Coaxial catadioptric infrared cryogenic optical system and its assembly and testing method

Abstract

The application discloses a coaxial folding and reflecting type infrared cryogenic optical system and a mounting, adjusting and testing method thereof. The optical system is composed of a main optical system, a main system support frame, a rear optical path infrared lens group, an infrared lens group lens seat and a rear optical path mounting bottom plate. In order to improve the adjusting precision and simplify the adjusting freedom, precise adjusting equipment such as an interferometer, a laser tracker, a centering instrument and a theodolite is adopted, the main optical system and the rear optical path infrared lens group are first adjusted as independent units and then are precisely integrated and tested. In order to reduce the testing period and cost of the cryogenic optical system, a testing compensation lens is inserted into the cryogenic optical path, so that the cryogenic optical system can be imaged and tested and the image quality can be evaluated in a normal temperature and pressure environment. The method has the advantages of simple principle, high precision, short period and low cost and is suitable for the mounting, adjusting and testing of the coaxial folding and reflecting type infrared cryogenic optical system.

Description

Technical Field

[0001] This invention relates to the field of infrared cryogenic optical system assembly, adjustment, and testing, and particularly to a coaxial catadioptric infrared cryogenic optical system and its assembly, adjustment, and testing method. Background Technology

[0002] With the development of infrared remote sensing technology, the requirements for the detection sensitivity of infrared optical systems are becoming increasingly stringent. Low-temperature optics technology has become the main means to reduce background noise and improve detection sensitivity in infrared optical systems. Due to the physical properties of thermal expansion and contraction, the relative positions of optical and mechanical components in low-temperature optical systems change significantly compared to their room-temperature state, which will affect the imaging quality of the optical system. To reduce this impact, low-temperature optical systems require higher assembly and adjustment precision.

[0003] Limited by the width of the crosshairs and mechanical positioning errors, the traditional assembly and adjustment method of controlling the relative positions of optomechanical components by installing crosshairs in the central hole of the primary mirror and the rear optical path lens group can no longer meet the assembly and adjustment accuracy requirements of optical systems at low temperatures. In addition, the existing low-temperature vacuum environment simulators in China are complex and bulky, have long cooling times and high costs, making the assembly, adjustment and testing of low-temperature optical systems difficult and expensive. Summary of the Invention

[0004] The present invention aims to provide a catadioptric low-temperature infrared optical system and its assembly, adjustment and testing method to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A coaxial catadioptric infrared cryogenic optical system mainly consists of a main optical system and a rear optical path, which are connected by a carbon fiber truss structure.

[0007] The main optical system includes: secondary mirror assembly 1, main system support frame 2, primary mirror assembly 3, primary mirror assembly support structure 7, secondary mirror assembly support structure 8, and secondary mirror assembly fixing structure 9;

[0008] The rear optical path includes: the first lens group 4, the second lens group 5, and the mounting base plate 6.

[0009] The primary lens assembly support structure 7 is fixedly connected to one end of the main system support frame 2, and the secondary lens assembly support structure 8 is fixedly connected to the other end of the main system support frame 2 through the secondary lens assembly fixing structure 9. The secondary lens assembly 1 is installed on the secondary lens assembly support structure 8, and the primary lens assembly 3 is installed on the primary lens assembly support structure 7. The rear optical path mounting base plate 6 is located on the side of the primary lens assembly support structure 7 away from the main system support frame 2. The rear optical path first lens group 4 and the rear optical path second lens group 5 are sequentially arranged on the rear optical path mounting base plate 6.

[0010] A method for assembling and testing a coaxial catadioptric infrared cryogenic optical system, the method comprising:

[0011] S1. Assembly and adjustment of the main optical system: The main optical system is assembled and adjusted using the optical path autocollimation method. Marker lens A is attached to the edge of the main system support frame to record the optical axis direction of the main system. After the assembly and adjustment is completed, the main optical system is thermally applied.

[0012] S2. Rear optical path lens assembly and adjustment: Using the mounting reference surface inside the lens mount as a reference, install the lens assembly into the lens mount in sequence. Use a two-way alignment instrument to test the tilt of the lens assembly and use a dial indicator to test the eccentricity of the lens assembly inside the lens mount. Correct the tilt of the lens assembly by grinding the lens spacer or shims the copper sheet, and correct the eccentricity of the lens assembly by adjusting the coaxiality of the lens assembly with the mounting reference surface.

[0013] S3. Lens Mount Installation and Adjustment: First, install the second lens group mount onto the rear optical path mounting base. Use the mounting reference surface inside the second lens group mount as the mounting reference for the rear optical path lens group mount, and define the central axis of this reference surface as the optical axis of the rear optical path. Attach marker mirror B to the rear optical path mounting base to record the direction of this optical axis. Move the position of the first lens group mount to adjust the distance and coaxiality of the two mounts. Use a laser tracker to test the distance and coaxiality of the two mounts. After installation, perform a heat treatment on the rear optical path.

[0014] S4. Assembly and Adjustment of Main Optical System and Rear Optical Path: With the main optical system in a fixed position, adjust the position of the rear optical path mounting plate to ensure that the distance and eccentricity between the two are within the tolerance range; use a laser tracker to test the distance and eccentricity between the main system and the rear optical path lens group, and adjust by grinding the shims; use a theodolite to monitor the direction of the optical axis of the main system and the optical axis of the rear optical path to ensure that the two optical axes are aligned, and perform whole-machine thermal installation after the assembly is completed;

[0015] S5. Test in normal temperature and pressure environment: Install a normal temperature test compensation lens in front of the second lens group of the rear optical path to compensate the image quality of the optical system. Use an infrared target simulator to test the imaging quality of the optical system under normal temperature and pressure environment.

[0016] S6. Testing in a low-temperature vacuum environment: Remove the room-temperature test compensation lens, place the optical system inside the low-temperature vacuum simulator, and use an infrared target simulator to test the imaging quality of the optical system; if defocusing occurs, the position of the detector focal plane can be finely adjusted through the focusing mechanism to optimize the low-temperature image quality of the optical system.

[0017] Preferably, the optical path self-collimation method in step S1 is as follows:

[0018] A standard plane mirror is placed at the front end of the main system, and an interferometer is placed behind the main mirror. The theoretical positions of the interferometer's focus and the main system's focus coincide. The converging rays emitted by the interference are reflected by the main system and become parallel rays. These parallel rays are reflected by the standard plane mirror and converge again through the main system to enter the interferometer to form an interferogram. The wavefront of the main system is evaluated through the interferogram.

[0019] Preferably, the marker mirror A in step S1 is a cube with three vertical reflective surfaces, the normals of which are parallel to the azimuth, pitch and rotation directions of the optical axis of the main optical system, respectively.

[0020] Preferably, in step S2, a mounting reference surface is machined inside the lens mount to serve as the mounting reference for the lens assembly.

[0021] Preferably, in step S3, the marker mirror B is a cube with three vertical reflective surfaces, and the normals of the three vertical surfaces are parallel to the azimuth, pitch and rotation directions of the optical axis, respectively.

[0022] Preferably, step S4 specifically includes the following sub-steps:

[0023] S401. The position of the main optical system is fixed. Adjust the position of the rear optical path mounting base plate. Use a laser tracker to test the distance and eccentricity between the main system and the rear optical path lens group. Adjust the distance and eccentricity to the preset value by grinding the shims.

[0024] S402. Place the theodolite 1 in front of the main system. The theodolite 1 should first be aligned with the marker mirror A on the main system support frame, and then rotated 180°.

[0025] S403. Place a standard plane mirror in front of the main system and adjust the azimuth and pitch of the standard plane mirror so that the theodolite 1 is auto-aligned with the standard plane mirror. At this time, the normal direction of the standard plane mirror is the direction of the optical axis of the main system.

[0026] S404. Place the theodolite 2 on the side of the main optical system and align its optical axis with the standard plane mirror.

[0027] S405. Place the theodolite 3 behind the marker mirror B on the rear optical path mounting base plate, adjust the azimuth and pitch of the theodolite 3, and make the theodolite 2 rotate α° and then autoalign with the theodolite 3.

[0028] S406. Rotate the azimuth of the theodolite 3 by 180°-α° so that it is aimed at the marker mirror B. Fine-tune the azimuth and pitch of the rear optical path mounting plate so that the optical axis of the theodolite 3 is self-aligned with the marker mirror B. At this time, the optical axis direction of the main optical system is consistent with the optical axis direction of the rear optical path.

[0029] S407. After fixing the optical path mounting base plate, perform whole-machine thermal implementation.

[0030] Preferably, the room temperature test compensation lens in step S5 is a meniscus lens with a large radius of curvature. This lens is installed at the front end of the second lens group in the rear optical path, with loose assembly and adjustment tolerances, and its positional accuracy can be ensured by a mechanical positioning structure.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] This invention discloses a method for assembling and testing a coaxial catadioptric infrared cryogenic optical system. The main optical system and the rear optical path infrared lens group are assembled and tested as independent optical path units, reducing the degrees of freedom and difficulty in assembling multi-element optical systems and simplifying the assembly process. Precision assembly equipment such as interferometers, centerers, laser trackers, and theodolites are used during the assembly process, significantly improving the assembly accuracy of the optical system and the temperature adaptability of the optomechanical system. By inserting a room-temperature test compensation mirror into the cryogenic optical path, the cryogenic optical system can be subjected to optical imaging tests and image quality evaluation under normal temperature and pressure conditions, reducing the difficulty, cycle time, and cost of testing cryogenic optical systems. This method has the advantages of simple principle, high precision, short cycle time, and low cost. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of a coaxial catadioptric infrared cryogenic optical system.

[0034] Figure 2 A flowchart for the assembly, adjustment, and testing of a coaxial catadioptric infrared cryogenic optical system;

[0035] Figure 3 This is a schematic diagram of the main optical system assembly and adjustment of a coaxial catadioptric infrared cryogenic optical system.

[0036] Figure 4 This is a cross-sectional view of the infrared lens group in the rear optical path of a coaxial catadioptric infrared cryogenic optical system.

[0037] Figure 5 This is a schematic diagram of the main system and the rear optical path docking of a coaxial catadioptric infrared cryogenic optical system.

[0038] The reference numerals in the accompanying drawings include:

[0039] 1. Secondary mirror assembly; 2. Main system support frame; 3. Primary mirror assembly; 4. Rear optical path first lens group; 5. Rear optical path second lens group; 6. Rear optical path mounting base plate; 7. Primary mirror assembly support structure; 8. Secondary mirror assembly support structure; 9. Secondary mirror assembly fixing structure; 10. Standard plane mirror A; 11. Marker mirror A; 12. Interferometer; 13. PI adjustment frame; 41. Rear optical path first lens group mount; 42. First lens group mount internal mounting reference surface; 43. Rear optical path first lens group spacer; 44. Infrared lens 1 assembly; 45. Infrared lens 2 assembly; 50. Room temperature test compensation mirror assembly; 51. Rear optical path second lens group mount; 52. Rear optical path second lens group spacer; 53. Second lens group mount internal mounting reference surface; 54. Infrared lens 4 assembly; 55. Infrared lens 3 assembly; 56. Room temperature test compensation mirror assembly limiting structure; 14. Standard plane mirror B; 15. Marker mirror B. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments:

[0041] like Figure 1 As shown, the coaxial catadioptric infrared low-temperature optical system provided in this embodiment of the invention mainly consists of a main optical system and a rear optical path, which are connected by a carbon fiber truss structure. The main optical system is fixed to the front end of the carbon fiber truss structure by the main system support frame 2, and the rear optical path is fixed to the rear end of the carbon fiber truss structure by the rear optical path mounting base plate 6.

[0042] The main optical system includes: secondary mirror assembly 1, main system support frame 2, primary mirror assembly 3, primary mirror assembly support structure 7, secondary mirror assembly support structure 8, and secondary mirror assembly fixing structure 9;

[0043] The rear optical path includes: the first lens group 4, the second lens group 5, and the mounting base plate 6.

[0044] The primary lens assembly support structure 7 is fixedly connected to one end of the main system support frame 2, and the secondary lens assembly support structure 8 is fixedly connected to the other end of the main system support frame 2 through the secondary lens assembly fixing structure 9. The secondary lens assembly 1 is installed on the secondary lens assembly support structure 8, and the primary lens assembly 3 is installed on the primary lens assembly support structure 7. The rear optical path mounting base plate 6 is located on the side of the primary lens assembly support structure 7 away from the main system support frame 2. The rear optical path first lens group 4 and the rear optical path second lens group 5 are sequentially arranged on the rear optical path mounting base plate 6.

[0045] like Figure 2 As shown, an embodiment of the present invention provides a method for assembling, adjusting, and testing a coaxial catadioptric infrared cryogenic optical system, which includes the following steps:

[0046] S1. Assembly and adjustment of the main optical system, the specific implementation method is as follows:

[0047] Building such Figure 3 The self-collimating optical path shown has a standard plane mirror A10 placed at the front end of the main system. The normal direction of the standard plane mirror A10 is consistent with the optical axis direction of the main mirror. The optical axis direction of the main mirror can be marked in the optical path for detecting the surface shape of the main mirror. An interferometer 12 is placed behind the main mirror assembly 3, with the focus of the interferometer 12 at the theoretical position of the focus of the main system.

[0048] The secondary mirror assembly 1 is installed onto the PI adjustment bracket 13. The position and angle of the secondary mirror assembly 1 are adjusted using the PI adjustment bracket 13 to ensure that the wavefront of the main system acquired by the interferometer 12 reaches the preset value. The secondary mirror assembly 1 is first fixed to the secondary mirror assembly support structure 8, and then fixed to the main system support frame 2 via the secondary mirror assembly fixing structure 9. The PI adjustment bracket 13 is then removed, completing the assembly and adjustment of the main optical system. A marker mirror A11 is attached to the edge of the main system support frame 2 to record the optical axis direction of the main system, and the main system is then subjected to thermal testing.

[0049] S2. Rear optical path lens assembly and adjustment. The cross-section of the rear optical path lens assembly is shown below. Figure 4 As shown, it consists of two parts: the first lens group of the rear optical path and the second lens group of the rear optical path.

[0050] The specific implementation method for assembling and adjusting the first lens of the rear optical path is as follows: First, place the first lens group mount 41 on the assembly and adjustment platform of the bidirectional alignment instrument, and install and fix the infrared lens 1 assembly 44 with the mounting reference surface 42 inside the mount as the reference; then, rotate the first lens group mount 41 180° and reset the optical axis, and then place the first lens group spacer 43 and the infrared lens 2 assembly 45 into the first lens group mount 41 in sequence, adjust the eccentricity and tilt of the infrared lens 2 assembly 45 to the preset value, and then fix it.

[0051] The specific implementation method for assembling and adjusting the second lens in the rear optical path is as follows: First, place the second lens group mount 51 on the assembly and adjustment platform of the two-way alignment instrument. Using the mounting reference surface 53 inside the mount as a reference, install and fix the infrared lens 4 assembly 54. Then, place the second lens group spacer 52 and the infrared lens 3 assembly 55 into the second lens group mount 51 in sequence. Test and adjust the eccentricity and tilt of the infrared lens 3 assembly 55. After meeting the preset value, fix it. The room temperature test compensation mirror assembly 50 is installed at the front end of the second lens group mount 51 in the rear optical path through the mechanical limiting structure 56.

[0052] S3. Infrared Lens Group Mount Assembly and Adjustment. Using the mounting reference surface 53 of the second lens group mount as the mounting reference, the central axis of this reference surface is defined as the optical axis of the rear optical path. A marker mirror B15 is attached to the rear optical path mounting base to record the direction of this optical axis; the first lens group mount 41 is moved to adjust the relative position of the two mounts, and a laser tracker is used to test the distance and coaxiality of the two mounts. After installation, the rear optical path is thermally tested.

[0053] S4. Assembly and adjustment of the main optical system and rear optical path. The assembly and adjustment method of the main optical system and rear optical path is as follows: Figure 5 As shown, the specific implementation steps are as follows:

[0054] Step S4 specifically includes the following sub-steps:

[0055] S401. Fix the main system, adjust the position of the rear optical path mounting base plate 6, use a laser tracker to test the distance and eccentricity between the main system and the first lens group 4 of the rear optical path, and adjust by grinding the shims.

[0056] S402. Place the theodolite 1 in front of the main system. The theodolite 1 should first be aligned with the marker mirror A11 on the main system support frame, and then rotated 180°.

[0057] S403. Place a standard plane mirror 14 in front of the main system and adjust the azimuth and pitch of the standard plane mirror 14 so that the theodolite 1 is self-aligned with the standard plane mirror 14.

[0058] S404. Place the theodolite 2 on the side of the main optical system, make its optical axis self-aligned with the standard plane mirror 14, and then rotate the theodolite 2 180°.

[0059] S405. Place the theodolite 3 behind the rear optical axis marker mirror B15, and adjust the azimuth and pitch of the theodolite 3 so that the theodolite 2 can be self-aligned with the theodolite 3 after rotating its azimuth by α°.

[0060] S406. Rotate the azimuth of the theodolite 3 by 180°-α° so that it is aimed at the optical axis marker mirror B15 of the rear optical path. Fine-tune the azimuth and pitch of the rear optical path mounting base plate 6 so that the optical axis of the theodolite 3 is self-aligned with the optical axis of the marker mirror B15. At this time, the optical axis direction of the main optical system is consistent with the optical axis direction of the rear optical path.

[0061] S407, after fixing the optical path mounting base plate 6, perform overall heat treatment;

[0062] S5. Testing under normal temperature and pressure. A normal temperature test compensation mirror 50 is installed in front of the second lens group in the rear optical path to compensate for the image quality of the optical system. The imaging quality of the optical system is tested under normal temperature and pressure using an infrared target simulator.

[0063] S6. Testing in a low-temperature vacuum environment. Remove the room-temperature test compensation mirror 50 and place the optical system in a low-temperature vacuum simulator. Use an infrared target simulator to test the imaging quality of the optical system. If defocusing occurs, the position of the detector focal plane can be finely adjusted through the focusing mechanism to optimize the low-temperature image quality of the optical system.

[0064] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A coaxial catadioptric infrared cryogenic optical system, characterized in that: The coaxial catadioptric infrared cryogenic optical system consists of a main optical system and a rear optical path, which are connected by a carbon fiber truss structure. The main optical system includes: a secondary mirror assembly (1), a main system support frame (2), a primary mirror assembly (3), a support structure for the primary mirror assembly (3), a support structure for the secondary mirror assembly (1), and a fixing structure for the secondary mirror assembly (1); The rear optical path includes: a first lens group (4), a second lens group (5), and a mounting base plate (6); The primary mirror assembly (3) support structure is fixedly connected to one end of the main system support frame (2), and the secondary mirror assembly (1) support structure is fixedly connected to the other end of the main system support frame (2) through the secondary mirror assembly (1) fixing structure. The secondary mirror assembly (1) is mounted on the secondary mirror assembly (1) support structure, and the primary mirror assembly (3) is mounted on the primary mirror assembly (3) support structure. The rear optical path mounting base plate (6) is located on the side of the primary mirror assembly (3) support structure away from the main system support frame (2). The rear optical path first lens group (4) and the rear optical path second lens group (5) are sequentially mounted on the rear optical path mounting base plate (6).

2. A method for assembling, adjusting, and testing a coaxial catadioptric infrared cryogenic optical system as described in claim 1, characterized in that: The assembly and testing method includes: Step S1, Assembly and Adjustment of the Main Optical System: The main optical system is assembled and adjusted using the optical path autocollimation method. Marker lens A is attached to the edge of the main system support frame to record the optical axis direction of the main system. After the assembly and adjustment is completed, the main optical system is thermally applied. Step S2, Rear optical path lens assembly and adjustment: Using the mounting reference surface inside the lens mount as a reference, install the lens assemblies into the lens mount in sequence. Use a two-way alignment instrument to test the tilt of the lens assembly and a dial indicator to test the eccentricity of the lens assembly inside the lens mount. Correct the tilt of the lens assembly by grinding the lens spacer or shims with copper shims, and correct the eccentricity of the lens assembly by adjusting the coaxiality between the lens assembly and the mounting reference surface. Step S3, Lens Mount Installation and Adjustment: First, install the second lens group mount onto the rear optical path mounting base plate. Use the mounting reference surface inside the second lens group mount as the mounting reference for the rear optical path lens group mount, and define the central axis of this reference surface as the optical axis of the rear optical path. Attach marker mirror B to the rear optical path mounting base plate to record the direction of this optical axis. Move the position of the first lens group mount to adjust the distance and coaxiality of the two mounts. Use a laser tracker to test the distance and coaxiality of the two mounts. After installation, perform a heat treatment on the rear optical path. Step S4, Assembly and Adjustment of Main Optical System and Rear Optical Path: Fix the position of the main optical system, adjust the position of the rear optical path mounting plate to ensure that the distance and eccentricity between the two are within the tolerance range; use a laser tracker to test the distance and eccentricity between the main system and the rear optical path lens group, and adjust by grinding the shims; use a theodolite to monitor the direction of the optical axis of the main system and the optical axis of the rear optical path to ensure that the two optical axes are aligned, and perform whole-machine thermal installation after the assembly is completed; Step S5, Testing in a normal temperature and pressure environment: Install a normal temperature test compensation lens in front of the second lens group in the rear optical path to compensate for the image quality of the optical system. Use an infrared target simulator to test the imaging quality of the optical system in a normal temperature and pressure environment. Step S6, Testing in a low-temperature vacuum environment: Remove the room-temperature test compensation lens, place the optical system inside the low-temperature vacuum simulator, and use an infrared target simulator to test the imaging quality of the optical system; if defocusing occurs, fine-tune the position of the detector focal plane through the focusing mechanism to optimize the low-temperature image quality of the optical system. The optical path self-collimation method described in step S1 is as follows: A standard plane mirror is placed at the front end of the main system, and an interferometer is placed behind the main mirror. The theoretical positions of the focus of the interferometer and the focus of the main system coincide. The converging rays emitted by the interference are reflected by the main system and become parallel rays. These parallel rays are reflected by the standard plane mirror and converge again through the main system to enter the interferometer to form an interferogram. The wavefront of the main system is evaluated by the interferogram. The marker mirror A mentioned in step S1 is a cube with three vertical reflective surfaces. The normals of the three vertical surfaces are parallel to the azimuth, pitch and rotation directions of the optical axis of the main optical system, respectively. In step S2, a mounting reference surface is machined inside the lens mount to serve as the mounting reference for the lens assembly. In step S3, the marker mirror B is a cube with three vertical reflecting surfaces, and the normals of the three vertical surfaces are parallel to the azimuth, pitch and rotation directions of the optical axis, respectively. Step S4 specifically includes the following sub-steps: Step S401: Fix the position of the main optical system, adjust the position of the rear optical path mounting base, use a laser tracker to test the distance and eccentricity between the main system and the rear optical path lens group, and adjust the distance and eccentricity to the preset value by grinding the shims. Step S402: Place the first theodolite in front of the main system. The first theodolite first self-aligns with the marker mirror A on the main system support frame, and then rotates 180°. Step S403: Place a standard plane mirror in front of the main system, and adjust the azimuth and pitch of the standard plane mirror so that the first theodolite is auto-aligned with the standard plane mirror. At this time, the normal direction of the standard plane mirror is the direction of the optical axis of the main system. Step S404: Place a second theodolite on the side of the main optical system and align its optical axis with the standard plane mirror. Step S405: Place the third theodolite behind the marker mirror B on the rear optical path mounting base plate, adjust the azimuth and elevation of the third theodolite, and make the second theodolite rotate α° and then autoalign with the third theodolite. Step S406: Rotate the azimuth of the third theodolite by 180°-α° so that it is aimed at the marker mirror B. Fine-tune the azimuth and pitch of the rear optical path mounting plate so that the optical axis of the third theodolite is self-aligned with the marker mirror B. At this time, the optical axis direction of the main optical system is consistent with the optical axis direction of the rear optical path. Step S407: Fix the optical path mounting base plate and perform overall heat treatment; In step S5, the room temperature test compensation lens is a meniscus lens with a large radius of curvature. This lens is installed at the front end of the second lens group in the rear optical path, with loose assembly and adjustment tolerances, and its positional accuracy is ensured by a mechanical positioning structure.

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