring arm to fit the axis of the primary mirror 5. The three surfaces of the base plate reference planes B, C, and D are used as references. The eccentricity of the primary mirror 5 is adjusted according to the measured distance x between the axis of the primary mirror 5 and the base plate reference plane C, the horizontal distance y between the axis of the primary mirror 5 and the base plate reference plane B, and the horizontal distance z between the back reference plane A of the primary mirror 5 and the base plate reference plane D, ensuring that x = 251 ± 0.05 mm, y = 185 ± 0.05 mm, and z = 576 ± 0.05 mm. At this point, the rough adjustment of the eccentricity of the primary mirror 5 is completed, and the subsequent assembly is carried out based on the primary mirror 5.

[0093] Determine whether the angle deviation between the back reference plane A of the primary mirror 5 and the base reference plane D is less than or equal to 10″, and whether the eccentricity tolerance of the primary mirror 5 is less than or equal to 0.05 mm. If so, execute step 2, otherwise readjust the angle and / or eccentricity of the primary mirror 5.

[0094] 2. Transferring the back reference surface A of the primary mirror 5. After the angle and eccentricity adjustment of the primary mirror 5 is completed, the first theodolite 22, the second theodolite 23 and the third theodolite 24 are used to transfer the reference of the back reference surface A of the primary mirror 5 to the third mirror 12 and the secondary mirror 7.

[0095] like Figure 4 As shown, firstly, the three mirrors 12 are transferred to the reference, and the first theodolite 22 is aligned with the back reference surface A of the primary mirror 5 to characterize the normal direction (i.e., the reference direction) of the back reference surface A of the primary mirror 5. Then, the first theodolite 22 is rotated 90° counterclockwise by a two-dimensional translation platform and then aligned with the second theodolite 23, and the reference direction of the back reference surface A of the primary mirror 5 is transferred to the second theodolite 23. Then, the second theodolite 23 is rotated 90° counterclockwise by a two-dimensional translation platform, and its optical axis can characterize the normal direction of the back reference surface A of the primary mirror 5, and then the second theodolite 23 is used to adjust the reference of the three mirrors 12.

[0096] like Figure 5 As shown, the secondary mirror 7 is transferred to the reference. After the first theodolite 22 at A (the position corresponding to the primary mirror 5) is rotated 90° clockwise using a two-dimensional translation platform, the first plane reflector 18 at the base plate reference surface D is used to self-align with it to calibrate the reference direction. Then the first theodolite 22 is translated from A to B (the position corresponding to the third mirror 12) using a two-dimensional translation platform. During the translation process, the first theodolite 22 has a slight angular offset. Therefore, it is necessary to rotate the first theodolite 22 at B 90° clockwise and self-align with the second plane reflector 19 located on the optical path of the first theodolite 22 to ensure that the reference direction remains unchanged. After the first theodolite 22 at B is rotated 90° counterclockwise, it is aimed at the third theodolite 24 near the base plate reference surface E, and the back reference surface A of the primary mirror 5 is transferred to the third theodolite 24. Finally, the first theodolite 22 is used to adjust the reference of the secondary mirror 7.

[0097] 3. Roughly position the secondary mirror assembly. Reserve the secondary mirror assembly installation holes on the bottom plate 17. The secondary mirror assembly is fixedly connected to the installation holes of the bottom plate 17 through the threaded holes of the secondary mirror two-dimensional adjustment mechanism 9, the secondary mirror two-dimensional adapter frame 10 and the secondary mirror support seat 11, which is used for preliminary positioning of the secondary mirror assembly.

[0098] As shown in FIG6(a), the angle of the secondary mirror 7 is adjusted. Theoretically, the angle between the back reference plane A of the primary mirror 5 and the base plate reference plane B is 90°, and the angle between the back reference plane of the secondary mirror 7 and the base plate reference plane B is 89.62°. Therefore, the theoretical difference between the angle between the back reference plane of the secondary mirror 7 and the back reference plane A of the primary mirror 5 is 90-89.62=0.38°. The optical axis of the third theodolite 24 has characterized the back reference direction of the primary mirror 5. By rotating it counterclockwise by 0.38°, the back reference direction of the secondary mirror 7 can be determined. According to the back reference direction of the secondary mirror 7, the angle between the back reference plane of the secondary mirror 7 and the back reference plane A of the primary mirror 5 is adjusted and self-aligned with the third theodolite 24, which can ensure that the theoretical difference between the angle between the back reference plane of the secondary mirror 7 and the back reference plane A of the primary mirror 5 is 0.38°, and the rough positioning of the angle of the secondary mirror 7 is completed.

[0099] As shown in Fig. 6(b), the eccentricity of the secondary mirror 7 is adjusted. The outer cylindrical surface of the secondary mirror 7 is measured by a three-coordinate measuring arm to fit the axis of the secondary mirror 7. The three surfaces of the base plate reference planes B, C, and E are used as references. The eccentricity of the secondary mirror 7 is adjusted according to the measured horizontal distance y1 between the axis of the secondary mirror 7 and the reference plane B, the distance x1 between the axis of the secondary mirror 7 and the reference plane C of the base plate, and the horizontal distance z1 between the back reference plane of the secondary mirror 7 and the reference plane E of the base plate, ensuring that x1 = 251 ± 0.05 mm, y1 = 390.03 ± 0.05 mm, and z1 = 507 ± 0.05 mm. At this point, the rough adjustment of the eccentricity of the secondary mirror 7 is completed.

[0100] Determine whether the angle deviation between the back reference plane of the secondary mirror 7 and the back reference plane A of the primary mirror 5 is less than or equal to 10″, and whether the eccentricity tolerance of the secondary mirror 7 is less than or equal to 0.05 mm. If so, execute step 4, otherwise readjust the angle and / or eccentricity of the secondary mirror 7.

[0101] Although the angle deviation has reached the tolerance requirement, since the processing deviation between each polishing reference surface (i.e., the back reference surface A of the primary mirror 5, the back reference surface of the secondary mirror 7, and the back reference surface of the third mirror 12) and its optical axis is 30″, the imaging quality of the off-axis three-mirror optical system cannot be guaranteed according to the error of the positioning angle of the polishing surface.

[0102] 4. Roughly position the three-mirror assembly. Reserve the installation holes for the three-mirror assembly on the bottom plate 17, and the three-mirror assembly is fixedly connected to the installation holes of the bottom plate 17 through the threaded holes of the three-mirror two-dimensional adapter 14, the three-mirror two-dimensional adjustment mechanism 15 and the three-mirror support seat 16;

[0103] As shown in FIG7(a), the angle of the three mirrors 12 is adjusted. Theoretically, the angle between the back reference plane A of the primary mirror 5 and the base plate reference plane B is 90°, and the angle between the back reference plane of the three mirrors 12 and the base plate reference plane F is 90.2°. Therefore, the theoretical difference between the angle between the back reference plane of the three mirrors 12 and the back reference plane A of the primary mirror 5 is 90-90.2=-0.2°. The optical axis of the second theodolite 23 has characterized the back reference direction of the primary mirror 5. Rotating it clockwise by 0.2° can determine the back reference direction of the three mirrors 12. According to the back reference direction of the three mirrors 12, the angle between the back reference plane of the three mirrors 12 and the back reference plane A of the primary mirror 5 is adjusted and self-aligned with the second theodolite 23, which can ensure that the angle between the back reference plane of the three mirrors 12 and the back reference plane A of the primary mirror 5 is 0.2° in theory, and the angle positioning of the three mirrors 12 is completed.

[0104] As shown in Fig. 7(b), the eccentricity of the three mirrors 12 is adjusted. The three-coordinate measuring arm is used to measure the outer cylindrical surface of the three mirrors 12 and fit the axis of the three mirrors 12. The three surfaces of the base plate reference planes C, D, and F are used as references. The eccentricity of the three mirrors 12 is adjusted according to the measured distance x2 between the axis of the three mirrors 12 and the reference plane C, the horizontal distance z2 between the back reference plane of the three mirrors 12 and the base plate reference plane D, and the horizontal distance y2 between the axis of the three mirrors 12 and the base plate reference plane F, ensuring that x2 = 251 ± 0.05 mm, y2 = 195.68 ± 0.05 mm, and z2 = 370.24 ± 0.05 mm; at this point, the eccentricity adjustment of the three mirrors 12 is completed.

[0105] Repeat the above operation to repeatedly adjust the angle and eccentricity of the three mirrors 12 to ensure that the eccentricity tolerance of the three mirrors 12 is less than or equal to 0.05 mm and the angle deviation between the back reference plane of the three mirrors 12 and the back reference plane A of the main mirror 5 is less than or equal to 10″.

[0106] Determine whether the angle deviation between the back reference plane of the three mirrors 12 and the back reference plane A of the main mirror 5 is less than or equal to 10", and whether the eccentricity tolerance of the three mirrors 12 is less than or equal to 0.05mm. If so, execute step 2], otherwise readjust the angle and / or eccentricity of the three mirrors 12.

[0107] Although the angle deviation has reached the tolerance requirement, since the processing deviation between each polishing reference surface (i.e., the back reference surface A of the primary mirror 5, the back reference surface of the secondary mirror 7, and the back reference surface of the third mirror 12) and its optical axis is 30″, the imaging quality of the off-axis three-mirror optical system cannot be guaranteed according to the error of the positioning angle of the polishing surface.

[0108] Finally, the primary mirror assembly, secondary mirror assembly, and tertiary mirror assembly must be precisely adjusted based on the wave aberration measurement results. The above process only eliminates large deviations.

[0109] 5. Adjust the lens group posture and perform image quality correction.

[0110] After the rough positioning of the primary mirror 5, the secondary mirror 7, and the third mirror 12 is completed through the above steps, the theoretical value of the angle between the third plane reflector 20 and the back reference plane A of the primary mirror 5 is designed to be 9°. The positioning method of the third plane reflector 20 is specifically as follows: the first theodolite 22 is self-aligned with the back reference plane A of the primary mirror 5 to derive the reference direction. The first theodolite 22 is moved from A to B (i.e., the side of the primary mirror 5 away from the three mirrors 12), and then the first theodolite 22 at B is rotated clockwise by 9° to represent the normal direction of the third plane reflector 20. The third plane reflector 20 is self-aligned with the first theodolite 22 at B to complete the positioning of the third plane reflector 20, the purpose of which is to determine the relative position of the third plane reflector 20 and the primary mirror 5, and to perform wave phase difference detection.

[0111] like Figure 8 As shown, a 4D laser interferometer 21 is set at a position corresponding to the three mirrors 12, and the interference principle is used to detect the wave aberration of the off-axis three-mirror optical system. The position of the 4D laser interferometer 21 is uniquely determined by the position of the third plane reflector 20. However, when there are deviations in the radius of curvature, aspheric coefficient, verticality of the polishing reference surface and the optical axis of the primary mirror 5, the secondary mirror 7, and the three mirrors 12, only assembling the primary mirror 5, the secondary mirror 7, and the three mirrors 12 according to the theoretical position may not necessarily guarantee that the imaging quality of the off-axis three-mirror optical system meets the requirements. The Zemax optical design software is used to analyze the wave aberration of the off-axis three-mirror optical system, and it is found that the angle of the secondary mirror 7 and the three mirrors 12 is very sensitive to the image quality. Therefore, a 4D laser interferometer 21 must be used to detect the wave aberration of the off-axis three-mirror optical system, and the angle of the secondary mirror 7 and the three mirrors 12 must be slightly adjusted according to the measurement results to ensure that the off-axis three-mirror optical system has good imaging quality.

[0112] In summary, since the installation of the off-axis three-mirror optical system involves multiple components and the adjustment of multiple degrees of freedom, the precise misalignment cannot be obtained empirically, and the sensitivity matrix algorithm is required to solve the misalignment.

[0113] exist Figure 8 There is a corresponding relationship between the posture of the third plane reflector 20 and the field of view. The posture of the third plane reflector 20 determined in the above steps corresponds to the f1 field of view measured by the 4D laser interferometer 21. The f1 field of view is the initial state of the third reflector. The third plane reflector 20 is rotated 1° clockwise from the f1 field of view to correspond to the f2 field of view measured by the 4D laser interferometer 21. The third plane reflector 20 is rotated 1° counterclockwise from the f1 field of view to correspond to the f3 field of view measured by the 4D laser interferometer 21. The third plane reflector 20 is rotated 1° upward from the f1 field of view to correspond to the f4 field of view measured by the 4D laser interferometer 21. The third plane reflector 20 is rotated 1° downward from the f1 field of view to correspond to the f5 field of view measured by the 4D laser interferometer 21.

[0114] The mathematical relationship between the full-field Zernike wave aberration coefficient and the 12 degrees of freedom of the secondary mirror 7 and the tertiary mirror 12 is expressed as follows:

[0115]

[0116] In formula (1), ΔZ is the full-field Zernike wave aberration coefficient matrix, M is the misalignment, and A is the sensitivity matrix; in the misalignment freedom matrix M, dx1 / dy1 / dz1 / tx1 / ty1 / tz1 are the six degrees of freedom of the secondary mirror 7, and dx1 / dy1 / dz1 / tx1 / ty1 / tz1 respectively represent the translation of the secondary mirror 7 along the three axes x1, y1, and z1 and the rotation around these three axes, see Figure 6. dx2 / dy2 / dz2 / tx2 / ty2 / tz2 are the six degrees of freedom of the three mirrors 12, and dx2 / dy2 / dz2 / tx2 / ty2 / tz2 respectively represent the translation of the three mirrors 12 along the three axes x2, y2, and z2 and the rotation around these three axes, see Figure 7.

[0117] In the full-field Zernike wave aberration coefficient matrix ΔZ, elements Z15-Z19 represent the Zernike coefficients of the f1 field of view, among which Z15\Z16 represent the astigmatism of the f1 field of view, Z17\Z18 represent the coma of the f1 field of view, and Z19 represents the spherical aberration of the f1 field of view; Z25-Z29 represent the Zernike coefficients of the f2 field of view, among which Z25\Z26 represent the astigmatism of the f2 field of view, Z27\Z28 represent the coma of the f2 field of view, and Z29 represents the spherical aberration of the f2 field of view, and so on.

[0118] In the sensitivity matrix A, each element represents the differential of the wave aberration with respect to the degree of freedom in each field of view. Taking the first element as an example, it represents the first-order partial differential of the f1 field of view Z15 with respect to the secondary mirror 7dx1 degree of freedom. Its value can be calculated using Zemax software.

[0119] The size of the sensitivity matrix A is 25×12, which represents the relationship between the 12 degrees of freedom of the secondary mirror 7 and the 12-mirror group of the third mirror and the 25 Zernike wave aberration coefficients of the five fields of view f1-f5:

[0120]

[0121] Through the analysis of Zemax optical design software, it is found that the tx1 / ty1 rotational degrees of freedom of the secondary mirror 7 and the tx2 / ty2 rotational degrees of freedom of the tertiary mirror 12 are sensitive to wave aberration. Therefore, these four degrees of freedom are selected for image quality correction.

[0122] 6. Calculate the misalignment. The adaptive damped least squares method is an algorithm for solving the misalignment based on the sensitivity matrix. Its basic principle is to dynamically adjust the damping coefficient according to the input measured wavefront data, quickly traverse the damping coefficient range, and find the coefficient that minimizes the deviation. In this way, the accuracy and reliability of the correction results can be ensured while avoiding over-adjustment. The algorithm is based on the Python module and Zemax optical design software, and real-time communication is used for closed-loop simulation verification. The input is the full-field Zernike wave aberration coefficient matrix ΔZ of the off-axis three-mirror optical system, and the misalignment M is solved by the initial damping coefficient ρ. Use the Python module and Zemax optical design software for joint simulation, bring the misalignment M into the Zemax optical design software, adjust the primary mirror assembly, secondary mirror assembly, and three-mirror assembly to the misaligned state, and obtain the full-field Zernike wave aberration coefficient matrix ΔZ of the simulated off-axis three-mirror optical system. Using the above algorithm, dynamically adjust the damping coefficient ρ and solve the new misalignment, quickly traverse the range of the damping coefficient, select the damping coefficient that minimizes the input and output deviations, and complete the calculation.

[0123] like Figure 8 As shown, a 4D laser interferometer 21 is used to construct a detection optical path of an off-axis three-mirror optical system. According to the full-field Zernike wave aberration coefficient matrix ΔZ, the misalignment M of the secondary mirror 7 and the third mirror 12 is calculated by formula (3):

[0124] M=(A T A+ρI) -1 A T ΔZ (3)

[0125] Among them, A T represents the transposed matrix of the sensitivity matrix A; ρ is the damping coefficient; I represents the identity matrix;

[0126] 8. According to the misalignment amount M, the secondary mirror two-dimensional adjustment mechanism 9 and the three-mirror two-dimensional adjustment mechanism 15 are used to precisely adjust the angles of the secondary mirror 7 and the three-mirror 12 to complete the final assembly of the off-axis three-mirror optical system. The experimental results of the full-field wave aberration of the off-axis three-mirror optical system are as follows: Fig. 9 As shown, the wavefront error RMS is better than 0.05λ@632.8nm, and the image quality meets the requirements.

[0127] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A precision assembly method for an off-axis three-mirror optical system, wherein the off-axis three-mirror optical system comprises a primary mirror assembly, a secondary mirror assembly and a three-mirror assembly; characterized in that: The following steps are involved: 1] Taking the primary mirror assembly as the reference, roughly position the primary mirror assembly, secondary mirror assembly and triple mirror assembly respectively until the angle deviation of each mirror assembly is less than or equal to 10″ and the eccentricity tolerance is less than or equal to 0.05mm; 2] Use Zemax optical design software to analyze the wavefront aberration of the off-axis three-mirror optical system, and adjust the rotational freedom of the secondary mirror assembly and the three-mirror assembly on the x-axis and y-axis according to the wavefront aberration to perform image quality correction; 3] The full-field Zernike wave aberration coefficient matrix ΔZ and the sensitivity matrix A are analyzed using Zemax optical design software, and the misalignment M of the secondary mirror (7) and the tertiary mirror (12) is calculated: M=(A T A+ρI) -1 A T ΔZ Among them, A T is the transposed matrix of the sensitivity matrix A; ρ is the damping coefficient; I represents the unit matrix; 4] The angles of the secondary mirror (7) and the tertiary mirror (12) are precisely adjusted according to the misalignment amount M, so that the full-field wave aberration of the off-axis three-mirror optical system is less than a threshold value, thereby completing the precise assembly of the off-axis three-mirror optical system.

2. The precision assembly method of the off-axis three-mirror optical system according to claim 1, characterized in that: Step 1] Specifically: 1.1) Roughly position the primary mirror assembly; 1.2) transferring the reference of the back reference surface A of the primary mirror (5) in the primary mirror assembly to the tertiary mirror (12) and the secondary mirror (7); 1.3) Roughly position the secondary mirror assembly; 1.4) Roughly position the three-mirror assembly.

3. The precision assembly method of the off-axis three-mirror optical system according to claim 2, characterized in that: Step 1.1) is specifically: 1.1.

1. Preliminary positioning of the main mirror assembly: a mounting hole for the main mirror assembly is reserved on the bottom plate (17), and the main mirror assembly is fixedly connected to the mounting hole of the bottom plate (17) through the threaded hole of the main mirror base (1); 1.1.2, set the reference surface; The upper surface of the bottom plate (17) in the off-axis three-mirror optical system is used as the bottom plate reference surface C; A protruding side wall surface of the bottom plate (17) close to the main mirror (5) and parallel to the main mirror (5) serves as a bottom plate reference surface D; A protruding side wall surface of the bottom plate (17) away from the main mirror (5) and parallel to the main mirror (5) serves as a bottom plate reference surface E; A side wall surface of the bottom plate (17) close to the primary mirror (5) and perpendicular to the primary mirror (5) serves as a bottom plate reference surface B; A side wall surface of the bottom plate (17) away from the main mirror (5) and perpendicular to the main mirror (5) serves as a bottom plate reference surface F; 1.1.

3. Adjust the angle of the primary mirror (5); The first plane reflector (18) is arranged at the base plate reference surface D, and the first theodolite (22) and the first plane reflector (18) are used for self-alignment to derive a reference direction; then, the angle between the back reference surface A of the primary mirror (5) and the base plate reference surface D is adjusted according to the reference direction, so that the back reference surface A of the primary mirror (5) is perpendicular to the optical axis of the first theodolite (22), and is thus parallel to the base plate reference surface D; 1.1.

4. Adjust the eccentricity of the primary mirror (5); Using a three-coordinate measuring arm to measure the outer cylindrical surface of the primary mirror (5) and fit the axis of the primary mirror (5), taking the three surfaces of the base plate reference surfaces B, C, and D as references, adjusting the eccentricity of the primary mirror (5) according to the measured distance x between the axis of the primary mirror (5) and the base plate reference surface C, the horizontal distance y between the axis of the primary mirror (5) and the base plate reference surface B, and the horizontal distance z between the back reference surface A of the primary mirror (5) and the base plate reference surface D; 1.1.

5. Determine whether the angle deviation between the back reference plane A of the primary mirror (5) and the base reference plane D is less than or equal to 10″, and whether the eccentricity tolerance of the primary mirror (5) is less than or equal to 0.05 mm. If so, execute step 1.2), otherwise return to step 1.1.

3.

4. The precision assembly method of the off-axis three-mirror optical system according to claim 3, characterized in that: Step 1.2) is specifically: 1.2.

1. Performing reference transfer on the three mirrors (12); The first theodolite (22) is used to align with the back reference surface A of the primary mirror (5) to indicate the reference direction of the back reference surface A of the primary mirror (5); the first theodolite (22) is rotated 90° counterclockwise to aim at the second theodolite (23) to transmit the reference direction of the back reference surface A of the primary mirror (5) to the second theodolite (23); the second theodolite (23) is then rotated 90° counterclockwise to indicate the reference direction of the back reference surface A of the primary mirror (5) using its optical axis; 1.2.

2. Performing reference transfer on the secondary mirror (7); The first theodolite (22) is rotated 90° clockwise and aligned with the first plane reflector (18) at the base plate reference surface D to calibrate the reference direction. The first theodolite (22) is then translated to a position corresponding to the three mirrors (12). The first theodolite (22) is then rotated 90° clockwise and aligned with the second plane reflector (19) located on the optical path of the first theodolite (22) to ensure that the reference direction remains unchanged. The first theodolite (22) is then rotated 90° counterclockwise to align with the third theodolite (24) near the base plate reference surface E to transfer the back reference surface A of the main mirror (5) to the third theodolite (24).

5. The precision assembly method of the off-axis three-mirror optical system according to claim 4, characterized in that: Step 1.3) is specifically: 1.3.

1. Preliminary positioning of the secondary mirror assembly: a mounting hole for the secondary mirror assembly is reserved on the bottom plate (17), and the secondary mirror assembly is fixedly connected to the mounting hole of the bottom plate (17) through the threaded hole of the secondary mirror support seat (8); 1.3.

2. Adjust the angle of the secondary mirror (7); According to the theoretical difference M between the angles of the back reference plane of the secondary mirror (7) and the back reference plane A of the primary mirror (5), the third theodolite (24) is rotated counterclockwise by 0.38° to determine the back reference direction of the secondary mirror (7), and the angles of the back reference plane of the secondary mirror (7) and the back reference plane A of the primary mirror (5) are adjusted according to the back reference direction of the secondary mirror (7) and are self-aligned with the third theodolite (24); 1.3.

3. Adjust the eccentricity of the secondary mirror (7); Using a three-coordinate measuring arm to measure the outer cylindrical surface of the secondary mirror (7) and fit the axis of the secondary mirror (7), taking the three surfaces of the base plate reference planes B, C, and E as references, adjusting the eccentricity of the secondary mirror (7) according to the measured horizontal distance y1 between the axis of the secondary mirror (7) and the reference plane B, the distance x1 between the axis of the secondary mirror (7) and the base plate reference plane C, and the horizontal distance z1 between the back reference plane of the secondary mirror (7) and the base plate reference plane E; 1.3.

4. Determine whether the angle deviation between the back reference plane of the secondary mirror (7) and the back reference plane A of the primary mirror (5) is less than or equal to 10", and whether the eccentricity tolerance of the secondary mirror (7) is less than or equal to 0.05 mm. If so, execute step 1.4), otherwise return to step 1.3.

2.

6. The precision assembly method of the off-axis three-mirror optical system according to claim 5, characterized in that: Step 1.4) is specifically: 1.4.

1. Preliminary positioning of the three-mirror assembly: a mounting hole for the three-mirror assembly is reserved on the bottom plate (17), and the three-mirror assembly is fixedly connected to the mounting hole of the bottom plate (17) through the threaded hole of the three-mirror support seat (16); 1.4.

2. Adjust the angles of the three mirrors (12); According to the theoretical difference N between the angles of the back reference plane of the three mirrors (12) and the back reference plane A of the primary mirror (5), the second theodolite (23) is rotated clockwise by 0.2° to determine the back reference direction of the three mirrors (12), and the angles of the back reference plane of the three mirrors (12) and the back reference plane A of the primary mirror (5) are adjusted according to the back reference direction of the three mirrors (12) and are self-aligned with the second theodolite (23); 1.4.

3. Adjust the eccentricity of the three mirrors (12); Using a three-coordinate measuring arm to measure the outer cylindrical surface of the three mirrors (12) and fit the axis of the three mirrors (12), taking the three surfaces of the base plate reference planes C, D, and F as references, adjusting the eccentricity of the three mirrors (12) according to the measured horizontal distance x2 between the axis of the three mirrors (12) and the reference plane C, the horizontal distance z2 between the back reference plane of the three mirrors (12) and the base plate reference plane D, and the horizontal distance y2 between the axis of the three mirrors (12) and the base plate reference plane F; 1.4.

4. Determine whether the angle deviation between the back reference plane of the third mirror (12) and the back reference plane A of the main mirror (5) is less than or equal to 10", and whether the eccentricity tolerance of the third mirror (12) is less than or equal to 0.05 mm. If so, proceed to step 2], otherwise return to step 1.4.

2.

7. The precision assembly method of the off-axis three-mirror optical system according to claim 6, characterized in that: Step 2] Specifically: 2.1) Build a wavefront aberration detection system to obtain the wavefront aberration of the off-axis three-mirror optical system; 2.2) using Zemax optical design software to analyze the wavefront aberration of the off-axis three-mirror optical system, and screening out the tx1 / ty1 rotational freedom of the secondary mirror (7) and the tx2 / ty2 rotational freedom of the third mirror (12), wherein tx1 represents the rotation amount of the secondary mirror (7) along the x1 axis, ty1 represents the rotation amount of the secondary mirror (7) around the y1 axis, tx2 represents the rotation amount of the third mirror (12) around the x2 axis, and ty2 represents the rotation amount of the third mirror (12) around the y2 axis; 2.3) Adjust the rotation amount of tx1 / ty1 of the secondary mirror (7) and tx2 / ty2 of the tertiary mirror (12) to achieve image quality correction.

8. The precision assembly method of the off-axis three-mirror optical system according to claim 7, characterized in that: Step 2.1) is specifically: 2.1.

1. Using the first theodolite (22) and the reference surface A on the back of the primary mirror (5) for self-alignment, a reference direction is derived; 2.1.

2. Maintaining the reference direction, translate the first theodolite (22) to the side of the primary mirror (5) away from the third mirror (12), and rotate the first theodolite (22) clockwise by 9° to indicate the normal direction of the third plane reflector (20) disposed on the side of the base plate reference plane E; 2.1.

3. Adjust the third plane reflector (20) and align it with the first theodolite (22) to complete the positioning of the third plane reflector (20); 2.1.

4. A 4D laser interferometer (21) is arranged at a position corresponding to the three mirrors (12), and the 4D laser interferometer (21) is used to emit laser light to the three mirrors (12) and detect the wave aberration of the off-axis three-mirror optical system.

9. The precision assembly method of the off-axis three-mirror optical system according to claim 8, characterized in that: In step 3], the calculation of the misalignment M of the secondary mirror (7) and the tertiary mirror (12) adopts the adaptive damping least square method; The size of the sensitivity matrix A is 25×12; the size of the full-field Zernike wave aberration coefficient matrix ΔZ is 25×1.

10. The precision assembly method of the off-axis three-mirror optical system according to claim 9, characterized in that: In step 4], the threshold is 0.05λ@632.8nm.

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