A high-precision real-time detection method for splicing sub-mirror edge error based on an observed target

Abstract

The high-precision real-time detection method for the edge error of the mosaic sub-mirror based on an observation target comprises the following steps: a sub-pupil phase difference imaging system is designed between the mosaic sub-mirrors; and an algorithm for estimating the edge error between the sub-mirrors is executed based on the sub-pupil phase difference imaging system. The high-precision real-time detection method for the edge error of the mosaic sub-mirror based on an observation target measures the edge error of a reflecting surface without calibrating a zero point, and the calculation method is simple and reliable, so that the real-time detection of the edge error can be realized. Moreover, the method has good anti-noise performance and the advantage of not being affected by a complex working environment. The method can be applied not only in the active adjustment stage but also in the common phase maintaining stage.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of edge error detection of a spliced sub-mirror, and particularly relates to a high-precision real-time detection method for edge error of a spliced sub-mirror based on an observation target. BACKGROUND

[0002] A large-aperture astronomical telescope is an important tool for astronomers to study major frontier scientific problems, and its performance is mainly reflected in light collecting ability and resolution ability. The larger the aperture of the telescope, the stronger the light collecting ability and the higher the resolution. Due to the constraints of mirror processing, mirror detection, structural scheme, transportation and adjustment, and post-maintenance, the aperture of a single main mirror cannot continue to grow, so the idea of a spliced mirror is proposed, that is, a large-aperture equivalent mirror is formed by splicing many small-aperture mirror surfaces, and the spliced main mirror is co-phased by adjusting the pose state of the sub-mirror to achieve the diffraction limit.

[0003] High-precision edge error detection of the sub-mirror is the key to achieving co-phasing of the telescope using the spliced sub-mirror active optical technology. The active optical co-phasing control of the spliced sub-mirror includes two stages of active adjustment and co-phasing maintenance. In the active adjustment stage, the edge error between the sub-mirrors is detected by using optical sensing technology, and in the co-phasing maintenance stage, the edge error between the sub-mirrors is detected in real time by using a mechanical and electrical type edge sensor.

[0004] The current optical sensing technology for edge error between the spliced sub-mirrors mainly includes point spread function (PSF) based edge error sensing technology, curvature wavefront detection technology, four-pyramid detection technology, dispersion fringe detection technology, Zernike phase contrast detection technology, and optimization technology based on phase difference method. The basic principle of the dispersion fringe detection technology is to place a grating dispersion element at the exit pupil position, and to fit and extract the co-phasing error of the sub-mirror through the interference pattern light intensity distribution between the sub-mirrors. The principle of the phase difference method is to collect two or more images with phase difference, and to solve the wavefront error according to the light intensity distribution information of the images. The former is only used for estimation of the overall surface shape of the optical system, and the splicing error of the sub-mirror is extracted from the overall surface shape.

[0005] The existing optical type sub-mirror edge error detection technology mostly requires that the detection target is a point light source, and in order to achieve high-precision detection, the target also needs to have sufficient brightness. Moreover, these detection technologies need optimization method, iteration or fitting to solve the co-phasing error, and the calculation amount is large, and the detection real-time performance is poor. Therefore, they can only be used as detection schemes for active adjustment and auxiliary sensing schemes for mechanical and electrical type sensors, and cannot be used in the co-phasing maintenance stage.

[0006] The edge error change of the back of the spliced sub-mirror is reflected by the electromechanical edge sensor in the prior art, the edge error of the reflecting surface cannot be directly measured, and the edge error of the reflecting surface can be converted only by calibrating the zero point; with large-aperture telescopes being applied to more complex observation environments, such as large solar telescopes, active optical technology of spliced sub-mirrors is also required, and it is difficult to realize long-time co-phase maintenance of the spliced main mirror only by using the electromechanical edge sensor.

[0007] Therefore, the technical personnel in the art are committed to developing a spliced sub-mirror edge error high-precision real-time detection method based on an observation target, aiming at solving the defects in the prior art. SUMMARY

[0008] In view of the above defects in the prior art, the technical problem to be solved by the present application is that in the current edge error detection technology, the electromechanical edge sensor reflects the edge error change of the back of the spliced sub-mirror, the edge error of the reflecting surface cannot be directly measured, and the edge error of the reflecting surface can be converted only by calibrating the zero point, and in a complex working environment, it is difficult to realize long-time co-phase maintenance only by using the electromechanical sensor; the current optical edge error detection technology is limited by the detection target, the algorithm is complex, the detection real-time performance is poor, and is not suitable for application in the co-phase maintenance stage.

[0009] To achieve the above object, the present application is a spliced sub-mirror edge error high-precision real-time detection method based on an observation target, comprising the following steps:

[0010] Step 1: designing a sub-pupil phase difference imaging system between the spliced sub-mirrors;

[0011] Step 2: on the basis of the imaging system in step 1, executing an algorithm for estimating the edge error between the sub-mirrors;

[0012] In step 2, the cost function defined by the algorithm is:

[0013]

[0014] The cost function is expressed by a sine function as:

[0015]

[0016] Where I f , I d are the Fourier spectra of the short-exposure images of the observation target before and after focusing respectively; H f , H d are the OTFs of the sub-pupil with the edge error between the sub-mirrors before and after focusing respectively; P is the edge error between the sub-mirrors, and u, v are the coordinates in the Fourier frequency domain; the value range of the edge error P between the sub-mirrors is [-λ / 4, λ / 4], and λ is the observation center wavelength.

[0017] The cost function is fitted using a sine function. When the sub-mirror edge error is in the range of [-λ / 4, λ / 4], P when e reaches its minimum value is the sub-mirror edge error.

[0018] The sub-pupil design in step 1 is as follows: Figure 1 As shown, the sub-pupil is designed with a circular aperture, with a diameter between 6cm and 10cm, and the edge of the sub-mirror is located in the middle of the sub-pupil;

[0019] The stitched sub-pupil phase difference imaging system in step 1, such as Figure 2 As shown, the sub-pupil phase difference imaging system can simultaneously obtain short-exposure images of two observation targets with different defocus amounts in the sub-pupil, and estimate the sub-mirror edge error in real time from the short-exposure images of these two observation targets;

[0020] The defocus aberration PV of the two channels of the sub-pupil phase difference imaging system does not exceed one wavelength and is not less than 0.5 wavelengths. One channel is in front of the focal plane and the other channel is behind the focal plane. The distances of the two channels from the focal plane are equal.

[0021] The principle of the sub-mirror edge error detection algorithm in step 2 is as follows: by defining a cost function, the value of the cost function is calculated using multiple pre-designed OTF templates and the Fourier spectrum of the short exposure image, and the cost function value is fitted with a sine function to estimate the edge error between sub-mirrors.

[0022] The specific detection process of the algorithm executed in step 2 is as follows:

[0023] Step S1: Calculate the relationship between the sub-pupil and the edge of the sub-lens to determine the front and rear defocus positions;

[0024] Step S2: Based on the detection range, simulate and generate pre-focus and post-focus OTF templates;

[0025] Step S3: Obtain the short exposure images of the sub-pupil before and after focus, and calculate the Fourier spectrum of the short exposure images;

[0026] Step S4: Calculate the cost function value using the Fourier spectrum of the short exposure image and the OTF template;

[0027] Step S5: Fit the cost function value with a sine function to estimate the height of the sub-mirror edge;

[0028] In step S1, calibrating the relationship between the sub-pupil and the edge of the sub-lens is determined by accurately measuring the defocus amount of the pre-focus and post-focus channels to pinpoint the front and rear defocus positions.

[0029] Step S2 is performed after the front and rear defocus positions are determined in step S1.

[0030] The detection range of the step S2 is the value range of the inter-sub-mirror edge error P, wherein the P value takes the templates of the designed sub-pupil at equal intervals, and then the OTF of each template before and after focusing can be calculated;

[0031] The detection range of the step S2 is [-λ / 4, λ / 4], in the detection range, the OTF templates of the sub-pupil are designed by taking 11 values of the inter-sub-mirror edge error at equal intervals, and then the OTF templates before and after focusing of each template can be calculated according to the value of the inter-sub-mirror edge error;

[0032] After the step S3 obtains the short-exposure images before and after focusing of the sub-pupil, the Fourier spectrum of the images before and after focusing is calculated;

[0033] In the step S3, the exposure time of the short-exposure image of the observation target is less than 30 ms;

[0034] In the step S4, the OTF of the template obtained in the step S2 and the Fourier spectrum of the short-exposure image obtained in the step S3 are used to calculate a group of cost function values through the definition of the cost function;

[0035] In the step S5, the inter-sub-mirror edge error P is taken as a variable, the cost function values are fitted by using a sine function, the phase at the minimum value is extracted, and then the extracted phase is converted into the inter-sub-mirror edge error;

[0036] By adopting the above scheme, the observation target-based high-precision real-time detection method for the edge error of the spliced sub-mirror has the following advantages:

[0037] (1) The observation target-based high-precision real-time detection method for the edge error of the spliced sub-mirror detects the edge error of the spliced sub-mirror by using the observation target, and measures the edge error of the reflecting surface, so that the zero point does not need to be calibrated, and compared with the mechanical and electrical edge sensor, the method has the advantage of not being affected by a complex working environment;

[0038] (2) The observation target-based high-precision real-time detection method for the edge error of the spliced sub-mirror adopts the trigonometric function fitting algorithm in the estimation of the edge error of the spliced sub-mirror, the calculation method is simple and reliable, the real-time detection of the edge error can be performed, and the anti-noise performance is good; the method can be applied not only in the active adjustment stage but also in the common phase maintaining stage, and the application scenarios are enriched;

[0039] In summary, the high-precision real-time detection method for edge error of spliced sub-mirror based on observation target disclosed by the application measures the edge error of the reflecting surface, does not need to calibrate zero point, can be applied in the active adjustment stage and the co-phase maintaining stage, and has the advantage of not being affected by complex working environment; and the algorithm for edge error between sub-mirrors adopted by the method is simple and reliable in calculation method, can realize real-time detection of edge error, and has good noise resistance.

[0040] The concept, specific technical solutions and technical effects of the application will be further described in combination with specific embodiments, so as to fully understand the purpose, features and effects of the application. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is a sub-pupil design structure diagram of the spliced sub-mirror between the high-precision real-time detection method for edge error of spliced sub-mirror based on observation target disclosed by the application;

[0042] Figure 2 is a phase difference detection imaging system diagram of the spliced sub-pupil of the high-precision real-time detection method for edge error of spliced sub-mirror based on observation target disclosed by the application;

[0043] Figure 3 is a flow chart of the execution algorithm of the high-precision real-time detection method for edge error of spliced sub-mirror based on observation target disclosed by the application;

[0044] Figure 4 is a sine curve diagram of the cost function of the high-precision real-time detection method for edge error of spliced sub-mirror based on observation target disclosed by the application;

[0045] Figure 2 In the figure, 1 is a telescope focal plane, 2 is a collimator, 3 is a sub-pupil mask, 4 is an imaging mirror, 5 is a light splitting prism, 6 is a detection system focal plane, 7 is a detector of a rear defocus plane, and 8 is a detector of a front defocus plane. DETAILED DESCRIPTION

[0046] The following describes a plurality of preferred embodiments of the application, so that the technical content of the application is clearer and easier to understand. The application can be embodied in many different forms of embodiments, and these embodiments are described by way of example only, and the protection scope of the application is not limited to the embodiments mentioned in the text.

[0047] Embodiment 1: Real-time detection of edge error of spliced sub-mirror by using the method of the application

[0048] Firstly, step 1, a sub-pupil phase difference imaging system is designed between the spliced sub-mirrors, the sub-pupil design of this embodiment 1 is as shown in Figure 1 The sub-pupil design is a circular aperture with a diameter of 6 cm, and the edge of the sub-mirror is located in the middle of the sub-pupil.

[0049] Sub-pupil phase difference imaging system is then established, as shown in Figure 2 The sub-pupil phase difference imaging system can simultaneously obtain observation target short exposure images of two different defocus amounts of the sub-pupil, and estimate the inter-sub-mirror edge error in real time from the two observation target short exposure images. In this embodiment 1, the observation target is a point light source, and the exposure time of the two different defocus amount images of the sub-pupil is 30 ms.

[0050] After step 1 is performed and the construction of the inter-sub-mirror designed sub-pupil phase difference imaging system is completed, step 2 is performed to estimate the inter-sub-mirror edge error.

[0051] The principle of the algorithm executed in step 2 is to define a cost function, calculate the value of the cost function by using the designed multiple templates and the Fourier spectrum of the short exposure image, and estimate the inter-sub-mirror edge error by using a sine function to fit the cost function value.

[0052] The cost function is defined as follows:

[0053]

[0054] Where I f , I d are the Fourier spectra of the front and rear observation target short exposure images, respectively; H f , H d are the front and rear OTF templates of the sub-pupil with a specified inter-sub-mirror edge error; P is the specified inter-sub-mirror edge error, and u and v are the Fourier domain coordinates. The value range of the inter-sub-mirror edge error P is [-λ / 4, λ / 4], and λ is the observation center wavelength. In this embodiment 1, the cost function defined above is fitted by using a sine function:

[0055]

[0056] As can be seen from the defined cost function, when the cost function e reaches the minimum value, P is the measured inter-sub-mirror edge error.

[0057] The specific detection process steps of the algorithm executed in step 2 are as follows:

[0058] Step S1, calibrate the relationship between the sub-pupil and the sub-mirror edge, and determine the front and rear defocus positions.

[0059] Step S2, according to the detection range [-λ / 4, λ / 4], the sub-mirror edge error takes N equally spaced values, and N OTF templates of the front and rear sub-pupil are calculated.

[0060] Step S3, obtain the front and rear short exposure images of the sub-pupil, and calculate the Fourier spectrum thereof.

[0061] Step S4, calculate N cost function values of the Fourier spectrum of the short exposure image and the OTF template through the definition of the cost function;

[0062] Step S5, fit the N cost function values with a sine function to estimate the height of the edge of the sub-mirror;

[0063] The step S1, the relationship between the scaled pupil and the edge of the sub-mirror, is determined by accurately measuring the defocus amount of the pre-focal and post-focal channels to the pre-focal and post-focal positions;

[0064] The step S2 is performed after the pre-focal and post-focal positions are determined in the step S1;

[0065] N in the step S2 is different according to the calculation capacity and the calculation speed requirement, and can be 11 or 21 in general;

[0066] The detection range of the step S2 is [-λ / 4, λ / 4], and the edge error of the sub-mirror takes N equally spaced values in the detection range to calculate N OTF templates of the pre-focal and post-focal sub-pupils;

[0067] After the pre-focal and post-focal short exposure images of the sub-pupil are obtained in the step S3, the Fourier spectrum of the pre-focal and post-focal images is calculated;

[0068] In the step S4, a group of cost function values can be calculated through the definition of the cost function by using the OTF template obtained in the step S2 and the Fourier spectrum of the short exposure image obtained in the step S3;

[0069] In the step S5, the cost function value is fitted with a sine function with the edge error P of the sub-mirror as the variable, the phase at the minimum value is extracted, and then the extracted phase is converted into the edge error of the sub-mirror; Figure 4

[0070] After the above steps are performed, the embodiment 1 completes the high-precision real-time detection of the edge error of the spliced sub-mirror.

[0071] Embodiment 2, real-time detection of the edge error of the spliced sub-mirror by using the method of the present application

[0072] Except that the observation target is an extended source, the rest of the operations are completely consistent with the operations related to the embodiment 1; the high-precision real-time detection of the edge error of the spliced sub-mirror obtained by finally executing the algorithm is also consistent with the result obtained in the embodiment 1.

[0073] Comparative example 1, real-time detection of the edge error of the spliced mirror by using the phase difference wavefront detection technology

[0074] ​The observation target of the present comparative example 1 is a point light source, and the focal plane image and the out-of-focus image are collected, and the edge error of the sub-mirror is solved by the optimization method (BFGS), the algorithm execution process is complex, the calculation amount is large, the detection real-time performance is poor, and the anti-noise performance is poor, and it is difficult to apply to the phase-keeping stage.

[0075] The comparative example 2 uses a capacitive edge sensor to splice the edge error of the sub-mirror for real-time detection

[0076] The present comparative example 2 is limited by the detection principle, and detects the edge error change of the back surface of the spliced sub-mirror, cannot directly measure the edge error of the reflecting surface, needs to calibrate the zero point of the sensor, and is easily affected by the change of the environment temperature and humidity in the real-time detection process, and therefore is difficult to apply to a complex working environment.

[0077] Comparative analysis: through the analysis and comparison of the examples 1 and 2 and the comparative examples 1 and 2, it can be concluded that the OTF template before and after the focus of the sub-pupil is designed in advance, the cost function value of the OTF template and the Fourier spectrum of two different out-of-focus images is calculated according to the defined cost function, and the cost function value is fitted by a sine function; the method is simple to calculate, greatly improves the calculation efficiency, and can detect the edge error in real time; the detection target of the examples can be a point light source or an extended source, and has good anti-noise performance, and can still be used in a complex working environment, and has a wide application scene; and the examples 1 and 2 measure the edge error of the reflecting surface in the implementation, do not need to calibrate the zero point, and because the algorithm calculation method is simple, it can be applied not only in the active adjustment stage, but also in the phase-keeping stage.

[0078] In summary, the present patent technical solution measures the edge error of the reflecting surface, does not need to calibrate the zero point, has a simple and reliable calculation method, can realize real-time detection of the edge error, has good anti-noise performance, has the advantage of not being affected by the complex working environment, and can be applied not only in the active adjustment stage, but also in the phase-keeping stage.

[0079] The above describes the preferred embodiments of the present application in detail. It should be understood that those skilled in the art can make many modifications and changes without creative labor according to the concept of the present application. Therefore, any technical solution obtained by logical analysis, reasoning or limited test on the basis of the prior art according to the concept of the present application shall be within the protection scope determined by the claims.

Claims

1. A high-precision real-time detection method for splicing sub-mirror edge error based on an observed target, characterized in that, It comprises the following steps: Step 1: Design a sub-pupil phase difference imaging system between the spliced sub-mirrors, wherein the sub-pupil is designed as a circular aperture, and the edge of the sub-mirror is located in the middle of the sub-pupil; Step 2: On the basis of the imaging system in step 1, an algorithm for estimating the edge error between the sub-mirrors is executed; The principle of the algorithm executed in step 2 is that the value of the cost function is calculated by using a plurality of templates designed and the Fourier spectrum of the short exposure image, the value of the cost function is fitted by using a sine function, and the edge error between the sub-mirrors is estimated; the cost function defined by the algorithm is: where I f , I b are the Fourier spectra of the pre- and post-focal observed target short-exposure images, respectively; H f , H b are the pre- and post-focal OTF templates of the sub-pupil that specifies the inter-sub-mirror edge error; p is the specified inter-sub-mirror edge error, and u, v are the coordinates in the Fourier frequency domain; the value range of the inter-sub-mirror edge error p is [-λ / 4, λ / 4], and λ is the observation central wavelength; The cost function fitted by using the sine function is: As can be seen from the defined cost function, when the cost function e reaches the minimum value, p is the measured edge error between the sub-mirrors.

2. The high-precision real-time detection method for the edge error of the spliced sub-mirror based on the observation target according to claim 1, characterized in that: the sub-pupil in step 1 is designed as a circular aperture with a diameter of 6 cm to 10 cm, and the edge of the sub-mirror is located in the middle of the sub-pupil; the spliced sub-pupil phase difference imaging system in step 1 simultaneously obtains short exposure images of the observation target at two different defocusing amounts of the sub-pupil, and the edge error between the sub-mirrors is estimated in real time from the short exposure images of the observation target at the two different defocusing amounts; the exposure time of the short exposure images of the observation target at the two different defocusing amounts of the sub-pupil needs to be less than 30 ms; the defocusing amount difference PV of the two channels of the sub-pupil phase difference imaging system is not more than one wavelength, and is not less than 0.5 wavelengths, one channel is in front of the focus, and one channel is behind the focus, and the distances of the two channels to the focal plane are equal.

3. The high-precision real-time detection method for the edge error of the spliced sub-mirror based on the observation target according to claim 1, characterized in that: the specific detection process steps of the algorithm executed in step 2 are as follows: Step S1: Calibrate the relationship between the sub-pupil and the edge of the sub-mirror, and determine the front and rear defocusing positions; Step S2: According to the detection range, simulate to generate the OTF templates in front of and behind the focus; Step S3: Obtain the short exposure images in front of and behind the focus of the sub-pupil, and calculate the Fourier spectrum of the short exposure images; Step S4: Calculate the cost function value from the Fourier spectrum of the short exposure images and the OTF templates; Step S5: Fit the cost function value by using a sine function, and estimate the edge height of the sub-mirror.

4. The high-precision real-time detection method for the edge error of the spliced sub-mirror based on the observation target according to claim 3, characterized in that: In step S1, the relationship between the sub-pupil and the edge of the sub-mirror is calibrated by accurately measuring the defocusing amounts of the front and rear channels to determine the front and rear defocusing positions; Step S2 is performed after the front and rear defocusing positions are determined in step S1; The detection range of step S2 is within half a wavelength and the value range of the edge error p between the sub-mirrors, and the OTF templates in front of and behind the focus of the sub-pupil are designed by taking the values of p at equal intervals.

5. The high-precision real-time detection method for the edge error of the spliced sub-mirror based on the observation target according to claim 3, characterized in that: After the short exposure images in front of and behind the focus of the sub-pupil are obtained in step S3, the Fourier transform of the images in front of and behind the focus is calculated; The step S4 calculates a group of cost function values by using the OTF template obtained in the step S2 and the Fourier spectrum of the short-exposure image obtained in the step S3; The step S5 fits the cost function values with a sinusoidal function by taking the inter-sub-mirror edge error p as a variable, extracts the phase at the minimum value, and then converts the extracted phase into the inter-sub-mirror edge error.

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