Large aperture optical imaging system secondary mirror high-precision calibration method

Abstract

The present application relates to a kind of large aperture optical imaging system secondary mirror high-precision calibration method, belong to optical imaging system technical field, this method is by installing two measuring cameras in the left and right of primary mirror, the target strip on same secondary mirror is imaged, according to binocular vision principle, the secondary mirror offset is solved, so as to complete calibration according to the offset of secondary mirror, the solving precision of rotation angle can reach angle second level, the solving precision of translation amount can reach nm level, with the advantages of high secondary mirror calibration precision, after high-precision calibration, the image quality of imaging system can be improved, and lay the foundation for subsequent state compensation and adjustment of imaging system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical imaging system, in particular to a large aperture optical imaging system secondary mirror high-precision calibration method. BACKGROUND

[0002] Due to the comprehensive influence of factors such as satellite launch vibration, gravity release, microgravity and thermal environment change, the camera frame will be deformed, and the position of each component will be out of adjustment. In a three-mirror optical system, the size of the secondary mirror is the smallest, but the position tolerance is the most stringent, and a small amount of adjustment will seriously affect the final image quality. Therefore, in the running process, in order to ensure the image quality, high-precision measurement of the out-of-adjustment and deformation of the camera components is required to compensate and adjust the camera state subsequently. SUMMARY

[0003] In order to solve the problem of high-precision calibration of the camera components to facilitate compensation and adjustment of the camera state, the present application provides a large aperture optical imaging system secondary mirror high-precision calibration method.

[0004] A large aperture optical imaging system secondary mirror high-precision calibration method, the method comprising the following steps:

[0005] Step 1: uniformly place N cross target strips on the secondary mirror surface;

[0006] Step 2: according to the number of image elements after camera detector splicing, initially arrange the camera detector in a center-symmetrical manner from left to right after splicing, and preliminarily determine the center image element of the middle position detector as the initial principal point O;

[0007] Step 3: place two measuring cameras on the primary mirror XY surface, fix one measuring camera on each left and right side, and establish the conversion relationship between the mirror surface coordinate system and the camera coordinate system;

[0008] Step 4: align the light pipe exit with the measuring camera, adjust the positions of the light pipe and the measuring camera to enable the target strip to be clearly imaged on the camera focal plane, and then fix the light pipe;

[0009] Step 5: calibrate the installation matrix of the two measuring cameras, respectively denoted as R l and R r ;

[0010] Step 6: perform steps 7 to 9 as follows to perform the first measurement and correction;

[0011] Step 7: record the imaging positions of the target strip at the initial time in the two measuring cameras and Wherein, (x0, y0, -f) is the position of the camera primary mirror principal point, and N is the number of imaging points;

[0012] Step 8: record the imaging position of the target strip imaging element in the imaging position of the two measurement cameras at time t and

[0013] Step 9: solve the translation amount Lx, Ly, Lz of the camera and the rotation angle ψ around the Z axis, and the specific solving process is as follows:

[0014] Step 9-1: according to the simultaneous imaging of the two measurement cameras on the ith target strip at time t, solve the coordinate position (Xt i , Yt i , Zt i ) of the ith target strip imaging object in the I system, and the calculation formula is as follows:

[0015]

[0016]

[0017]

[0018] Where (X sl , Y sl , Z sl ), (X sr , Y sr , Z sr ) are the point sets of the coordinates of all target strips in the I system by the left and right measurement cameras at time t;

[0019]

[0020] Step 9-2: solve the relative translation amount Lx, Ly, Lz of the secondary mirror in three directions at time t according to formula (5):

[0021]

[0022] Where c1-c9 are parameters in the rotation matrix;

[0023] Step 9-3: list three error equations for each point:

[0024]

[0025] Solve by least squares method to calculate parameters c1-c9, and then solve the angle ψ of the secondary mirror around Z;

[0026] Step 10: reverse the rotation angle ψ of the secondary mirror around the Z axis;

[0027] Step 11: keep the angle of the secondary mirror around the Z axis unchanged, and perform the second measurement and correction process, and repeat steps 7-9 to solve the rotation angle θ of the secondary mirror around the Y axis;

[0028] Step 12: Rotate the secondary mirror in the opposite direction by an angle θ around the Y-axis;

[0029] Step 13: Keeping the angles of the secondary mirror around the Z and Y axes constant, perform the third measurement and calibration process, repeating steps 7 to 9, and calculate the rotation angle of the secondary mirror around the X axis.

[0030] Step 14: Rotate the secondary mirror in the opposite direction around the X-axis.

[0031] The present invention provides a high-precision calibration method for a secondary mirror in a large-aperture optical imaging system. By installing two measuring cameras near the left and right sides of the primary mirror, the target strip on the same secondary mirror is imaged. Based on the principle of binocular vision, the offset of the secondary mirror is calculated, thereby calibrating the secondary mirror. This method can achieve an accuracy of arcseconds for calculating the rotation angle and an accuracy of nanometers for calculating the translation amount. It has the advantage of high calibration accuracy for the secondary mirror. The secondary mirror after high-precision calibration can improve the image quality of the imaging system and lay the foundation for subsequent state compensation and adjustment of the imaging system. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the installation of the secondary mirror and the primary mirror in an embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram of the coordinate system of the secondary mirror, primary mirror, and camera in an embodiment of the present invention. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0035] This embodiment provides a high-precision calibration method for secondary mirrors in a large-aperture optical imaging system, which includes the following steps:

[0036] Step 1: Place N cross-shaped target strips evenly on the secondary mirror surface, for example, N=12;

[0037] Step 2: Based on the number of pixels after the camera detectors are stitched together, in the initial stage, the camera detectors are arranged in a centrally symmetrical manner from left to right after stitching, and the center pixel of the detector in the middle position is initially defined as the initial principal point O.

[0038] Step 3: As Figure 1 As shown, two measuring cameras are placed on the XY plane of the primary mirror, one on each side. After fixing them in place, the transformation relationship between the mirror coordinate system and the camera coordinate system is established, as follows: Figure 2As shown in the figure, the mirror coordinate system includes a primary mirror coordinate system and a secondary mirror coordinate system, the primary mirror coordinate system is denoted as I system, the secondary mirror coordinate system is denoted as C system, and the coordinate systems of the measuring cameras installed on the left and right sides of the primary mirror are denoted as C1 system and C2 system respectively;

[0039] Step 4: Align the light pipe light outlet with the measuring camera, adjust the positions of the light pipe and the measuring camera, so that the target strip can be clearly imaged on the camera focal plane, and then keep the light pipe fixed;

[0040] Step 5: Calibrate the installation matrix of the left and right two measuring cameras, denoted as R l and R r respectively;

[0041] Step 6: Perform steps 7 to 9 as follows to perform the first measurement and correction process, and after the correction is completed, perform step 10.

[0042] Step 7: At the initial time, record the imaging positions of the target strip imaging pixels in the two measuring cameras and where (x0, y0, -f) is the position of the camera primary mirror principal point, and N is the number of target strips, i.e. the number of imaging points. It is assumed that the initial time image coordinates are as shown in Table 1.

[0043] Table 1 Initial time image coordinates

[0044]

[0045]

[0046] Step 8: At time t, record the imaging positions of the target strip imaging pixels in the two measuring cameras and It is assumed that the t time image coordinates are as shown in Table 2.

[0047] Table 2 t time image coordinates

[0048] X Y Z 138.8772 1495.261 196.032 61.33436 1500.244 194.8407 -1072.29 1046.027 194.8101 -922.128 1181.191 195.1275 -1488.12 153.7223 196.837 -909.605 -1188.25 195.2638 -374.128 -1450.18 196.9459 637.5509 -1358.36 195.3647 918.8211 -1187.49 197.3651 800.1153 -1270.03 195.6808 1425.307 481.1653 195.1783 895.9689 1207.413 195.2504

[0049] Step 9: Calculate the translation amount Lx, Ly, Lz of the camera and the rotation angle ψ around the Z axis. The specific calculation process is as follows:

[0050] Step 9-1: According to the simultaneous imaging of the two measuring cameras on the i-th target strip at time t, calculate the coordinate position (Xt i , Yt i , Zt i ) of the i-th target strip imaging object point in the I system, and the calculation formula is as follows:

[0051]

[0052]

[0053]

[0054] wherein (Xt i , Yt i , Zt i ) is the coordinate position of the i-th target bar in the I system, (X sl , Y sl , Z sl ), (X sr , Y sr , Z sr ) are the point sets of all target bars in the I system measured by the left and right cameras at time t, respectively, and (X, Y, Z) is the coordinate set of the object target points, wherein:

[0055]

[0056] Step 9-2: calculation of the secondary mirror translation amount.

[0057] At time t, Lx, Ly, and Lz are the relative translation amounts of the secondary mirror in three directions, which can be directly solved by the variation of the centroid of N points, and the calculation formula is as follows:

[0058]

[0059] wherein c1-c9 are parameters in the rotation matrix R. Since the rotation angles of the three axes are small, the rotation matrix R is close to the unit matrix, and here, R≈1; the centroid of N points (X0 i , Y0 i , Z0 i ) is (X center0 , Y center0 , Z center0 ), the centroid of N points (Xt i , Yt i , Zt i ) is (X centert , Y centert , Z centert ), and Lx=X centert -X center0 , Ly=Y centert -Y center0 , and Lz=Z centert -Z center0 .

[0060] The translation amount solving result is: 4.1337, 1.323, -4.024.

[0061] Step 9-3: calculation of the secondary mirror rotation amount.

[0062] To solve the c1-c9 parameters in the equation, three error equations are listed for each point:

[0063]

[0064]

[0065] To minimize the error sum of all points, the least square method is used to solve the equation, i.e. The parameters c1-c9 are calculated by minimizing the equation. Then, the angle ψ of the secondary mirror around Z is calculated according to the three-axis rotation matrix shown in equation (7) and the relationship between θ and ψ:

[0066] ψ = arctan (c2 / c1)

[0067] θ = -arcsin (c3)

[0068]

[0069] Step 10: According to the calculation result of step 9, ψ = -7.2337e-05 arcseconds, adjust the angle ψ of the secondary mirror around the Z axis in the opposite direction.

[0070] Step 11: Keep the angle of the secondary mirror around the Z axis unchanged, perform the second measurement and correction process, and cycle steps 7-9 to calculate the rotation angle θ of the secondary mirror around the Y axis, θ = 7.8693e-05 arcseconds.

[0071] Step 12: According to the calculation result of step 11, adjust the angle θ of the secondary mirror around the Y axis in the opposite direction.

[0072] Step 13: Keep the angles of the secondary mirror around the Z and Y axes unchanged, perform the third measurement and correction process, and cycle steps 7-9 to calculate the rotation angle φ of the secondary mirror around the X axis, φ = -1. 0000e-05 arcseconds.

[0073] Step 14: According to the calculation result of step 13, adjust the angle φ of the secondary mirror around the X axis in the opposite direction.

[0074] The principle of the present application is to use a cyclic measurement and adjustment process, i.e. first calculate the translation of the secondary mirror and the angle ψ around Z; then adjust the secondary mirror according to the calculated value, use two measurement cameras to perform the second measurement, at this time, the state of the secondary mirror is only two rotation angles θ; according to the above least square, calculate the Y axis rotation angle θ, and adjust the secondary mirror according to the calculation result; finally, perform the third measurement to calculate the X axis rotation angle ​​The angle solution is completed, and the secondary mirror is calibrated. The rotation angle solution precision of the method of the application can reach the angle second level, the translation amount solution precision can reach the nm level, and the method has the advantages of high secondary mirror calibration precision. The secondary mirror after high-precision calibration can improve the image quality of the imaging system, and lay a foundation for subsequent state compensation and adjustment of the imaging system.

[0075] The technical features of the above-described embodiments can be combined in any manner. To make the description concise, all possible combinations of the technical features in the above-described embodiments are not described, but as long as the combinations of the technical features do not exist, they should be considered as the scope of the description.

[0076] The above-described embodiments only express several implementation manners of the application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled persons in the art, some modifications and improvements can be made without departing from the concept of the application, and these belong to the protection scope of the application. Therefore, the protection scope of the patent of the application should be subject to the appended claims.

Claims

1. A large aperture optical imaging system secondary mirror high precision calibration method, characterized in that, The method comprises the following steps: Step 1: uniformly placing N cross targets on the secondary mirror surface; Step 2: according to the number of pixels after splicing of the camera detector, initially arranging the camera detector in a center-symmetrical manner from left to right after splicing, and preliminarily determining the central pixel of the detector at the middle position as the initial main point O; Step 3: placing two measuring cameras on the primary mirror XY surface, fixing one measuring camera on the left side and one on the right side, and establishing the conversion relationship between the mirror coordinate system and the camera coordinate system; wherein the mirror coordinate system comprises a primary mirror coordinate system and a secondary mirror coordinate system, and the primary mirror coordinate system is denoted as I system, and the secondary mirror coordinate system is denoted as C system; Step 4: aligning the light pipe light outlet with the measuring camera, adjusting the positions of the light pipe and the measuring camera, so that the target can be clearly imaged on the camera focal plane, and then the light pipe is fixed; Step 5: Calibrate the installation matrix of the two measurement cameras, respectively denoted as R l and R r ; Step 6: performing steps 7 to 9 to perform the first measurement and correction; Step 7: record the imaging position of the target strip imaging pixel in the imaging position of the two measurement cameras at the initial moment and wherein (x0, y0, -f) is the position of the camera main mirror principal point, and N is the number of imaging points. Step 8: record the imaging position of the target strip imaging pixel in the two measurement cameras at time t and ; Step 9: solving the translation amount Lx, Ly, Lz of the camera and the rotation angle ψ around the Z axis, and the specific solving process is as follows: Step 9-1: According to the two measurement cameras at time t, the imaging of the i-th target strip is simultaneously solved, and the coordinate position (Xt i , Yt i , Zt i ) of the imaging object point of the i-th target strip in the I system is calculated, and the calculation formula is as follows: ； ； ； where (X sl ,Y sl ,Z sl ), (X sr ,Y sr ,Z sr ) are the point sets of all target strips in the I system measured by the left and right cameras at time t, respectively. ； Step 9-2: solving the relative translation amount Lx, Ly, Lz of the secondary mirror surface in three directions at time t according to formula (5): ； Wherein, c1-c9 are parameters in the rotation matrix; Step 9-3: listing three error equations for each point: ； Solving by least square method, calculating the parameters c1-c9, and then solving the angle ψ of the secondary mirror around Z; Step 10: reversing the angle ψ of the secondary mirror around the Z axis; Step 11: keeping the angle of the secondary mirror around the Z axis unchanged, performing the second measurement and correction process, and repeating steps 7 to 9 to solve the rotation angle θ of the secondary mirror around the Y axis; Step 12: reversing the angle θ of the secondary mirror around the Y axis; Step 13: keeping the angles of the secondary mirror around the Z and Y axes unchanged, performing a third measurement and correction process, and repeating steps 7 to 9 to solve the rotation angle of the secondary mirror around the X axis ; Step 14: Reverse rotation angle of the secondary mirror around the X axis .

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