A collimator optical axis perpendicular to the reference plane calibration method

Abstract

This invention relates to the field of optical equipment technology and discloses a calibration method for ensuring the optical axis of a collimator is perpendicular to a reference plane. The method includes the following steps: placing a first theodolite and a second theodolite sequentially from front to back on an optical platform and leveling them; placing the collimator and an optical hexahedron parallel to the right side of the first and second theodolites, and rotating the first and second theodolites 90° to align them parallel to the optical axis of the collimator; aligning the first theodolite and the collimator horizontally, activating the autocollimation function of the second theodolite, and refining the bottom of the flat crystal mounting base by adjusting the autocollimation image deviation until the autocollimation image coincides; rotating the collimator mounting base 90° so that the optical axis of the collimator is upward, aiming the second theodolite at the flat crystal reflecting surface, and refining the bottom of the collimator mounting base according to the elevation angle data to achieve vertical calibration of the optical axis. This invention, through multiple iterative refinements, can control the calibration angle deviation to below 0.3", achieving sub-arcsecond-level vertical calibration of the collimator's optical axis.

Description

Technical Field

[0001] This invention relates to the field of optical equipment technology, and in particular to a calibration method for a collimator whose optical axis is perpendicular to a reference plane. Background Technology

[0002] In recent years, with the continuous development of optoelectronic technology in my country, the testing requirements for airborne optoelectronic detection equipment have been increasing, posing requirements for high-precision calibration and inspection of optical systems. How to accurately and efficiently calibrate ground detection equipment and ensure its testing accuracy is a key challenge. Especially in the field of airborne optoelectronic systems (such as infrared imagers and lidar), while resolution and operating range have significantly improved, the testing accuracy of ground detection equipment is limited by the perpendicularity error of the collimator calibration reference. For example, in a test at an altitude of 5000 meters, a certain type of UAV-borne optoelectronic platform experienced a target positioning deviation of 15 meters due to a 2" tilt of the optical axis. Traditional laser interferometry calibration methods rely on high-cost equipment and are difficult to adapt to dynamic environments; therefore, a high-precision, low-cost vertical calibration method is urgently needed. Summary of the Invention

[0003] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0004] A calibration method for ensuring the optical axis of a collimator is perpendicular to a reference plane includes the following steps:

[0005] S1. Place the first theodolite and the second theodolite on the optical platform from front to back. Set the first theodolite and the second theodolite relative to each other and level them. After leveling, turn on the autocollimation function of the first theodolite. Adjust the second theodolite so that its crosshairs coincide with the autocollimation image of the first theodolite to establish the initial optical axis reference.

[0006] S2. Install the collimator on the collimator mounting base via the collimator adapter. Fix the optical hexahedron flat crystal on the front end of the collimator adapter base via the flat crystal mounting base, and position it in front of the collimator. Place the collimator mounting base vertically on the optical platform and to the right of the first and second theodolites. Rotate the first and second theodolites 90° towards the collimator mounting base so that their optical axis is parallel to the optical axis of the collimator.

[0007] S3. Turn on the collimator, observe its crosshairs with the first theodolite and adjust the position of the collimator until the crosshairs of the first theodolite and the crosshairs of the collimator coincide, and complete the horizontal alignment. Turn on the autocollimation function of the second theodolite, observe the autocollimation image deviation of the optical hexahedral flat crystal reflection, and repair the bottom of the flat crystal mounting base until the autocollimation image coincides.

[0008] S4. Rotate the collimator mounting base 90° so that the optical axis of the collimator is upward. Aim the second theodolite at the flat crystal reflecting surface and adjust the bottom of the collimator mounting base according to the elevation angle data to achieve vertical calibration of the optical axis.

[0009] Furthermore, the first theodolite and the second theodolite are optical theodolites, and in step S1, the tilt angle of the first theodolite and the second theodolite is adjusted by ≤3″ each time.

[0010] Furthermore, in step S2, the flatness of the mounting surface of the collimator mounting base is ≤1μm, and the locking torque of the optical hexahedral flat crystal is 5N·m±0.2N·m during installation.

[0011] Furthermore, both the first and second theodolites have mounting bases on their lower surfaces. The mounting bases are placed on the optical platform, and stepper motors are embedded in the mounting bases. The output shafts of the stepper motors extend out of the upper surface of the mounting bases and connect to the lower surfaces of the corresponding first and second theodolites.

[0012] Furthermore, in step S3, the position of the collimator is adjusted using a process of "coarse centering → fine centering → coarse leveling → fine leveling".

[0013] Furthermore, in step S3, the flat crystal mounting base is iteratively repaired multiple times, including three repair processes: rough repair, semi-rough repair, and fine repair.

[0014] Furthermore, in step S4, the bottom surface of the parallel light tube mounting base is polished by diamond turning, with a cutting depth of ≤1μm and a surface roughness Ra≤0.025μm each time.

[0015] The beneficial effects of this invention are:

[0016] 1. Using the intersection measurement of the first and second theodolites can effectively reduce single-point errors and improve measurement accuracy.

[0017] 2. During the leveling process of the first and second theodolites, adjusting the tilt angle by ≤3″ each time can significantly reduce the interference of base deformation on the measurement system and reduce instrument attitude drift caused by gravity gradient.

[0018] 3. When rotating the first and second theodolites, control their angular velocity within the range of ≤1° / s; low-speed rotation can significantly reduce the acceleration impact of mechanical components (such as bearings and gears), avoid the lag of small displacements caused by inertial torque, thereby avoiding inertial errors and improving measurement accuracy.

[0019] 4. Through multiple iterations and refinements, the calibration angle deviation can be controlled to below 0.3", achieving sub-arcsecond-level vertical calibration of the collimator optical axis. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of the invention.

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the leveling structure for the first and second theodolites;

[0023] Figure 2 A schematic diagram of the structure of the first and second theodolites after their optical axes are paralleled with the collimator;

[0024] Figure 3 This is a schematic diagram of the structure after the collimator is placed vertically.

[0025] Figure 4 This is a cross-sectional view showing the connection between the first theodolite and the optical platform. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0027] Unless otherwise defined, the technical or scientific terms used in this disclosure shall have the ordinary meaning understood by one of ordinary skill in the art to which this disclosure pertains. The terms "first," "second," and similar terms used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships, and these relative positional relationships may change accordingly when the absolute position of the described object changes.

[0028] like Figures 1-4 As shown, a calibration method for a collimator with its optical axis perpendicular to a reference plane includes the following steps:

[0029] S1. Place the first theodolite and the second theodolite on the optical platform from front to back, set the first theodolite and the second theodolite relative to each other and level them. After leveling, turn on the autocollimation function of the first theodolite and adjust the second theodolite so that its crosshairs coincide with the autocollimation image of the first theodolite to establish the initial optical axis reference.

[0030] The first and second theodolites are optical theodolites. The optical theodolites can be leveled by adjusting their three-axis leveling knobs to center the horizontal bubble. Traditional laser interferometry calibration relies on a single laser theodolite and a mechanical reference plane for calibration. However, single-point observations with a single laser theodolite are easily affected by angle measurement errors, with an error angle ≥ 0.5″. In this invention, the use of dual theodolite intersection measurement (first and second theodolites) effectively reduces single-point errors and improves measurement accuracy. In this invention, during the leveling process, each adjustment of the tilt angle ≤ 3″ significantly reduces the interference of base deformation on the measurement system and minimizes instrument attitude drift caused by gravity gradients. When adjusting the theodolites, due to the mechanical structure of the optical theodolite, certain errors occur during mechanical transmission, causing the theodolite base tilt error. This base tilt error is positively correlated with the adjusted tilt angle.

[0031] △θ=k·sinφ

[0032] Where K is the error propagation coefficient, representing the amount of error caused by each 1 arcsecond tilt angle (k = 0.08″), and φ is the adjusted tilt angle. When the adjusted tilt angle is ≤3″, the base tilt error Δθ ≤ 0.24″, and the error propagation is limited to the sub-arcsecond level (<0.25″). This ensures that the parallelism error between the optical axes of the first and second theodolites after leveling is <0.5″. At this point, the redundancy of the dual theodolite joint observation can offset the random errors of a single instrument (such as vibration and airflow disturbance), thereby improving the overall measurement accuracy to 1.4 times that of the single-machine mode.

[0033] S2. Install the collimator on the collimator mounting base via the collimator adapter. Fix the optical hexahedron flat crystal on the front end of the collimator adapter base via the flat crystal mounting base, and position it in front of the collimator. Place the collimator mounting base vertically on the optical platform and to the right of the first and second theodolites. Rotate the first and second theodolites 90° toward the collimator mounting base so that their optical axis is parallel to the optical axis of the collimator.

[0034] When installing optical hexahedral flats, optical hexahedral flats with a λ / 20 flatness are used. These flats utilize a fused silica substrate with a thermal expansion coefficient as low as 0.52 × 10⁻⁶. -6 With a temperature difference of ±5℃, the deformation is ≤0.03μm, which can significantly reduce the impact of temperature drift on detection. Furthermore, it remains stable over a wide temperature range (-100~+200℃), avoiding detection errors caused by thermal stress. When installing the optical hexahedral flat crystal, the mounting surface of the flat crystal mount is checked with a dial indicator to ensure that the flatness of the mounting surface is ≤1μm. During installation, the locking torque of the optical hexahedral flat crystal is ensured to be 5N·m±0.2N·m. If the flatness of the mounting surface exceeds 1μm, it will cause uneven contact between the optical hexahedral flat crystal and the flat crystal mount, generating additional stress and amplifying the overall flatness error. Too small a locking torque (<4.8N·m) will cause the optical hexahedral flat crystal to loosen, introducing vibration errors; too large a torque (>5.2N·m) may cause plastic deformation of the flat crystal mount. In addition, the mounting surface can be pre-treated before installing the optical hexahedral flat crystal. Aviation gasoline can be used to clean the mounting surface to remove oil and particles with a diameter ≤0.5μm, avoiding false flatness readings. When applying torque, first pre-tighten to 3 N·m, then calibrate the flatness, and finally tighten to 5 N·m, releasing residual stress in stages.

[0035] When rotating the first and second theodolites, their rotation can be synchronously driven by a stepper motor or manually, with the angular velocity controlled within the range of ≤1° / s. Low-speed rotation significantly reduces the acceleration impact on mechanical components (such as bearings and gears), avoiding slight displacement lag caused by inertial torque, thus preventing inertial errors and improving measurement accuracy. Preferably, in this invention, the first and second theodolites are controlled by a stepper motor. Compared to manual control, the stepper motor enables precise control of the first and second theodolites, effectively reducing human error. Furthermore, the stepper motor employs micro-step control (e.g., 1 / 16 microstepping) combined with a precision lead screw (0.8mm / turn), achieving a single-step angular displacement ≤0.05°, effectively offsetting transmission chain backlash. Low-speed operation also reduces motor heating and electromagnetic interference, mitigating the impact of temperature drift.

[0036] In the prior art, optical theodolites generally do not have stepper motors installed. In this invention, in order to realize stepper motor control of the rotation of the first theodolite and the second theodolite, a mounting base is provided on the lower end face of the first theodolite and the second theodolite. The mounting base is a regular geometric body with a smooth bottom surface, such as a cuboid or cylinder. The mounting base can be placed parallel and stably on the optical platform. Furthermore, a stepper motor is embedded in the mounting base. The output shaft of the stepper motor extends out of the upper end face of the mounting base and connects to the lower end face of the corresponding first theodolite and second theodolite.

[0037] S3. Turn on the collimator, observe its crosshairs with the first theodolite and adjust the position of the collimator until the crosshairs of the first theodolite and the crosshairs of the collimator coincide, and complete the horizontal alignment. Turn on the autocollimation function of the second theodolite, observe the autocollimation image deviation of the optical hexahedral flat crystal reflection, and repair the bottom of the flat crystal mounting base until the autocollimation image coincides.

[0038] The first theodolite serves as the reference device, with its telescope containing a built-in crosshair reticle as the reference line. When adjusting the position of the collimator, the crosshairs emitted by the collimator are observed through the telescope. When the centers of the crosshairs coincide, it indicates that the optical axis of the collimator is collinear with the line of sight of the first theodolite. At this point, the beam output by the collimator is perfectly parallel, which meets the requirements for subsequent measurements.

[0039] The collimator outputs a parallel beam of light. By adjusting its focal length, the crosshairs are clearly imaged on the theodolite reticle. When the two crosshairs coincide, it proves that the light output from the collimator has neither diverged nor converged, forming an ideal parallel beam.

[0040] When adjusting the position of the collimator, the process of "coarse centering → fine centering → coarse leveling → fine leveling" is adopted. Combined with the intuitive crosshair comparison within the field of view of the first theodolite, alignment can be completed quickly, reducing the time spent on repeated adjustments.

[0041] After the collimator is adjusted, the autocollimation function of the second theodolite is activated. The autocollimation function achieves self-collimation by introducing a flat optical crystal to deflect the light rays, thereby eliminating spherical aberration and other aberrations, and improving image quality. The positional deviation Δd of the crosshair image reflected by the optical hexahedral flat optical crystal reflects the aberrations of the system. According to the principles of geometric optics, there is a certain relationship between the magnitude and direction of the aberrations and the height component of the aberrations.

[0042]

[0043] Wherein, aberration Δd·S represents the product of the magnitude and direction of the aberration, where Δd is the positional deviation, S is the pixel size (unit: mm / pixel), representing the actual physical size corresponding to each pixel; and f represents the focal length of the imaging system.

[0044] In this invention, the preferred values ​​are S = 0.005 mm / pixel and f = 40 mm, meaning that the actual physical size of each pixel is 5 micrometers. The smaller the pixel size, the higher the sensor's sensitivity to position deviation (Δd). When Δd = 1 pixel, the corresponding actual displacement is only 5 micrometers, which can achieve sub-micrometer level displacement resolution. A shorter focal length can balance the field of view and resolution, avoiding excessively large system size or narrow field of view due to excessively long focal length. It can ensure sufficient magnification to resolve minute displacements while avoiding the risks of system bulkiness and optical path misalignment caused by long focal lengths.

[0045] In the process of repairing the crystal flat mount, the accuracy of the repair is improved by gradually converging the errors in stages through multiple iterations. In this invention, it is preferable to use three iterations to repair the crystal flat mount, including rough repair, semi-rough repair, and fine repair. During rough repair, the surface of the mount and the working surface of the crystal flat are thoroughly cleaned with a mixture of high-purity alcohol or ether to avoid impurities interfering with the detection. A diamond dresser or surface grinder is used to quickly remove high points and burrs at the bottom of the mount, reducing the deviation to the sub-millimeter level. During semi-rough repair, micron-level deviations are corrected step by step using manual grinding or piezoelectric ceramic driven tools. During fine repair, nanoscale polishing tools (such as ion beam polishing) or closed-loop feedback systems (such as laser interferometers) are used to reduce the residual deviation to within 0.02″ angular error.

[0046] S4. Rotate the collimator mounting base 90° so that the optical axis of the collimator is upward. Aim the second theodolite at the flat crystal reflecting surface and adjust the bottom of the collimator mounting base according to the elevation angle data to achieve vertical calibration of the optical axis.

[0047] To reduce mechanical errors during the rotation of the collimator mount, such as eccentricity and tilt, the rotational angular velocity of the mount can be controlled within the range of ≤1° / s, minimizing the acceleration impact on mechanical components and reducing mechanical errors. Furthermore, the adjustment of the collimator mount is calculated by observing the elevation angle deviation Δβ of the reflected image from the optical hexahedral flat crystal using a second theodolite.

[0048] Optical path difference formula: Δs=2L(n-1)·tan(Δβ)≈2L(n-1)·Δβ (approximation for small angles)

[0049] Relationship between height deviation and optical path difference:

[0050]

[0051] Where L is the effective length of the optical hexahedral flat along the optical axis, and n is the refractive index of the optical hexahedral flat.

[0052] In this invention, the optical hexahedral flat crystal is made of K9 glass, with L = 100 mm and n = 1.5167. Setting L = 100 mm standardizes the reference length, simplifies the testing process, and reduces calibration complexity. Due to the optical path amplification effect introduced by the refractive index n, the speed of light propagation in the medium decreases, and the equivalent path lengthens. Therefore, geometric deviations need to be corrected using (n-1) terms to avoid systematic errors caused by neglecting the material's refractive index.

[0053] In this invention, the bottom surface of the collimator mounting base is machined using diamond turning for finishing, with each cut depth ≤1μm and a surface roughness Ra≤0.025μm. Diamond turning for finishing the bottom surface achieves ultra-high precision (Ra≤0.025μm) and stable machining results through nanoscale cutting edge geometry optimization and micrometer-level dynamic parameter matching. Specifically, the nanoscale cutting edge radius of the diamond tool (e.g., 20nm) concentrates cutting stress near the cutting edge, causing the material to form continuous chips through shear slip, reducing micro-tears. The micrometer-level cutting depth (≤1μm) ensures the cutting layer thickness is smaller than the material grain size, increases the shear angle, and shrinks the plastic deformation region to the submicrometer level, suppressing micro-cracks on the machined surface. Furthermore, the low coefficient of friction of the diamond tool (μ≈0.1) reduces cutting heat accumulation, preventing the formation of an amorphous damage layer on the workpiece surface due to high-temperature phase transformation.

[0054] Through repeated iterations and refinements of the above calibration method, the calibration angle deviation can be controlled to below 0.3", achieving sub-arcsecond-level vertical calibration of the collimator optical axis.

[0055] (1) Unless otherwise defined, the same reference numerals in the embodiments and drawings of this disclosure have the same meaning.

[0056] (2) The accompanying drawings of the embodiments of this disclosure only involve the structures involved in the embodiments of this disclosure. Other structures can be referred to the general design.

[0057] (3) For clarity, components or areas are enlarged in the drawings used to describe embodiments of the present disclosure. It will be understood that when an element is referred to as being “above” or “below” another element, the element may be “directly” located “above” or “below” the other element, or there may be an intermediate element.

[0058] The above description is merely a specific embodiment of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this disclosure should be included within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A calibration method for ensuring the optical axis of a collimator is perpendicular to a reference plane, characterized in that: Includes the following steps: S1. Place the first theodolite and the second theodolite on the optical platform from front to back. Set the first theodolite and the second theodolite relative to each other and level them. After leveling, turn on the autocollimation function of the first theodolite. Adjust the second theodolite so that its crosshairs coincide with the autocollimation image of the first theodolite to establish the initial optical axis reference. S2. Install the collimator on the collimator mounting base via the collimator adapter. Fix the optical hexahedron flat crystal on the front end of the collimator adapter base via the flat crystal mounting base, and position it in front of the collimator. Place the collimator mounting base vertically on the optical platform and to the right of the first and second theodolites. Rotate the first and second theodolites 90° towards the collimator mounting base so that their optical axis is parallel to the optical axis of the collimator. S3. Turn on the collimator, observe its crosshairs with the first theodolite and adjust the position of the collimator until the crosshairs of the first theodolite and the crosshairs of the collimator coincide, and complete the horizontal alignment. Turn on the autocollimation function of the second theodolite, observe the autocollimation image deviation of the optical hexahedral flat crystal reflection, and repair the bottom of the flat crystal mounting base until the autocollimation image coincides. S4. Rotate the collimator mounting base 90° so that the optical axis of the collimator is upward. Aim the second theodolite at the flat crystal reflecting surface and adjust the bottom of the collimator mounting base according to the elevation angle data to achieve vertical calibration of the optical axis.

2. The calibration method for a collimator optical axis perpendicular to a reference plane according to claim 1, characterized in that: The first and second theodolites are optical theodolites. In step S1, the tilt angle of the first and second theodolites is adjusted to ≤3″ each time.

3. The calibration method for a collimator optical axis perpendicular to a reference plane according to claim 1, characterized in that: In step S2, the flatness of the mounting surface of the collimator mounting base is ≤1μm, and the locking torque of the optical hexahedral flat crystal is 5N·m±0.2N·m during installation.

4. The calibration method for a collimator optical axis perpendicular to a reference plane according to claim 3, characterized in that: Both the first and second theodolites have mounting bases on their lower ends. The mounting bases are placed on the optical platform and have stepper motors embedded in them. The output shafts of the stepper motors extend out of the upper end of the mounting bases and connect to the lower ends of the corresponding first and second theodolites.

5. The calibration method for a collimator optical axis perpendicular to a reference plane according to claim 1, characterized in that: In step S3, the position of the collimator is adjusted using the process of "coarse centering → fine centering → coarse leveling → fine leveling".

6. The calibration method for a collimator optical axis perpendicular to a reference plane according to claim 5, characterized in that: In step S3, the flat crystal mounting base is iteratively repaired multiple times, including three repair processes: rough repair, semi-rough repair, and fine repair.

7. The calibration method for a collimator optical axis perpendicular to a reference plane according to claim 1, characterized in that: In step S4, the bottom surface of the parallel light tube mounting base is polished by diamond turning, with a cutting depth of ≤1μm and a surface roughness Ra≤0.025μm each time.

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