Method for constructing spatial positioning pyramid to measure phase field curvature

Abstract

The invention relates to a method for constructing a spatial positioning pyramid to measure the field curvature of a view field, which comprises the following steps of: selecting the view field to be measured, and determining the spatial position of a 4D Twyman interferometer relative to the focal plane of an optical system to be measured; detecting the spherical surface of the focus marking target ball, adjusting the position of the focus marking target ball, and pulling interference fringes to zero-order fringes; constructing a spatial positioning pyramid; taking down a focus mark target ball; switching to another to-be-measured view field, measuring a target ball mark point through a laser tracker, and calculating a focus coordinate; and measuring each field of view to be measured one by one, and fitting the focal plane curvature of the focal plane of the optical system to be measured according to a measurement result. According to the method disclosed by the invention, the spatial position of the interferometer is calibrated while an interference collimation light path is not blocked; the three trackers are used for building a station to measure coordinates of spatial points, measurement errors can be greatly reduced, the positions of the trackers can be changed according to the focal plane position of the camera, and higher applicability is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical alignment / optical detection, and particularly relates to a method for constructing a spatial positioning pyramid to measure the field curvature of a phase field. Background Art

[0002] With the continuous development of space optical remote sensing science, while the swath of space optical payloads is getting larger and larger, their ground resolution is also getting higher and higher. Therefore, it is required that the optical system has a larger aperture and field of view. However, as the field of view of the optical system continues to increase, the convergence points of light rays in different fields of view are all close to the Airy disk, but their convergence points do not coincide with the theoretical image points in the optical axis direction, resulting in the image plane presenting a curved form, that is, field curvature aberration (Petzval). Due to the existence of field curvature, the image formed by the optical system is clear on the axis and gradually blurred off-axis in the meridional or sagittal direction, resulting in a decrease in the edge field of view modulation transfer function (MTF), seriously affecting the imaging quality. For a transmissive optical system, in order to correct the field curvature of the optical system and improve the transfer function of the edge field of view, a scheme of gluing a flat-field lens at the tail of the lens is generally adopted. For an off-axis three-reflection optical system (TMA), since it cannot be provided with a flat-field lens at the central optical axis like the RC-Cassegrain system, its field curvature cannot be eliminated, and the field curvature is generally compensated by the arc splicing of the focal plane detector. Therefore, the detection of the field curvature distortion of the optical system is particularly important, and its detection result can not only evaluate the alignment accuracy of the system, but also provide guidance for the focal plane splicing.

[0003] The Chinese patent document with the publication number of 106404352B, the publication date of January 11, 2019, and the name of "A Method for Measuring the Distortion and Field Curvature of a Large Field-of-View Telescope Optical System" relies on a wavefront sensor to construct a closed-loop positioning system, switches the field of view by using an adjustment mechanism, and then fits the actual image plane of the optical system. This method constructs a criterion function through the decentration, tilt and defocus item data of the wavefront sensor, and calculates the image point position coordinates based on the relative movement amount of the wavefront sensor adjustment table. This method can realize the field curvature measurement of a space camera, but it needs to set different adjustment structures according to different sizes of the field of view, does not have universality, and requires a large parallel light source. For a large-aperture space camera, this method has high costs and a complex system, and the measurement result is easily affected by the movement accuracy of the adjustment table, resulting in the coupling of the calculated result of the image point coordinates and the movement error of the adjustment mechanism.

[0004] The Chinese patent document, Publication No. 104034352A, published on September 10, 2014, and titled "Method for Measuring Spatial Phase Curvature Using a Laser Tracker and Interferometry," combines a laser tracker with an interferometer for the first time. By measuring a target sphere in the wavefront collimation detection optical path, the focal plane image point position information is obtained, and the field curvature is subsequently fitted. This method blocks the collimation optical path when switching fields of view, and the spatial position of the interferometer cannot be calibrated, resulting in the measurement results being unable to correlate with field of view information. Furthermore, due to the measurement accuracy limitations of a single laser tracker, the measurement results still contain significant errors.

[0005] Therefore, it is necessary to improve the original spatial camera field curvature measurement method so that the field of view information can be associated without blocking the detection light path. Summary of the Invention

[0006] The present invention aims to solve the technical problems in the prior art and provides a method for constructing a spatial positioning pyramid to measure the curvature of a camera field.

[0007] In order to solve the above technical problems, the technical solutions of the present invention are as follows:

[0008] A method for constructing a spatially positioned pyramid to measure camera field curvature, wherein the applicable system includes: a focal plane curvature test adjustment frame, a 4D Twiman interferometer, an interferometer adjustment frame, a tracker cage adjustment frame, three laser trackers, three measurement target balls and target ball mounts, a focus mark target ball, a one-dimensional shearing stage, and an air bearing stage; the focal plane of the optical system to be measured is set in front of the 4D Twiman interferometer;

[0009] The focal plane curvature test adjustment frame is used to switch the field of view of the optical system and has three degrees of freedom in orthogonal directions. The 4D Twyman interferometer and interferometer adjustment frame are used to interferometrically measure the wavefront of the optical system. In conjunction with the plane mirror at the optical system's light entrance, it can achieve full-field collimation interferometry.

[0010] The air bearing stage is fixed to the focal plane curvature test adjustment frame; the 4D Twiman interferometer is connected to the air bearing stage via the interferometer adjustment frame; the 4D Twiman interferometer is used to adjust three degrees of freedom; a focus mark target is set in front of the 4D Twiman interferometer, and the bottom of the target is adjusted by a one-dimensional shear stage to detect the focus coordinates. The focus coordinate measurement point is located at the focus of the 4D Twiman interferometer.

[0011] The three laser trackers are fixed to the tracker cage adjustment frame respectively; the three measurement target balls are fixed to the upper surface of the 4D Twyman interferometer through a target ball mount respectively;

[0012] The method comprises the following steps:

[0013] Step 1: Select the field of view to be measured through the focal plane curvature test adjustment frame, and adjust the interferometer adjustment frame to determine the spatial position of the 4D Twyman interferometer relative to the focal plane of the optical system to be measured;

[0014] Step 2: Use the 4D Twyman interferometer to detect the spherical surface of the focus marking target ball, adjust the position of the focus marking target ball and pull the interference fringes to the zero-order fringes; at this time, the center position of the focus marking target ball is the focal point coordinates of the 4D Twyman interferometer and the focal plane coordinates of the optical system to be measured;

[0015] Step 3: Set up stations using three laser trackers, measure the spatial position relationship between the three measurement target balls and the focus marking target ball, and construct a spatial positioning pyramid;

[0016] Step 4: Remove the focus marking target ball;

[0017] Step 5: Switch to another field of view to be measured, position the field of view to be measured through the focal plane curvature test adjustment frame and the interferometer adjustment frame, measure the measurement target ball marking points through the laser tracker, and combine with the spatial positioning pyramid to calculate the focal point coordinates;

[0018] Step 6: Repeat Step 5, measure each field of view to be measured one by one, and fit the focal plane curvature of the focal plane of the optical system to be measured according to the measurement results.

[0019] In the above technical solution, a detection flat mirror is provided at the light incident port of the optical system, and this detection flat mirror is used to realize optical interference collimation measurement.

[0020] In the above technical solution, Step 3 is specifically as follows:

[0021] When the 0-order interference fringes appear in the 4D Twyman interferometer, measure the spherical surface shape of the focus marking target ball and adjust the spatial position of the 4D Twyman interferometer. When the coefficient of the power term is less than a certain threshold, use three laser trackers to set up stations, measure the target ball measurement points at the positions where the focus marking target ball and the three measurement target balls are located, and construct a spatial positioning pyramid.

[0022] In the above technical solution, the threshold of the coefficient of the power term is 0.1.

[0023] In the above technical solution, when setting up stations using three laser trackers, the global RMS of the station setup accuracy is better than 0.005 mm.

[0024] In the above technical solution, Step 5 is specifically as follows:

[0025] Adjust the spatial position of the 4D Twyman interferometer so that when the coefficient of the power term of the autocollimation interference optical path is less than a certain threshold, measure the three target ball measurement points at the positions where the three measurement target balls are located respectively, and use the spatial position invariance of the spatial positioning pyramid to calculate the actual image point coordinates under this field of view to be measured.

[0026] In the above technical solution, the threshold of the power term coefficient of the self-collimation interference optical path is 0.1.

[0027] The present invention has the following beneficial effects:

[0028] The method for constructing a spatial positioning pyramid to measure the field curvature of a phase camera of the present invention calibrates the spatial position of the interferometer while not blocking the interference collimation optical path; by using three trackers to set up stations to measure the coordinates of spatial points, the measurement error can be greatly reduced, and the positions of the trackers can be changed according to the position of the camera focal plane, having higher applicability.

[0029] The method for constructing a spatial positioning pyramid to measure the field curvature of a phase camera of the present invention improves the test accuracy of the existing field curvature measurement method, establishes a spatial position relationship between the measurement result and the optical path reference, and uses the tracker to set up stations and the spatial positioning pyramid to greatly improve the positioning accuracy of the laser tracker without significantly modifying the existing field curvature measurement method. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0031] Figure 1 FIG. is a schematic structural diagram of a measurement system applicable to the method for constructing a spatial positioning pyramid to measure the field curvature of a phase camera of the present invention.

[0032] Figure 2 FIG. is a schematic diagram of the positioning target ball measurement points in the measurement system applicable to the method for constructing a spatial positioning pyramid to measure the field curvature of a phase camera of the present invention.

[0033] Figure 3 FIG. is a schematic diagram of the spatial positioning pyramid in the measurement system applicable to the method for constructing a spatial positioning pyramid to measure the field curvature of a phase camera of the present invention.

[0034] The reference numerals in the drawings are represented as:

[0035] 1 - Focal plane curvature test and adjustment frame; 2 - 4D Twyman interferometer; 3 - Interferometer adjustment frame;

[0036] 4 - Focal plane of the optical system to be measured; 5 - Tracker cage adjustment frame; 6 - Laser tracker;

[0037] 7 - Measurement target ball; 8 - Target ball seat; 9 - Focus marking target ball; 10 - One-dimensional shear table; 11 - Air bearing table; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The inventive concept of the present invention is:

[0039] The present invention's method for constructing a spatial positioning pyramid to measure camera field curvature calibrates the interferometer position using four target sphere measurement points. The spatial relationship between the target sphere measurement points and the interferometer focus remains fixed. After switching the field of view, only the coordinates of the three target sphere measurement points need to be remeasured to obtain the focal coordinates based on the constructed spatial positioning pyramid. This ensures that the optical path is unobstructed during interferometric collimation measurement, and the positional relationship of the spatial positioning pyramid is treated as a systematic error and uniformly eliminated in subsequent data processing.

[0040] The present invention will be described in detail below with reference to the accompanying drawings.

[0041] like Figure 1 As shown, the method of constructing a spatial positioning pyramid to measure the field curvature of a camera of the present invention is applicable to a system comprising: a focal plane curvature test adjustment frame 1, a 4D Twyman interferometer 2, an interferometer adjustment frame 3, a tracker cage adjustment frame 5, three laser trackers 6, three measuring target balls 7 and target ball seats 8, a focus mark target ball 9, a one-dimensional shearing stage 10, and an air bearing stage 11; the focal plane 4 of the optical system to be measured is set in front of the 4D Twyman interferometer 2.

[0042] in:

[0043] The focal plane curvature test adjustment frame 1 is used to switch the field of view of the optical system and has three degrees of freedom in orthogonal directions. The 4D Twyman interferometer 2 and the interferometer adjustment frame 3 are used to interferometrically measure the wavefront of the optical system. Combined with the plane mirror at the light entrance of the optical system, they can achieve full-field collimation interferometry measurement.

[0044] An air bearing platform 11 is fixed to the focal plane curvature test adjustment frame 1; a 4D Twimann interferometer 2 is connected to the air bearing platform 11 via the interferometer adjustment frame 3. The 4D Twimann interferometer 2 can be adjusted over a wide range of three degrees of freedom. The focal plane 4 of the optical system to be measured is placed in front of the 4D Twimann interferometer 2. A detection plane mirror is placed at the light entrance of the entire optical system to perform optical interference alignment measurement. A focus mark target sphere 9 is placed directly in front of the 4D Twimann interferometer 2. Below it, a one-dimensional shearing stage 10 is used to adjust the focus coordinates. The focus coordinate measurement point is located at the focus of the 4D Twimann interferometer 2. After constructing the spatial positioning pyramid, the focus mark target sphere 9 and the one-dimensional shearing stage 10 are removed, and the three measurement targets spheres 7 are fixed during the measurement process.

[0045] Three laser trackers 6 are fixed to the tracker cage adjustment frame 5 by screws respectively; three measuring target balls 7 are fixed to the upper surface of the 4D Twiman interferometer 2 by gluing through a target ball seat 8 respectively, which are used to calibrate the spatial position of the 4D Twiman interferometer 2 and construct the three vertices of the spatial pyramid.

[0046] The method of constructing a spatial positioning pyramid to measure the curvature of a camera according to the present invention is as follows: Figure 2As shown, the measurement method combines an interference collimation optical path with a total station measurement optical path of a tracker. The specific steps are as follows:

[0047] Step 1: Select the field of view to be measured through the focal plane curvature test adjustment frame 1, and adjust the interferometer adjustment frame 3 to determine the spatial position of the 4D Twyman interferometer 2 relative to the focal plane 4 of the optical system to be measured;

[0048] Step 2: Use the 4D Twyman interferometer 2 to detect the spherical surface of the focus marking target ball 9, adjust the position of the focus marking target ball 9 and pull the interference fringes to the zero-order fringes; at this time, the center position of the focus marking target ball 9 is the focus coordinate of the 4D Twyman interferometer 2, and also the coordinate of the focal plane 4 of the optical system to be measured;

[0049] Step 3: Use three laser trackers 6 to set up a station to measure the spatial position relationship between the three measurement target balls 7 and the focus marking target ball 9, and construct a spatial positioning pyramid;

[0050] Step 4: Remove the focus marking target ball 9, and the three measurement target balls 7 remain fixed;

[0051] Step 5: Switch to another field of view to be measured, position the field of view to be measured through the focal plane curvature test adjustment frame 1 and the interferometer adjustment frame 3, measure the marking points of the measurement target ball 7 through the laser tracker 6, and combine the spatial positioning pyramid to calculate the focus coordinates;

[0052] Step 6: Repeat Step 5 to measure each field of view to be measured one by one, and fit the focal plane curvature of the focal plane 4 of the optical system to be measured according to the measurement results.

[0053] Specifically:

[0054] The method for measuring the field curvature of a spatial positioning pyramid in the present invention is as follows. In the interference measurement optical path of the 4D Twyman interferometer 2, four measurement points are located through the tracker cage adjustment frame 5. Three of them are located on the upper surface of the 4D Twyman interferometer 2 and are used to calibrate the spatial position of the 4D Twyman interferometer 2. The other measurement point is located by detecting the center of the sphere of the focal point marking target ball 9 on the spherical surface of the 4D Twyman interferometer 2. When the 0-level interference fringe appears in the 4D Twyman interferometer 2, the surface shape of the spherical surface of the focal point marking target ball 9 is measured and the spatial position of the 4D Twyman interferometer 2 is adjusted. When the power term coefficient is less than 0.1, three laser trackers 6 are used to set up a station to measure the positions of the four target ball measurement points (the positions of the focal point marking target ball 9 and the three measurement target balls 7). The global RMS of the station setup accuracy is better than 0.005 mm (the station setup accuracy is related to the selected position of the transfer point. Generally, an RMS less than 0.005 mm represents a reliable result). A spatial positioning pyramid is constructed based on the four target ball measurement points. The bottom surface of the spatial positioning pyramid represents the spatial position of the 4D Twyman interferometer 2, and its vertex is the actual image point coordinates of the focal plane 4 of the optical system to be measured. The focal plane curvature test adjustment frame 1 is used to switch to another field point to be measured. After switching the field, the focal point marking target ball 9 at the front end of the 4D Twyman interferometer 2 is removed, and the spatial position of the 4D Twyman interferometer 2 is adjusted so that the power term coefficient of the autocollimation interference optical path is less than 0.1 (the power term coefficient is related to optical processing, and theoretically, the smaller it is, the better). The three measurement points on the upper surface of the 4D Twyman interferometer 2 (the positions of the three measurement target balls 7) are measured, and based on the invariance of the spatial position of the spatial positioning pyramid, the actual image point coordinates in this field to be measured are calculated. According to the above steps, each field to be measured is measured one by one, and the field curvature of the optical system can be fitted based on the calculated image point coordinates.

[0055] The measurement principle of the method for measuring the field curvature by constructing a spatial positioning pyramid in the present invention is as follows: In the interference collimation detection optical path, target ball measurement points are inserted. By using the converging focal point of the interferometer lens to coincide with the center of the target ball, at this time, three laser trackers are used to set up a station to measure the positions of the four target ball measurement points, and a spatial positioning pyramid is constructed, as Figure 3 shown. Among them, three target ball measurement points (the positions of the three measurement target balls 7) are used to calibrate the spatial position of the 4D Twyman interferometer 2, and thus the relevant field information can be associated. The other target ball measurement point (the position of the focal point marking target ball 9) is used as the focal point of the 4D Twyman interferometer 2 and is also the image point of the focal plane 4 of the optical system to be measured in this field. The coordinates of the four target ball measurement points (the positions of the focal point marking target ball 9 and the three measurement target balls 7) are measured by using the tracker to set up a station, and a spatial positioning pyramid is constructed. During the full-field measurement process, since the spatial position relationship between the lens focal point and the interferometer remains unchanged, when switching the field subsequently, only the positions of the three target ball measurement points (the positions of the three measurement target balls 7) need to be measured, and the position of the other target ball measurement point (the position of the focal point marking target ball 9), that is, the spatial coordinates of the image point of the focal plane 4 of the optical system to be measured, can be deduced through the positioning pyramid.

[0056] Tests show that the method of the present invention is practical and effective.

[0057] Using this method to measure the XX-1 large-aperture wide-angle camera, around the test field to be measured, measure the fields of view of 0, ±0.3, ±0.5, ±0.8, ±1. At each field of view, take 2 measurements on the left and right sides respectively, and a total of 27 fields of view are measured to fit the focal plane curvature of the optical system.

[0058] During the measurement process, the power value of each field of view is taken as the average of three measurements to avoid the influence of reasons such as air flow and micro-vibration. The measurement uncertainty is less than 0.03, and the global RMS of the tracker station setup is better than 0.005 mm.

[0059] The measurement results show that the focal plane curvature (including the sagitta value) of the optical system is approximate to the theoretical design value, and the detector can be spliced according to the theoretical design value of the focal plane.

[0060] The method of the present invention for constructing a spatial positioning pyramid to measure the field curvature of a phase camera calibrates the spatial position of the interferometer while not blocking the interference collimation optical path; using three trackers to set up a station to measure the spatial point coordinates can greatly reduce the measurement error, and the tracker station positions can be changed according to the focal plane position of the camera, having higher applicability.

[0061] The method of the present invention for constructing a spatial positioning pyramid to measure the field curvature of a phase camera improves the test accuracy of the existing field curvature measurement method, and establishes a spatial position relationship between the measurement result and the optical path reference. Using the tracker station setup and the spatial positioning pyramid can greatly improve the positioning accuracy of the laser tracker without significantly modifying the existing field curvature measurement method.

[0062] Obviously, the above embodiments are only examples for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for constructing a spatial positioning pyramid to measure the phase field curvature of a camera, characterized in that The applicable system includes: a focal plane curvature test adjustment frame (1), a 4D Twyman interferometer (2), an interferometer adjustment frame (3), a tracker cage adjustment frame (5), three laser trackers (6), three measuring target balls (7) and target ball seats (8), a focus mark target ball (9), a one-dimensional shearing table (10) and an air-floating table (11); the focal plane (4) of the optical system to be measured is set in front of the 4D Twyman interferometer (2); The focal plane curvature test adjustment frame (1) is used to switch the field of view of the optical system and has three degrees of freedom in orthogonal directions. The 4D Twyman interferometer (2) and the interferometer adjustment frame (3) are used to interferometrically measure the wavefront of the optical system. In conjunction with the plane mirror at the light entrance of the optical system, collimated interferometric measurement of the entire field of view can be achieved. An air-floating platform (11) is fixed on a focal plane curvature test adjustment frame (1); a 4D Twyman interferometer (2) is connected to the air-floating platform (11) via an interferometer adjustment frame (3); the 4D Twyman interferometer (2) is used to adjust three degrees of freedom; a focus mark target ball (9) is provided in front of the 4D Twyman interferometer (2), and a one-dimensional shearing platform (10) is adjusted below the focus mark target ball (9) to detect the focus coordinates, and the focus coordinate measurement point is located at the focus of the 4D Twyman interferometer (2); Three laser trackers (6) are respectively fixed to the tracker cage adjustment frame (5); three measuring target balls (7) are respectively fixed to the upper surface of the 4D Twiman interferometer (2) through a target ball seat (8); The method comprises the following steps: Step 1: Select the field of view to be measured by using the focal plane curvature test adjustment frame (1), and adjust the interferometer adjustment frame (3) to determine the spatial position of the 4D Twyman interferometer (2) relative to the focal plane (4) of the optical system to be measured; Step 2: Using the 4D Twyman interferometer (2) to detect the spherical surface of the focus mark target ball (9), adjusting the position of the focus mark target ball (9) and pulling the interference fringes to the zero-order fringes; at this time, the center position of the focus mark target ball (9) is the focus coordinate of the 4D Twyman interferometer (2) and the coordinate of the focal plane (4) of the optical system to be measured; Step 3: Use three laser trackers (6) to establish a station, measure the spatial position relationship between the three measurement target balls (7) and the focus mark target ball (9), and construct a spatial positioning pyramid; Step 4: Remove the focus mark target ball (9); Step 5: Switch to another field of view to be measured, locate the position of the field of view to be measured by using the focal plane curvature test adjustment frame (1) and the interferometer adjustment frame (3), measure the mark point of the measurement target sphere (7) by using the laser tracker (6), and calculate the focus coordinates by combining the spatial positioning pyramid; Step 6: Repeat step 5, measure each field of view to be measured one by one, and fit the focal plane curvature of the focal plane (4) of the optical system to be measured based on the measurement results.

2. The method for constructing a spatial positioning pyramid to measure the field curvature of a camera according to claim 1, characterized in that, A detection plane mirror is provided at the light entrance of the optical system, and the detection plane mirror is used to realize optical interference collimation measurement.

3. The method for constructing a spatial positioning pyramid to measure the field curvature of a camera according to claim 1, characterized in that, Step 3 is as follows: When the 0th-order interference fringe appears in the 4D Twyman interferometer (2), measure the spherical surface shape of the focal point marking target ball (9) and adjust the spatial position of the 4D Twyman interferometer (2). When the coefficient of the power term is less than a certain threshold, use three laser trackers (6) to set up a station, measure the target ball measurement points at the positions where the focal point marking target ball (9) and the three measurement target balls (7) are located, and construct a spatial positioning pyramid.

4. The method for constructing a spatial positioning pyramid to measure the field curvature of a camera according to claim 3, wherein The threshold of the coefficient of the power term is 0.

1.

5. The method for constructing a spatial positioning pyramid to measure the field curvature of a camera according to claim 3, wherein When using three laser trackers (6) to set up a station, the global RMS of the station setup accuracy is better than 0.005 mm.

6. The method for constructing a spatial positioning pyramid to measure the field curvature of a camera according to claim 1, wherein Step 5 is specifically as follows: Adjust the spatial position of the 4D Twyman interferometer (2) so that when the coefficient of the power term of the autocollimation interference optical path is less than a certain threshold, measure the three target ball measurement points at the positions where the three measurement target balls (7) are located respectively, and calculate the actual image point coordinates in the field of view to be measured by using the spatial position invariance of the spatial positioning pyramid.

7. The method for constructing a spatial positioning pyramid to measure the field curvature of a camera according to claim 6, wherein The threshold of the coefficient of the power term of the autocollimation interference optical path is 0.1.

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