A calibration method for all-day star detection field-of-view gated imaging system

Abstract

The invention discloses a calibration method for a star detection field gated imaging system at all times of the day, the method comprising: constructing a multi-starlight simulator using a light source, a multi-star point plate and a collimator, wherein the central star point of the multi-star point plate is located at the focus of the collimator; making the optical axis of the central gated field imaging channel coaxial with the optical axis of the collimator; the position of the central star point image on the detector is the principal point position of the central gated field imaging channel, and the position coordinates are used as the reference point of all gated fields on the detector; switching to other gated fields, adjusting and recording the azimuth and pitch angles of the turntable, so that the central star point image is located at the reference point position on the detector; realizing the calibration of the coordinate relationship between gated fields of view through the azimuth and pitch angles of the turntable, and realizing the calibration of the focal length and distortion of each gated field of view through the invariance of the star pair angle distance of the multi-star map in each gated field of view. The invention can solve the calibration problem of optical imaging systems with multiple optical axes and multiple imaging channels.

Description

Technical Field

[0001] The invention belongs to the technical field of all-day star sensor optical imaging, and in particular relates to a calibration method for an all-day star detection field-of-view gating imaging system. Background Art

[0002] As a high-precision attitude-sensitive measuring instrument, star sensors are often used in the navigation of spacecraft such as satellites and spacecraft. In recent years, star sensors have gradually developed from space-based platform applications to near-Earth platform applications. Compared with star sensors used in space, near-Earth platform all-day star sensors face the interference of strong sky background light, and usually use spectral filtering and small field of view imaging systems to suppress the sky background light. Since the field of view is very small, a two-dimensional turntable is required to achieve tracking and detection of a single star. Therefore, this type of star sensor is essentially a star tracker, which has weaknesses such as a motion mechanism, poor reliability, and poor autonomy, and has many limitations in the application of miniaturized platforms.

[0003] The optical imaging system based on field of view gating technology uses a large field of view telescope to collect stars, and utilizes microlenses and microswitch arrays to achieve rapid gating of the instantaneous field of view. It can simultaneously obtain large field of view detection and strong sky background light suppression capabilities. It has the advantages of small size, light weight, and strong autonomy, and is very suitable for the application of all-day star sensors on near-Earth platforms.

[0004] However, unlike the traditional star sensor optical system, this system integrates a microlens array, and each microlens unit corresponds to an imaging channel of a gated field of view. Since each gated field of view of this system will cover the entire detector target surface after passing through the back-end amplification imaging system, the micro-switch array only turns on one switch at a time to detect a gated field of view. It is worth noting that the celestial coordinate relationship corresponding to the star image of different gated fields on the detector target surface is different, and this coordinate relationship is closely related to the position coordinate of the primary image plane corresponding to the center of each microlens unit.

[0005] Due to the processing and assembly errors of the microlens array, the actual position of the center of the microlens unit may deviate from the theoretical position, which will cause a certain error in the direction vector corresponding to the center of each gated field of view. Moreover, the focal length and distortion distribution of each imaging channel may also be different. Therefore, in order to achieve accurate reconstruction of the star map in a larger field of view under bright daylight background, it is necessary to calibrate the field of view gated imaging system.

[0006] Traditional star sensor calibration methods are based on the pinhole model or the actual model based on it, using the undetermined coefficient method or the internal parameter method, using a single starlight simulator in a ground laboratory with a two-dimensional turntable to collect data to calibrate the principal point, focal length and distortion of the star sensor. However, this calibration method is only applicable to the star sensor optical system with a single optical axis, and cannot be applied to the calibration of the field-of-view gated imaging system with multiple optical axes and multiple imaging channels. Summary of the invention

[0007] The technical problem to be solved by the present invention is: in view of the problem that the existing star sensor calibration method cannot be applied to the imaging system with multiple optical axes and multiple imaging channels, a calibration method for the field-of-view gated imaging system of all-day star detection is proposed. According to the imaging principle of the field-of-view gated imaging system, the method adopts a multi-starlight simulator and a turntable, and realizes the calibration of the field-of-view gated imaging system by constructing the coordinate relationship between the gated fields of view and within the gated fields of view.

[0008] The technical solution adopted by the present invention to solve the technical problem is: a calibration method for a field-of-view gating imaging system for all-day star detection, the method comprising the following steps:

[0009] Step S1. A multi-starlight simulator is constructed using a light source, a multi-star point plate and a collimator, wherein the central star point of the multi-star point plate is located at the focus of the collimator, and the focal length of the collimator and the star point position of the multi-star point plate are both known;

[0010] Step S2. Adjust the relative positions of the autocollimator, the turntable and the collimator so that the autocollimator is parallel to the optical axis of the collimator, and the pitch axis and azimuth axis of the turntable are perpendicular to the optical axis of the collimator; use the reference plane of the field of view gating imaging system to place the field of view gating imaging system on the turntable, only open the center gating field of view, adjust the installation position of the field of view gating imaging system on the turntable and fix it so that the optical axis of the center gating field of view is coaxial with the optical axis of the collimator, and the position of the turntable is defined as the relative zero position;

[0011] Step S3. Collect a multi-star map, in which the position of the central star point image on the detector is the principal point position of the central gated field of view of the field of view gated imaging system. Use the real-time star point image centroid positioning software to calculate and record the position coordinates of the principal point on the detector, and use the position coordinates as the reference point O of all gated fields of view on the detector. Use the invariance of the star pair angular distance of the multi-star map on the detector to calibrate the focal length and distortion of the central gated field of view;

[0012] Step S4. close the central gated field of view, open another gated field of view, adjust the azimuth and elevation angle of the turntable to make the multi-star image enter this gated field of view, use the real-time star point image centroid positioning software to make the central star point image be located at the reference point O of the gated field of view on the detector, record the position coordinates of the multi-star image at this time, calibrate the focal length and distortion of the gated field of view using the invariance of the star pair angular distance of the multi-star image on the detector, and record the azimuth angle α0 and elevation angle β0 of the turntable;

[0013] Step S5. Change other gated fields of view and repeat step S4 to complete the calibration of the focal length and distortion of all gated fields of view; at the same time, record the turntable azimuth α corresponding to the reference point O on the detector for all gated fields of view. mn and the pitch angle β mn , and complete the collection of multiple star maps;

[0014] Step S6. Take the reference point O of the center gated field of view on the detector as the principal point position of the field of view gated imaging system. When the center gated field of view is turned on, the star point image falling on the detector reference point O corresponds to (0, 0, 1) T When other gated fields are turned on, the star image of the reference point O on the detector corresponds to (cosα mn sinβ mn ,sinα mn ,cosα mn cosβ mn ) T The direction vector is used to complete the calibration of the coordinate relationship between different gated fields of view.

[0015] Furthermore, the number of star points in the multi-star point plate in step S1 is not less than 9, and the star points are evenly distributed with the central star point as the origin, and the field of view size corresponding to the star point distribution should not be larger than the size of the gated field of view;

[0016] Furthermore, the reference plane in step S2 is perpendicular to the optical axis of the central gating field of view of the field-of-view gating imaging system, and the reference plane and the autocollimator are used to make the optical axis of the central gating field of view of the field-of-view gating imaging system parallel to the optical axis of the collimator;

[0017] The optical imaging system based on field-of-view gating technology is a new imaging system for all-day star sensor applications, which contains key components such as microlens arrays. In the actual processing and assembly of the microlens array, due to the eccentricity error or assembly error of the microlens unit, the direction vector corresponding to the star point image on the detector will have a large error, which will have a great impact on the measurement accuracy of the system. So far, there has been no report on the calibration method of the field-of-view gating imaging system.

[0018] Compared with the existing star sensor technology, the present invention has the following advantages:

[0019] 1. The existing star sensor calibration method is only applicable to the star sensor optical system of a single optical axis, and cannot be applied to the calibration of the field of view gated imaging system of multiple optical axes and multiple imaging channels. The calibration method of the present invention aims at the imaging characteristics of the field of view gated imaging system, and corrects the coordinate relationship between the gated fields of view and within the gated fields of view, thereby realizing the calibration of the field of view gated imaging system of multiple optical axes and multiple imaging channels;

[0020] 2. Taking into account the problems of large number of imaging channels and large number of sampling points of star image in the field of view selection imaging system, the present invention adopts the method of multi-starlight simulator to calibrate the coordinate relationship within the selection field of view, which can effectively speed up data collection and improve calibration efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 4 is a top view of a calibration test system for a field-of-view gated imaging system for all-day star detection in an embodiment of the present invention;

[0022] In the figure: 1 is a light source, 2 is a multi-star point plate, 3 is a collimator, 4 is a two-dimensional turntable, 5 is a field of view gating imaging system, and 6 is an autocollimator;

[0023] Figure 2 It is a flow chart of a calibration method of a field-of-view gating imaging system for all-day star detection according to the present invention;

[0024] Figure 3 is a star point distribution diagram on the multi-star point plate 2 in an embodiment of the present invention;

[0025] In the figure: 20 is the central star point on the multi-star point plate;

[0026] Figure 4 is the gated field of view sequence number of the field of view gated imaging system in the embodiment of the present invention;

[0027] In the figure: the numbers are arranged in order of rows and columns, where the center-selected field of view is numbered (4,4);

[0028] Figure 5 It is a schematic diagram of the coordinate system for the calibration test system of the center gated field of view of the field gated imaging system;

[0029] In the figure: 21 is the multi-star point plate plane coordinate system, 51 is the front telescope in the field of view gated imaging system, 52 is the microlens array in the field of view gated imaging system, 53 is the micro switch array in the field of view gated imaging system, 54 is the rear imaging objective lens of the field of view gated imaging system, 55 is the detector, (4,4) represents the center gated field of view, and 56 is the detector plane coordinate system when the center gated field of view (4,4) is imaged.

[0030] Figure 6It is a schematic diagram of the coordinate system for the calibration test system of other gated fields of view of the field-of-view gated imaging system;

[0031] In the figure: 21 is the multi-star point plate plane coordinate system, 51 is the front telescope in the field of view gated imaging system, 52 is the microlens array in the field of view gated imaging system, 53 is the micro switch array in the field of view gated imaging system, 54 is the rear imaging objective lens of the field of view gated imaging system, 55 is the detector, (4,5) is represented as the gated field of view, and 56 is the detector plane coordinate system when the gated field of view (4,5) is imaged. DETAILED DESCRIPTION

[0032] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. However, the following embodiments are limited to explaining the present invention, and the protection scope of the present invention should include the entire contents of the claims, and through the following embodiments, those skilled in the art can implement the entire contents of the claims of the present invention.

[0033] The embodiment of the present invention is a method for calibrating a field of view gated imaging system with an aperture of 100 mm, a focal length of 1.5 m, a total field of view of 5°×5°, a gated field of view of Φ0.4°, and 7×7 imaging channels.

[0034] like Figure 1 , which is a top view of the calibration test system for the all-day star detection field-of-view gated imaging system. Figure 2 The flowchart of the calibration method of the field-of-view gating imaging system for all-day star detection of the present invention. First, a multi-star pseudo star source is formed by using a light source 1, a multi-star point plate 2 and a collimator 3. The relative positions of the autocollimator 6, the two-dimensional turntable 4 and the collimator 3 are adjusted so that the autocollimator 6 is parallel to the optical axis of the collimator 3, and the plane normal direction of the two-dimensional turntable 4 is perpendicular to the optical axis of the collimator 3. The collimator 3 has an aperture of 200 mm, a focal length of 3 m, a field of view of Ф0.5°, a maximum relative distortion of less than 0.01%, and a distribution of star points on the multi-star point plate 2 as shown in the figure. Figure 3 As shown, the number of star points is 25, the diameter of the star points is 10 μm, the distance between the star points is 5 mm, and the central star point 20 is located at the focus of the parallel light tube 3;

[0035] Using the reference plane of the field of view gating imaging system 5, the field of view gating imaging system 5 is placed on the two-dimensional turntable 4. The sequence number of the gating field of view is specified as follows: Figure 4 As shown, only the central selected field of view (4, 4) is turned on, and the reflector installed on the reference plane of the field of view selection imaging system 5 is monitored in real time through the autocollimator 6. The installation position of the field of view selection imaging system 5 on the two-dimensional turntable 4 is adjusted and fixed so that the optical axis of the central selected field of view (4, 4) is parallel to the optical axis of the autocollimator 6 and the parallel light tube 3. At this time, the position of the two-dimensional turntable 4 is zero position (α0, β0) = (0°, 0°).

[0036] Figure 5 The diagram is a coordinate system diagram for calibrating and testing a field of view gating imaging system. The diagram includes: a multi-star point plate plane coordinate system 21, a front telescope 51 in the field of view gating imaging system, a microlens array 52 in the field of view gating imaging system, a microswitch array 53 in the field of view gating imaging system, a rear imaging objective lens 54 and a detector 55 in the field of view gating imaging system. The central microswitch in the microswitch array 53 is turned on, and the central gating field of view (4, 4) is selected. 56 is the detector plane coordinate system when the central gating field of view (4, 4) is imaged.

[0037] Collect multiple star images, in which the position of the central star image on the detector is the position of the principal point of the central gated field of view of the field of view gated imaging system. Use the real-time star image centroid positioning software to record the position coordinates (x0, y0) of the principal point on the detector, and use this position coordinate as the reference point O of all gated fields on the detector. The star image falling on the detector reference point O corresponds to (0, 0, 1) T The direction vector of .

[0038] First, use the multi-star map to calibrate the focal length and distortion in each selected field of view. The calibration process is as follows:

[0039] Since the distortion of the collimator is very small, the distortion of the multi-star point plate after collimation by the collimator is not considered. Since the center star point is located on the optical axis of the collimator, the position coordinate of the center star point on the multi-star point plate is marked as (x p0 ,y p0 )=(0,0), then the position coordinates in the multi-star point plate are (x pi ,y pi ) and the coordinates are (x pj ,y pj ) after being collimated by a collimator, the corresponding direction vectors can be recorded as:

[0040]

[0041]

[0042] Among them, f p is the focal length of the collimator. Then the star-to-star angular distance of the two star points on the multi-star point plate after being collimated by the collimator can be expressed as:

[0043]

[0044] in, is the direction vector v pi The transposed vector and direction vector v pj The product of .

[0045] In the detector coordinate system of the center-gated field of view (4,4), if the distortion in the center-gated field of view is considered, the position coordinates in the multi-star point plate are (x pi ,y pi ) and the coordinates are (x pj ,y pj ) is imaged by the imaging channel of the center-gated field of view (4,4) of the field-gated optical imaging system, and its coordinates on the detector are (x 44i ,y 44i ) and the coordinates are (x 44j ,y 44j ), the corresponding direction vectors can be expressed as:

[0046]

[0047]

[0048] Among them, D 44i and D 44j are the distortions at the locations of the two star point images in the central selected field of view, f 44 is the focal length of the imaging channel with the center-selected field of view (4,4). Since the tangential distortion is usually small, if only the radial distortion is considered, then Among them, k 441 and k 442 is the radial distortion coefficient of the distortion distribution in the center-selected field of view, r 44i and r 44j are the distances of the two star images from the reference point O(x0, y0), and

[0049] Then the star-to-star angular distance corresponding to the two star point images on the detector can be expressed as:

[0050]

[0051] By pij =d 44ij The focal length f of the center selected field of view can be achieved 44 and distortion coefficient k 441 and k 442 In the central gated field of view (4,4), the least squares fitting is performed on the angular distance measurement results of multiple pairs of stars to improve the calibration accuracy.

[0052] Close the center gated field of view (4,4), open another gated field of view (4,5), and adjust the azimuth and elevation angles of the turntable to bring the multi-star map into the gated field of view (4,5). Figure 6As shown, the central star image is located at the reference point O of the detector in the gated field of view (4,5) using real-time star image centroid positioning software, and a multi-star image is collected at this time. The focal length and distortion in the gated field of view (4,5) are calibrated using a calibration method similar to the focal length and distortion in the central gated field of view (4,4).

[0053] Change other gated fields of view (m, n) and repeat the above steps until the turntable azimuth angle α corresponding to the reference point O on the detector for all gated fields of view is recorded. mn and the pitch angle β mn , the focal length and distortion calibration for each selected field of view (m, n) can be completed.

[0054] Next, the position coordinate relationship between different gated fields of view is calibrated.

[0055] When the center-gated field of view (4,4) is turned on to observe multiple stars, the direction vector of the center star point on the multi-star point plate after being collimated by the collimator is v c44 =(0,0,1) T When switching to the gated field of view (4,5) to observe multiple stars, the azimuth and elevation angles of the turntable are α 45 and β 45 The direction vector of the center star point on the multi-star point plate after being collimated by the parallel light tube is v c45 =(cosα 45 sinβ 45 ,sinα 45 ,cosα 45 cosβ 45 ) T Therefore, the simulated star pair angular distance before and after the turntable rotates can be expressed as:

[0056]

[0057] In the detector coordinate system, when the center gated field of view (4,4) is turned on to observe multiple stars, since the position coordinates of the center star point image on the detector are specified as the reference point O, the direction vector corresponding to the center star point coordinates can also be expressed as:

[0058] v 440 =(0,0,1) T (8)

[0059] Since each gated field of view shares the same detector, and different gated fields of view correspond to different direction vectors at the center of the detector, it is necessary to expand the coordinate range on the detector plane according to the direction vector corresponding to each gated field of view at the center of the detector. For example, when only the gated field of view (4,5) is turned on, although the central star image point is still located at the reference point O, the direction vector of the central star image point is different from v440 It is not the same. Assume that the coordinates of the central star point on the detector plane expansion coordinates are marked as (x 450 ,y 450 ), then the direction vector corresponding to the center star point image coordinates of the gated field of view (4,5) on the detector can be expressed as:

[0060]

[0061] Among them, f 45 is the focal length of the imaging channel with the gated field of view (4,5).

[0062] The star-to-star angular distance can be expressed as:

[0063]

[0064] By c44c45 =d 440450 The position coordinates (x 450 ,y 450 ), thereby realizing the calibration of the position coordinate relationship between the gated field of view and the central gated field of view.

[0065] Change other gated fields of view (m, n) and repeat the above steps until the position coordinates (x mn0 ,y mn0 ) can be calibrated to complete the position coordinate relationship between different gated fields of view.

[0066] After calibrating the focal length and distortion of each gated field of view (m, n) and the position coordinate relationship between different gated fields of view, it can be obtained that when the gated field of view (m, n) is turned on, the coordinate on the detector is (x mni ,y mni ) in the extended coordinate system is:

[0067] X=x mn0 -x0+(x mni -x0)(1+D mni )

[0068] Y=y mn0 -x0+(y mni -x0)(1+D mni )

[0069] Among them, D mni The detector coordinates are (x mni ,y mni ) is the distortion at the position of the star point image.

[0070] The above description is only a specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any person familiar with the technology can understand and think of any changes or substitutions within the technical scope disclosed by the present invention, which should be included in the scope of the present invention.

Claims

1. A calibration method for a field-of-view gating imaging system for all-day star detection, characterized in that: The method comprises the following steps: Step S1. A multi-starlight simulator is constructed using a light source, a multi-star point plate and a collimator, wherein the central star point of the multi-star point plate is located at the focus of the collimator, and the focal length of the collimator and the star point position of the multi-star point plate are both known; Step S2. Adjust the relative positions of the autocollimator, the turntable and the collimator so that the autocollimator is parallel to the optical axis of the collimator, and the pitch axis and azimuth axis of the turntable are perpendicular to the optical axis of the collimator; use the reference plane of the field of view gating imaging system to place the field of view gating imaging system on the turntable, only open the center gating field of view, adjust the installation position of the field of view gating imaging system on the turntable and fix it so that the optical axis of the center gating field of view is coaxial with the optical axis of the collimator, and the position of the turntable is defined as the relative zero position; Step S3. Collect a multi-star map, in which the position of the central star point image on the detector is the principal point position of the central gated field of view of the field of view gated imaging system. Use the real-time star point image centroid positioning software to calculate and record the position coordinates of the principal point on the detector, and use the position coordinates as the reference point O of all gated fields of view on the detector. Use the invariance of the star pair angular distance of the multi-star map on the detector to calibrate the focal length and distortion of the central gated field of view; Step S4. close the central gated field of view, open another gated field of view, adjust the azimuth and elevation angle of the turntable to make the multi-star image enter this gated field of view, use the real-time star point image centroid positioning software to make the central star point image be located at the reference point O of the gated field of view on the detector, record the position coordinates of the multi-star image at this time, calibrate the focal length and distortion of the gated field of view using the invariance of the star pair angular distance of the multi-star image on the detector, and record the azimuth angle α0 and elevation angle β0 of the turntable; Step S5. Change other gated fields of view and repeat step S4 to complete the calibration of the focal length and distortion of all gated fields of view; at the same time, record the turntable azimuth α corresponding to the reference point O on the detector for all gated fields of view. mn and the pitch angle β mn , and complete the collection of multiple star maps; Step S6. Take the reference point O of the center gated field of view on the detector as the principal point position of the field of view gated imaging system. When the center gated field of view is turned on, the star point image falling on the detector reference point O corresponds to (0, 0, 1) T When other gated fields are turned on, the star image of the reference point O on the detector corresponds to (cosα mn sinβ mn ,sinα mn ,cosα mn cosβ mn ) T The direction vector is used to complete the calibration of the coordinate relationship between different gated fields of view.

2. The calibration method of the all-day star detection field-of-view gating imaging system according to claim 1 is characterized in that: The number of star points in the multi-star point plate in step S1 is not less than 9, and they are evenly distributed with the central star point as the origin, and the field size corresponding to the star point distribution should not be larger than the size of the gated field of view.

3. The calibration method of the all-day star detection field-of-view gating imaging system according to claim 1 is characterized in that: In step S2, the reference plane of the field-of-view gating imaging system is perpendicular to the optical axis of the central gating field of the field-of-view gating imaging system, and the reference plane and the autocollimator are used to make the optical axis of the central gating field of the field-of-view gating imaging system coaxial with the optical axis of the parallel light tube.

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