A high-precision assembling and adjusting method for off-axis three-mirror optical system based on partitioned holographic element

Abstract

The application discloses a high-precision assembling and adjusting method of an off-axis three-mirror optical system based on a partitioned computer-generated hologram element. The method takes a single partitioned computer-generated hologram element as the only optical reference of the whole assembling and adjusting process, cooperates with a plane mirror with an aperture similar to that of a primary mirror, adopts a multi-stage recursive assembling and adjusting strategy from the rear stage to the front stage and a full optical path closed-loop calibration mechanism, and sequentially completes the alignment of an interferometer and the CGH, three-mirror assembling and adjusting, secondary mirror assembling and adjusting, primary mirror assembling and adjusting, plane mirror auxiliary closed-loop calibration and system fine assembling and adjusting. The application replaces the traditional multiple-device scattered reference with a single optical reference, eliminates the reference transmission error from the root, solves the multi-mirror six-degree-of-freedom strong coupling misadjustment problem by combining the multi-stage recursion and closed-loop calibration, and has the advantages of unified reference, coarse and fine integration, simple process, high stability and the like, and is suitable for the assembling and adjusting of high-precision off-axis reflective optical systems such as space remote sensing loads and astronomical telescopes.

Description

Technical Field

[0001] This invention belongs to the field of optical precision assembly and adjustment technology, specifically relating to a high-precision assembly and adjustment method for an off-axis three-reflector optical system based on a multifunctional partitioned computational hologram (CGH). It is applicable to off-axis reflective optical systems with extremely high requirements for assembly and adjustment accuracy and stability, such as space remote sensing payloads, astronomical telescopes, and large-aperture optical inspection equipment. Background Technology

[0002] Off-axis three-mirror optical systems are increasingly widely used in space remote sensing, astronomical observation, and large-aperture optical detection due to their advantages such as no central obstruction, high optical efficiency, and wide spectral adaptability. The system consists of three off-axis aspherical mirrors: a primary mirror, a secondary mirror, and a tertiary mirror. These mirrors are strongly coupled with each other in six degrees of freedom: spacing, eccentricity (in two directions), pitch, yaw, and roll. Imaging quality is extremely sensitive to assembly and adjustment precision, typically requiring a full-field wavefront error RMS ≤ 0.1λ (λ = 632.8 nm).

[0003] The existing off-axis three-mirror optical system assembly and adjustment technology has the following technical bottlenecks:

[0004] 1. Dispersed reference points during assembly and adjustment lead to significant cumulative errors. Traditional assembly and adjustment methods employ multiple devices such as laser trackers, interferometers, and autocollimators working together. The lack of a unified reference point results in reference errors propagating sequentially during multi-mirror assembly and adjustment, making it difficult to consistently achieve diffraction-limited accuracy. Rough assembly and adjustment lacks an integrated reference carrier, requiring multiple equipment replacements and optical path reconstructions, resulting in a cumbersome and time-consuming process.

[0005] 2. Lack of multi-level recursive constraints leads to multi-mirror coupling misalignment. Existing assembly and adjustment methods mostly involve independent alignment of a single mirror, without establishing a step-by-step recursive mechanism where the subsequent stage constrains the preceding stage. The coupling attitudes between the three mirrors, secondary mirrors, and primary mirror are difficult to match accurately, which can easily lead to problems such as pointing deviation and difficulty in aberration convergence.

[0006] 3. Lack of multi-level convergence mechanism leads to poor system stability. Traditional assembly and adjustment methods cannot correct the accumulated errors of multi-level assembly and adjustment in real time, making it difficult to meet the needs of high-stability applications such as aerospace remote sensing and astronomical observation.

[0007] It is particularly important to note that existing technologies include detection and parameter control methods for single off-axis splicing sub-mirrors. These methods aim to control the optical parameters of a single mirror and use a CGH in conjunction with a laser tracker and a theodolite to measure the radius of curvature and off-axis measurement. However, these methods are only suitable for independent detection of a single mirror and cannot solve the problems of multi-level recursive alignment, wavefront convergence of the entire system, and long-term stability in a strongly coupled system of three mirrors. Summary of the Invention

[0008] This invention aims to overcome the shortcomings of existing off-axis three-mirror optical system assembly and adjustment technologies, such as dispersed references, large error accumulation, cumbersome processes, and low efficiency. It provides a high-precision assembly and adjustment method based on partitioned computation of holographic elements, realizing a single optical reference, integrated coarse and fine calibration, multi-level recursive constraints, and plane mirror-assisted closed-loop calibration. This significantly improves assembly and adjustment accuracy, efficiency, and system stability. While simplifying the process and suppressing error accumulation, it ensures that the system assembly and adjustment accuracy reaches the order of 0.1λ.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A high-precision assembly and adjustment method for an off-axis three-mirror optical system based on a partitioned computational holographic element (CGH) is proposed. The method uses a single partitioned computational holographic element (CGH) as the sole optical reference for the assembly and adjustment process, and is combined with a plane mirror with an aperture similar to that of the primary mirror. The method employs a multi-stage recursive assembly and adjustment strategy from the rear stage to the front stage, consisting of three mirrors → secondary mirror → primary mirror, and a closed-loop calibration mechanism for the entire optical path.

[0011] The assembly and adjustment method includes the following steps: alignment of the interferometer with the CGH, assembly and adjustment of the three mirrors, assembly and adjustment of the secondary mirror, assembly and adjustment of the primary mirror, auxiliary closed-loop calibration of the plane mirror, and fine assembly and adjustment of the system;

[0012] The multi-stage recursive adjustment strategy is as follows: using the reference beams of different functional areas of the CGH, the mirrors are adjusted step by step through the three mirrors, secondary mirrors, and primary mirrors, and the adjustment results of the subsequent mirrors are used to constrain the adjustment accuracy of the preceding mirrors.

[0013] The all-optical-path closed-loop calibration mechanism is as follows: the off-axis three-mirror optical system is calibrated by plane mirrors to form a closed calibration loop of "interferometer → CGH → three mirrors → secondary mirror → primary mirror → plane mirror → primary mirror → secondary mirror → three mirrors → CGH → interferometer". The pose of each mirror is iteratively adjusted based on the wavefront error of the system to correct the accumulated error in the multi-level recursive process.

[0014] The sole optical reference is: the optical reference is not changed throughout the entire process, the detection optical path is not reconstructed, and the six degrees of freedom constraints of all mirrors are based on the single CGH as a unified reference.

[0015] Furthermore, after assembly and adjustment, the system wavefront error RMS is ≤0.1λ, where λ=632.8nm, and the change in wavefront error after the system is left to stand for 4–12 hours is ≤0.02λ.

[0016] Furthermore, the partitioned CGH integrates multiple independently coded and crosstalk-free functional diffraction regions, each of which independently undertakes coarse alignment, attitude constraint, spacing constraint and wavefront detection functions.

[0017] The functional diffraction regions include: the main holographic region, the alignment holographic region, the reference point diffraction region, the three-mirror distance region, the three-mirror pointing region, the three-mirror position reference region, the secondary mirror distance region, the secondary mirror pointing region, the secondary mirror position reference region, the primary mirror distance region, the primary mirror pointing region, the primary mirror position reference region, and the plane mirror distance region and the plane mirror pointing region.

[0018] The distance region is used to project and converge spherical waves to the center of the mirror surface to constrain the mirror spacing; the position reference region is used to project crosshairs to the edge of the mirror to constrain the mirror position; the pointing region is used to guide the adjustment of the mirror pitch and azimuth angles; and the reference point diffraction region is used to project a three-dimensional spatial reference point to construct the mirror pitch and azimuth pointing reference.

[0019] Furthermore, the three-mirror assembly and adjustment steps include:

[0020] By utilizing the three-mirror distance region of CGH, a converging spherical wave is projected to the center of the three mirrors to achieve three-mirror distance constraint;

[0021] By using the three-mirror position reference area of ​​CGH to project crosshairs to the edges of the three mirrors, coarse alignment of the three mirrors can be achieved.

[0022] Using the reference point diffraction region of the CGH, a three-mirror aerial pointing reference point is constructed. The attitude of the three mirrors is adjusted so that the diffraction beam points reflected by the three mirrors coincide with the preset aerial reference point, thus completing the installation and adjustment of the pitch and azimuth of the three mirrors.

[0023] Furthermore, the secondary mirror assembly and adjustment step includes:

[0024] The diffracted light generated in the secondary mirror distance region of CGH is reflected by three mirrors and converged to the center of the secondary mirror, thereby achieving secondary mirror spacing constraint;

[0025] The diffracted light generated by the secondary mirror position reference region of CGH is reflected by the three mirrors and projected onto the edge of the secondary mirror, thereby achieving the position constraint of the secondary mirror;

[0026] Adjust the secondary mirror's attitude so that the convergence point of the diffracted beams reflected by the three mirrors and the secondary mirror coincides with the convergence point of the primary mirror's center preset by CGH, thus completing the installation and adjustment of the secondary mirror's pitch and azimuth.

[0027] Furthermore, the primary mirror assembly and adjustment steps include:

[0028] By utilizing the primary mirror distance region of the CGH to project and converge spherical waves to the center of the primary mirror, the primary mirror distance constraint is achieved;

[0029] The position of the primary mirror is constrained by projecting crosshairs onto the edge of the primary mirror using the primary mirror position reference area of ​​the CGH.

[0030] Adjust the attitude of the primary mirror so that the convergence point of the diffraction beams reflected by the three mirrors, secondary mirrors, and primary mirror coincides with the CGH-preset primary mirror aerial pointing reference point, thus completing the installation and adjustment of the primary mirror's pitch and azimuth.

[0031] Furthermore, the plane mirror-assisted closed-loop calibration step includes:

[0032] The diffracted light generated by the plane mirror distance region of CGH is reflected by the three mirrors, secondary mirrors and primary mirror and then converged to the center of the plane mirror, thus realizing the plane mirror spacing adjustment;

[0033] Adjust the orientation of the plane mirror so that the diffraction beam point reflected by the entire optical path coincides with the convergence point of the primary mirror preset by CGH, forming a closed verification loop.

[0034] Furthermore, the system fine-tuning steps include:

[0035] The system acquires full-aperture interference fringes using an interferometer and calculates wavefront aberrations.

[0036] Fine-tune the axial distance of each mirror to eliminate system defocus components;

[0037] Fine-tune the roll attitude of each mirror to eliminate astigmatic components of the system;

[0038] By correcting the accumulated systematic error from multiple recursion stages, the wavefront error is ultimately reduced to less than the preset error value.

[0039] Furthermore, the off-axis three-mirror optical system consists of three off-axis aspherical mirrors: a primary mirror, a secondary mirror, and a tertiary mirror, with a strong six-degree-of-freedom coupling relationship between each mirror;

[0040] The assembly and adjustment method differs from the independent testing and assembly and adjustment of a single off-axis sub-mirror, and must simultaneously satisfy multi-level optical path recursive alignment and wavefront convergence of the entire system.

[0041] Furthermore, each functional diffraction region of the CGH employs independent phase encoding to ensure that the diffracted beams in each region do not interfere with each other during propagation; combined with the central convergence point, the edge crosshairs, and the aerial pointing reference point, the full constraint of the six degrees of freedom of the reflector is achieved.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. Unified reference, suppressing error accumulation. Only a single CGH is used as the sole optical reference throughout the entire process, replacing the dispersed references of multiple devices. This avoids error accumulation during reference transfer and can stably achieve diffraction-limited assembly accuracy.

[0044] 2. Integrated coarse and fine alignment for high efficiency. A single CGH integrates multiple functions for coarse alignment, attitude calibration, and wavefront detection, eliminating the need to change equipment or reconstruct the optical path. The entire assembly and adjustment process is completed in one setup, significantly shortening the assembly and adjustment cycle.

[0045] 3. Independent coding for each zone, with no crosstalk in the optical path. Each functional diffraction region of the CGH is independently coded, has an independent optical path, and an independent function, without interfering with each other; combined with the central convergence point, the edge crosshairs, and the aerial pointing reference point, the six degrees of freedom of the mirror are precisely constrained.

[0046] 4. Multi-level recursion for precise coupling attitude matching. A step-by-step constraint adjustment strategy from the later stage to the earlier stage is adopted to establish the coupling constraint relationship between the three mirrors, secondary mirrors, and primary mirror, thus solving the problems of multi-mirror pointing deviation and aberration convergence.

[0047] 5. Closed-loop calibration ensures excellent system stability. An additional plane mirror enables closed-loop optical path verification, actively correcting accumulated errors from multiple assembly stages. After 4–12 hours of static setup, the wavefront drift is ≤0.02λ, meeting the requirements for high-stability applications such as aerospace remote sensing.

[0048] 6. The equipment is simple and highly versatile. It only requires an interferometer, a single CGH, and a plane mirror to complete the assembly and adjustment, making it compatible with various high-precision off-axis three-mirror optical systems such as space remote sensing, astronomical telescopes, and large-aperture optical inspection equipment. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of an off-axis three-mirror optical system.

[0050] Figure 2 This is a schematic diagram of the optical path for assembling an off-axis three-mirror optical system;

[0051] Figure 3 This is a schematic diagram of the layout of the diffraction region of the computational holographic element;

[0052] Figure 4 This is a flowchart of the assembly and adjustment process for an off-axis three-mirror optical system;

[0053] Figure 5 This is a schematic diagram of the alignment fringes between the interferometer and the computational holographic element;

[0054] Figure 6 This is a schematic diagram of a three-mirror dimming path;

[0055] Figure 7 This is a schematic diagram of the mirror-mounted dimming path;

[0056] Figure 8 This is a schematic diagram of the primary mirror mounting dimming path;

[0057] Figure 9 This is a schematic diagram of a plane mirror mounting and dimming path;

[0058] Figure 10 This is a schematic diagram of the optical path for system wave aberration testing. Detailed Implementation

[0059] The present invention will now be described in further detail with reference to the accompanying drawings.

[0060] A high-precision assembly and adjustment method for an off-axis three-mirror optical system based on a partitioned computational holographic element (CGH) is proposed. The system being assembled and adjusted is a centerless, large-field-of-view, diffraction-limited off-axis three-mirror space optical system (such as...). Figure 1 As shown, the system consists of three off-axis aspherical mirrors: a primary mirror, a secondary mirror, and a tertiary mirror. These three mirrors have a strong coupling relationship with six degrees of freedom: spacing, eccentricity, pitch, yaw, and roll. The assembly and adjustment must simultaneously satisfy the multi-level optical path recursive alignment and the wavefront convergence of the entire system.

[0061] This invention employs only one multi-functional partitioned CGH and one plane mirror with an aperture similar to the system's primary mirror. Using the single CGH as the sole unified optical reference, it adopts a recursive assembly and adjustment strategy from the later stages to the earlier stages, constraining the accuracy of the earlier stages through multi-stage recursion. A closed-loop verification loop is formed by using the plane mirror to assist in verifying the system's wavefront aberrations, correcting accumulated errors from multiple stages. The entire process does not involve changing the reference, reconstructing the optical path, or adding compensation components. This solves the four major bottlenecks of traditional assembly and adjustment: reference dispersion, error accumulation, coarse-fine separation, and coupling misalignment. Ultimately, the system achieves a wavefront error RMS ≤ 0.1λ (λ = 632.8 nm) and a wavefront drift ≤ 0.02λ after 4–12 hours of rest, making it suitable for highly stable scenarios such as space remote sensing and astronomical telescopes. This invention provides a high-precision assembly and adjustment method for an off-axis three-mirror optical system based on partitioned computational holographic elements. This method uses a single partitioned computational holographic element (CGH) as the sole optical reference, integrates coarse alignment and fine adjustment functions, and is equipped with a plane mirror with an aperture similar to that of the primary mirror. It adopts a multi-stage recursive adjustment strategy from the later stage to the earlier stage to complete the alignment of the interferometer and CGH, the three-mirror adjustment, the secondary mirror adjustment, the primary mirror adjustment, the plane mirror-assisted closed-loop calibration, and the system fine adjustment in sequence. The plane mirror is used to achieve full optical path closed-loop calibration.

[0062] The off-axis three-mirror optical path assembly system, as shown in Figure 2, mainly includes: an interferometer 1, a spatial filter 2, a computational holographic element 3, three mirrors of the off-axis three-mirror optical system 4, a secondary mirror of the off-axis three-mirror optical system 5, a primary mirror of the off-axis three-mirror optical system 6, and a plane mirror 7. The test spherical wave emitted from the interferometer is filtered by the spatial filter, diffracted by the computational holographic element, reflected by the three mirrors, reflected by the secondary mirror, and reflected by the primary mirror, before being incident perpendicularly on the plane mirror. According to the law of reflection, the test light is reflected back along the original optical path, and after being reflected by the primary mirror, secondary mirror, and three mirrors and diffracted by the computational holographic element, it returns to the interferometer and interferes with the internal reference spherical wave, forming interference fringes on the CCD surface. By processing the interference fringe data by a computer, the wavefront error of the optical system can be obtained.

[0063] The partitioned CGH integrates multiple independently coded, crosstalk-free functional diffraction regions (as shown in Figure 3). This invention utilizes different functional partitions on the computational holographic element to achieve high-precision assembly and adjustment of optical elements in an off-axis three-mirror optical system. The functional diffraction regions include: a main holographic region (diffraction region 3.1) for system wavelet aberration detection; an alignment holographic region (diffraction region 3.2) for aligning the computational holographic element relative to the interferometer position; a reference point diffraction region (diffraction regions 3.3A and 3.3B) for projecting the elevation and azimuth pointing references of the three mirrors and the primary mirror; a three-mirror distance and pointing holographic region (diffraction regions 3.4A and 3.4B) for adjusting the distance and pointing of the three mirrors; and a three-mirror position reference holographic region (diffraction regions 3.4C and 3.4D) for adjusting the position of the three mirrors. The system comprises four holographic regions: 3.4E, 3.4F (for adjusting the spacing and pointing of the secondary mirrors); 3.5A, 3.5B (diffraction regions); 3.5C, 3.5D, 3.5E, 3.5F (for adjusting the position of the secondary mirrors); 3.6A, 3.6B (for adjusting the spacing and pointing of the primary mirrors); 3.6C, 3.6D, 3.6E, 3.6F (for adjusting the position of the primary mirrors); 3.7A, 3.7B (for adjusting the spacing and pointing of the primary mirrors); and 3.7A, 3.7B (for adjusting the spacing and pointing of the plane mirrors). Each region independently performs coarse alignment, attitude constraint, spacing constraint, and wavefront detection functions, replacing the traditional method of dispersing references across multiple devices and eliminating reference transfer errors at their source.

[0064] The core of the CGH design in this invention lies in regional ray tracing and independent phase encoding. Specifically, it can be implemented according to the optical path structure shown in Figures 6-10, following these steps:

[0065] Region splitting and ray tracing: such as Figure 3As shown, the CGH surface is functionally divided into multiple independent regions. For each region, based on its projection target (a converging spherical wave or crosshair projected onto the mirror surface, or a mirror spatial reference point projected into three-dimensional space), ray tracing is performed using optical design software to calculate the optical path difference (OPD) corresponding to each sampling point within that region, thereby constructing the original phase distribution of that region. The phase of the sampling points in this region is:

[0066]

[0067] Independent phase coding: The original phase of each region is independently quantized and coded without interference, corresponding to the coarse alignment of each mirror and the zero-position interference fine-tuning of the system. During propagation, the diffracted light field in each region only generates the preset target spot, and there is no spatial crosstalk between the diffracted components in different regions. This enables a single CGH to complete the coarse alignment and fine-tuning of the off-axis three-mirror optical system. Furthermore, the coding design of each region ensures that the diffracted beams do not interfere with each other, guaranteeing the uniformity of the alignment reference.

[0068] The spatial coordinates of the three-mirror aerial pointing reference point, the primary mirror center convergence point, and the primary mirror aerial pointing reference point described in this invention are all defined based on the CGH reference coordinate system and obtained in physical space through the diffraction characteristics of CGH. The design and acquisition methods are as follows:

[0069] 1. Establish a unified CGH reference coordinate system.

[0070] The origin of the coordinate system is O(0, 0, 0), with the geometric center of the CGH substrate surface as the origin; the normal direction of the CGH surface as the Z-axis; and the horizontal and vertical directions are determined by the off-axis three-mirror system assembly layout as the X-axis and Y-axis, respectively. The theoretical positions and reference point coordinates of all optical elements are calculated in this coordinate system.

[0071] 2. Theoretical definition of reference point coordinates and phase calculation.

[0072] Based on the theoretical design parameters of the off-axis three-mirror optical system, the theoretical coordinates of each reference point are calculated in advance during the CGH design stage and converted into phase distribution data for the corresponding region of the CGH: the three-mirror aerial reference point P1 (X1, Y1, Z1) is located at the midpoint of the line connecting the center of the three mirrors and the center of the secondary mirror; the primary mirror central convergence point P2 (X2, Y2, Z2) is located at the geometric center of the primary mirror surface; the primary mirror aerial reference point P3 (X3, Y3, Z3) is located on the geometric dividing plane between the plane mirror and the primary mirror, and is in the normal direction of the plane mirror.

[0073] The three-dimensional coordinate information of the aforementioned reference points is encoded into the phase structure of different functional regions of the CGH. Specifically, through reverse ray tracing calculations, the spherical wavefront of the target spatial point is converted into a holographic fringe distribution on the CGH plane, enabling a specific region of the CGH to diffract and generate a wavefront converging at that theoretical coordinate when illuminated.

[0074] 3. Automatic acquisition of reference points based on interferometer alignment.

[0075] The physical acquisition and calibration process of the reference point during assembly and adjustment is as follows:

[0076] First, the CGH is aligned using a standard spherical or plane wave emitted by the interferometer. When the interferometer and CGH reach the preset alignment state (i.e., the zero-position interference state), the optical axis of the interferometer coincides with the Z-axis of the CGH reference coordinate system, and the origin of the optical path is locked.

[0077] At this point, the beam illuminates a specific functional area of ​​the CGH, and diffracts according to the pre-calculated phase distribution. Since the phase structure of the CGH encodes theoretical coordinate information, the diffracted beam automatically converges or points to a preset theoretical position in three-dimensional space. Therefore, the three-mirror aerial reference point P1, the primary mirror center convergence point P2, and the primary mirror aerial reference point P3 are precisely obtained in physical space by the diffracted beam of the CGH, without the need for additional mechanical calibration steps, and directly used as the reference target points for assembly and adjustment.

[0078] All reference points are determined during the CGH design phase and are strictly aligned with the CGH reference. During assembly and adjustment, the interferometer is aligned with the CGH, and each three-dimensional reference point is precisely diffracted by the corresponding diffraction region of the CGH, automatically converging to the preset theoretical spatial position.

[0079] Table 1. Optical System Assembly and Adjustment Stages and Accuracy Indicators

[0080]

[0081] The component assembly and adjustment accuracy judgment indicators for the coarse alignment and fine alignment stages of the optical system are shown in Table 1. The assembly and adjustment process can be generally divided into two stages:

[0082] 1. Coarse alignment stage

[0083] Coarse alignment aims to quickly establish the initial optical path, laying the foundation for subsequent fine alignment. This stage mainly includes three steps: mirror spacing, determining the position and orientation of each mirror.

[0084] Mirror spacing judgment: Visually observe the size of the light spot at the center of the mirror surface, using the smallest light spot as the criterion, and quickly adjust to prevent excessive defocusing of the system. After adjustment, the mirror spacing error should be approximately ±2mm.

[0085] Mirror position adjustment: When installing the three mirrors, secondary mirrors and primary mirror, use a steel ruler to visually determine and adjust the position of each mirror to ensure that the positional error is better than 0.5 mm, so as to prevent the system position from deviating too much and causing the inability to find the light later.

[0086] Mirror pointing adjustment: Using coordinate paper as an aid, the mirror pointing is adjusted by observing and judging the overlap between the reflected light spot and the reference point. During the adjustment process, it is necessary to ensure that the overlap accuracy of the light spot is better than 0.5 mm, thereby establishing a reliable initial optical path reference.

[0087] 2. Fine Decoration Adjustment Stage

[0088] During the fine-tuning stage, the system wavelet aberration is used as the core criterion. A laser interferometer is used to perform fine-tuning of the system. The specific operation is as follows:

[0089] Defocus and astigmatism correction: First, fine-tune the axial distance of each mirror to minimize the defocus component of the system; then fine-tune the position and roll attitude of each mirror to eliminate the astigmatic component of the system.

[0090] Final target achieved: Through the above fine-tuning, the system wavelet aberration RMS ≤ 0.1λ (λ=632.8nm) was finally achieved, ensuring that the system performance reaches the diffraction limit and eliminating the cumulative errors generated during coarse alignment and processing.

[0091] This invention employs a step-by-step, component-by-component assembly and adjustment strategy, sequentially aligning and constraining the position and orientation of each mirror in the off-axis three-mirror optical system. The assembly and adjustment flowchart is as follows: Figure 4 As shown, the specific steps are as follows:

[0092] Step 1: High-precision alignment of CGH and interferometer (establishment of unified benchmark)

[0093] The computational holographic element alignment region ensures that the spherical wave emitted from the interferometer returns along the original optical path, interfering with the reference wavefront inside the interferometer. As shown in Figure 5, by observing the interference fringes in the alignment region on the interferometer's CCD, the spatial position of the computational holographic element is adjusted until the tilt and defocus of the interference fringes are minimized, thus completing the spatial alignment of the computational holographic element and the interferometer.

[0094] This step establishes a unique benchmark for the entire process. Subsequent installation and adjustment of the three mirrors, secondary mirrors, primary mirrors, and plane mirrors are all based on this benchmark, which is different from the local benchmark in single-mirror inspection where "CGH only serves a single mirror".

[0095] Step 2: Adjust the position and orientation of the three lenses (first installation of the power amplifier, laying the foundation for the recursive process).

[0096] Spacing and position: such as Figure 6As shown, the distance region 3.4A of the holographic element's three mirrors is calculated. A converging spherical wave is projected onto the center of the three mirrors to achieve spacing constraint. Crosshairs are projected onto the edges of the three mirrors in the reference regions 3.4C, 3.4D, 3.4E, and 3.4F to achieve coarse alignment. During the coarse alignment stage: During installation, a steel ruler is used to visually assess and adjust the position of the three mirrors to ensure a positional error better than 0.5 mm. Visually, the light spot converging at the center of the three mirror surfaces is minimized, at which point the distance error is less than 2 mm.

[0097] Attitude Constraints: A three-mirror aerial pointing reference point is constructed using the diffraction region of the reference point. Graph paper is used as the receiving screen and placed at the aerial reference point position. The light spot on the graph paper is observed visually. The pitch and azimuth attitudes of the three mirrors are adjusted so that the light spot formed by the diffracted light in region 3.4B of CGH after reflection by the three mirrors coincides with the light spot pointing to the preset aerial pointing reference point at 3.3A. Visual judgment is aided by the graph paper scale to control the overlap accuracy of the two light spots to be better than 0.5mm, thus completing the pitch and azimuth pointing adjustment of the three mirrors.

[0098] Coupling control: Synchronously lock the attitude of the three mirrors to avoid affecting the optical paths of the secondary mirror, primary mirror, and plane mirror.

[0099] Step 3: Adjust the position and orientation of the secondary mirror (alignment across two stages).

[0100] Spacing and position: such as Figure 7 As shown, the diffracted light from the secondary mirror of the holographic element at a distance of 3.5A is reflected by the three mirrors and converges to the center of the secondary mirror, thus achieving the spacing constraint between the secondary mirrors. The diffracted light from the reference areas of the secondary mirror at 3.5C, 3.5D, 3.5E, and 3.5F is reflected by the three mirrors and projected onto the crosshair reference line at the edge of the secondary mirror, thus achieving the position constraint. During the coarse alignment stage: During installation, the position of the secondary mirror is adjusted visually with the aid of a steel ruler to ensure that its positional error is better than 0.5 mm; visually, the light spot converging to the center of the secondary mirror surface is minimized, at which point the distance error is less than 2 mm.

[0101] Attitude Constraint: Place the coordinate paper at the preset convergence point of the primary mirror and visually judge the overlap of the light spots on the coordinate paper. Adjust the pitch and azimuth attitude of the secondary mirror so that the light spot formed after the diffracted light in diffraction region 3.5B is reflected sequentially by the three mirrors and the secondary mirror, precisely coincides with the light spot at the preset reference point of the primary mirror center in diffraction region 3.6A, with an overlap accuracy better than 0.5mm. This completes the installation and adjustment of the pitch and azimuth of the secondary mirror. This process constrains the secondary mirror with the three mirrors as a reference, solving the coupling misalignment problem between the two mirrors.

[0102] Coupling control: Synchronously lock the attitude of the secondary mirror to avoid affecting the optical paths of the subsequent primary mirror and plane mirror.

[0103] Step 4: Adjust the position and orientation of the primary mirror (three-level recursion, complete the constraint of the entire mirror).

[0104] Spacing and position: such as Figure 8 As shown, the diffraction region 3.6A of the holographic element is calculated to project the diffracted and converging spherical wave onto the geometric center of the primary mirror surface, thus constraining the primary mirror spacing. The diffraction regions 3.6C, 3.6D, 3.6E, and 3.6F diffract the crosshairs onto the upper, lower, left, and right edges of the primary mirror, respectively, thus constraining the primary mirror position. During the coarse alignment stage: During installation, a steel ruler is used to visually assess and adjust the positions of the three mirrors, ensuring their positional error is better than 0.5 mm. Visual inspection is then performed to minimize the light spot converging at the center of the primary mirror surface, at which point the distance error is less than 2 mm.

[0105] Attitude Constraints: Adjust the pitch and azimuth attitude of the primary mirror so that the light spot formed by the diffracted light in diffraction region 3.6B after being reflected sequentially by the third mirror, secondary mirror, and primary mirror coincides with the light spot of the preset aerial reference point of the primary mirror pointed to by diffraction region 3.3B. This completes the installation and adjustment of the primary mirror's pitch and azimuth orientation. Visual judgment is aided by coordinate paper scales to control the overlap accuracy of the two light spots to be better than 0.5mm. This completes the step-by-step recursive constraint from the later stage (third mirror) to the earlier stage (primary mirror).

[0106] Coupling control: Synchronously lock the primary mirror attitude to avoid affecting the optical path of the subsequent plane mirror.

[0107] Step 5: Plane mirror-assisted closed-loop calibration (constructing an off-axis three-mirror system calibration loop)

[0108] The off-axis three-mirror optical system is calibrated with a plane mirror-assisted closed-loop calibration, forming a closed calibration loop of "interferometer → CGH → three mirrors → secondary mirror → primary mirror → plane mirror → primary mirror → secondary mirror → three mirrors → CGH → interferometer".

[0109] Spacing and position: such as Figure 9 As shown, the diffracted light from the 3.7 Å diffraction region of the holographic element is reflected sequentially by the three mirrors, secondary mirror, and primary mirror, converging at the geometric center of the plane mirror, thus achieving the alignment of the plane mirror spacing. During the coarse alignment stage: A steel ruler is used to visually adjust the position of the plane mirror, ensuring its positional error is better than 0.5 mm; visually, the light spot converging at the center of the plane mirror surface is minimized, at which point the distance error is less than 2 mm.

[0110] Attitude Constraint: Graph paper is used as the receiving screen, placed at the center convergence point of the primary mirror. The light spot on the graph paper is observed visually. The pitch and azimuth of the plane mirror are adjusted so that the light spot formed by the diffracted light in the diffraction region 3.7B after sequential reflection by the three mirrors, secondary mirror, primary mirror, and plane mirror coincides with the light spot at the preset center convergence point of the primary mirror, which is pointed to by the diffraction region 3.3B. This completes the pitch and azimuth adjustment of the plane mirror. The overlap accuracy of the two light spots is controlled to be better than 0.5mm by visual judgment aided by the graph paper scale.

[0111] Coupling control: Synchronously lock the pitch and azimuth attitude of the plane mirror to avoid affecting the closed-loop calibration optical path of the system.

[0112] Step 6: System fine-tuning and wavefront convergence (correcting multi-level recursive accumulated errors)

[0113] Fine alignment stage: After completing the aforementioned coarse alignment process, the process proceeds to precision closed-loop calibration. For example... Figure 10 As shown, wavefront data of the main holographic region is acquired using an interferometer, and the overall interference fringes of the CGH main holographic region are collected, no longer relying on the visually observed spot position. The system wavefront aberrations are then calculated using wavefront analysis software.

[0114] Axial adjustment: Fine-tune the axial distance of each mirror to minimize the defocus component of the system;

[0115] Attitude adjustment: Fine-tune the decenter / rotation attitude of each mirror to minimize the astigmatism component of the system;

[0116] The final system wavefront aberration RMS was achieved to be ≤ 0.1λ (λ = 632.8 nm), completing the overall system assembly and adjustment. After assembly and adjustment, the system was left to stand for 4–12 hours, and the change in wavefront error was ≤ 0.02λ.

[0117] In summary, this invention discloses a high-precision assembly and adjustment method for an off-axis three-mirror optical system based on a partitioned computational holographic element. This method uses a single partitioned computational holographic element as the sole optical reference for the entire assembly and adjustment process, in conjunction with a plane mirror with an aperture similar to the primary mirror. It employs a multi-stage recursive assembly and adjustment strategy from the later stages to the earlier stages, along with a full-path closed-loop calibration mechanism, to sequentially complete the alignment of the interferometer and CGH, the assembly and adjustment of the three mirrors, the secondary mirror assembly and adjustment, the primary mirror assembly and adjustment, the plane mirror-assisted closed-loop calibration, and the system fine-tuning. The partitioned CGH integrates multiple independently coded, crosstalk-free functional diffraction regions, including the main hologram, alignment hologram, reference point diffraction, and distance, position, and pointing references for each mirror. This enables integrated coarse alignment, attitude constraint, spacing constraint, and wavefront detection. By constraining the pose of the preceding mirrors with the subsequent mirrors, a closed verification loop is constructed: "CGH → three mirrors → secondary mirror → primary mirror → plane mirror → primary mirror → secondary mirror → three mirrors → CGH". Finally, an interferometer is used to collect the system's wavefront error and iteratively fine-tune the poses of each mirror to correct the accumulated error in the multi-stage recursive process. This invention replaces the traditional multi-device distributed reference with a single optical reference, eliminating reference transmission error at its source. Combined with multi-stage recursion and closed-loop calibration, it solves the problem of strong coupling misalignment in multi-mirror six-degree-of-freedom systems. Ultimately, the system wavefront error RMS ≤ 0.1 λ (λ = 632.8 nm), and the wavefront error change after 4–12 hours of static adjustment after assembly is ≤ 0.02 λ. This invention offers advantages such as unified benchmarks, integrated coarse and fine machining, a simple process, and high stability, making it suitable for the assembly and adjustment of high-precision off-axis reflective optical systems such as space remote sensing payloads and astronomical telescopes. The method achieves mirror-by-mirror step-by-step assembly and adjustment by calculating holographic elements in multiple zones, resulting in a simple assembly and adjustment process with clear constraints, significantly improving the accuracy and efficiency of off-axis three-mirror optical system assembly and adjustment.

[0118] The above embodiments are merely typical implementations of the present invention and are not intended to limit the present invention. All equivalent substitutions or improvements made within the scope of the claims of the present invention are within the protection scope of the present invention.

Claims

1. A high-precision assembly and adjustment method for an off-axis three-mirror optical system based on partitioned computational holographic elements, characterized in that, The method uses a single partitioned computational holographic element (CGH) as the sole optical reference for the assembly and adjustment process, and is combined with a plane mirror with an aperture similar to that of the primary mirror. It adopts a multi-stage recursive assembly and adjustment strategy from the rear stage to the front stage, consisting of three mirrors → secondary mirror → primary mirror, and a closed-loop calibration mechanism for the entire optical path. The assembly and adjustment method includes the following steps: alignment of the interferometer with the CGH, assembly and adjustment of the three mirrors, assembly and adjustment of the secondary mirror, assembly and adjustment of the primary mirror, auxiliary closed-loop calibration of the plane mirror, and fine assembly and adjustment of the system; The multi-stage recursive adjustment strategy is as follows: using the reference beams of different functional areas of the CGH, the mirrors are adjusted step by step through the three mirrors, secondary mirrors, and primary mirrors, and the adjustment results of the subsequent mirrors are used to constrain the adjustment accuracy of the preceding mirrors. The all-optical-path closed-loop calibration mechanism is as follows: the off-axis three-mirror optical system is calibrated by plane mirrors to form a closed calibration loop of "interferometer → CGH → three mirrors → secondary mirror → primary mirror → plane mirror → primary mirror → secondary mirror → three mirrors → CGH → interferometer". The pose of each mirror is iteratively adjusted based on the wavefront error of the system to correct the accumulated error in the multi-level recursive process. The sole optical reference is: the optical reference is not changed throughout the entire process, the detection optical path is not reconstructed, and the six degrees of freedom constraints of all mirrors are based on the single CGH as a unified reference.

2. The method according to claim 1, characterized in that, After assembly and adjustment, the wavefront error RMS of the system is ≤0.1λ, where λ=632.8nm, and the change in wavefront error after the system is left to stand for 4–12 hours is ≤0.02λ.

3. The method according to claim 1, characterized in that, The partitioned CGH integrates multiple independently coded and crosstalk-free functional diffraction regions, each of which independently undertakes coarse alignment, attitude constraint, spacing constraint and wavefront detection functions. The functional diffraction regions include: the main holographic region, the alignment holographic region, the reference point diffraction region, the three-mirror distance region, the three-mirror pointing region, the three-mirror position reference region, the secondary mirror distance region, the secondary mirror pointing region, the secondary mirror position reference region, the primary mirror distance region, the primary mirror pointing region, the primary mirror position reference region, and the plane mirror distance region and the plane mirror pointing region. The distance region is used to project and converge spherical waves to the center of the mirror surface to constrain the mirror spacing; the position reference region is used to project crosshairs to the edge of the mirror to constrain the mirror position; the pointing region is used to guide the adjustment of the mirror pitch and azimuth angles; and the reference point diffraction region is used to project a three-dimensional spatial reference point to construct the mirror pitch and azimuth pointing reference.

4. The method according to claim 3, characterized in that, The three-mirror assembly and adjustment steps include: By utilizing the three-mirror distance region of CGH, a converging spherical wave is projected to the center of the three mirrors to achieve three-mirror distance constraint; By using the three-mirror position reference area of ​​CGH to project crosshairs to the edges of the three mirrors, coarse alignment of the three mirrors can be achieved. Using the reference point diffraction region of the CGH, a three-mirror aerial pointing reference point is constructed. The attitude of the three mirrors is adjusted so that the diffraction beam points reflected by the three mirrors coincide with the preset aerial reference point, thus completing the installation and adjustment of the pitch and azimuth of the three mirrors.

5. The method according to claim 3, characterized in that, The secondary mirror assembly and adjustment steps include: The diffracted light generated in the secondary mirror distance region of CGH is reflected by three mirrors and converged to the center of the secondary mirror, thereby achieving secondary mirror spacing constraint; The diffracted light generated by the secondary mirror position reference region of CGH is reflected by the three mirrors and projected onto the edge of the secondary mirror, thereby achieving the position constraint of the secondary mirror; Adjust the secondary mirror's attitude so that the convergence point of the diffracted beams reflected by the three mirrors and the secondary mirror coincides with the convergence point of the primary mirror's center preset by CGH, thus completing the installation and adjustment of the secondary mirror's pitch and azimuth.

6. The method according to claim 3, characterized in that, The primary mirror assembly and adjustment steps include: By utilizing the primary mirror distance region of the CGH to project and converge spherical waves to the center of the primary mirror, the primary mirror distance constraint is achieved; The position of the primary mirror is constrained by projecting crosshairs onto the edge of the primary mirror using the primary mirror position reference area of ​​the CGH. Adjust the attitude of the primary mirror so that the convergence point of the diffraction beams reflected by the three mirrors, secondary mirrors, and primary mirror coincides with the CGH-preset primary mirror aerial pointing reference point, thus completing the installation and adjustment of the primary mirror's pitch and azimuth.

7. The method according to claim 3, characterized in that, The plane mirror-assisted closed-loop calibration steps include: The diffracted light generated by the plane mirror distance region of CGH is reflected by the three mirrors, secondary mirrors and primary mirror and then converged to the center of the plane mirror, thus realizing the plane mirror spacing adjustment; Adjust the orientation of the plane mirror so that the diffraction beam point reflected by the entire optical path coincides with the convergence point of the primary mirror preset by CGH, forming a closed verification loop.

8. The method according to claim 1, characterized in that, The system fine-tuning steps include: The system acquires full-aperture interference fringes using an interferometer and calculates wavefront aberrations. Fine-tune the axial distance of each mirror to eliminate system defocus components; Fine-tune the roll attitude of each mirror to eliminate astigmatic components of the system; By correcting the accumulated systematic error from multiple recursion stages, the wavefront error is ultimately reduced to less than the preset error value.

9. The method according to claim 1, characterized in that, The off-axis three-mirror optical system consists of three off-axis aspherical mirrors: a primary mirror, a secondary mirror, and a tertiary mirror. There is a strong six-degree-of-freedom coupling relationship between the mirrors. The assembly and adjustment method differs from the independent testing and assembly and adjustment of a single off-axis sub-mirror, and must simultaneously satisfy multi-level optical path recursive alignment and wavefront convergence of the entire system.

10. The method according to claim 3, characterized in that, Each functional diffraction region of the CGH employs independent phase coding to ensure that the diffracted beams in each region do not interfere with each other during propagation; combined with the central convergence point, the edge crosshairs, and the aerial pointing reference point, the full constraint of the six degrees of freedom of the reflector is achieved.

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