A method for region-enhanced wavefront detection of low-contrast extended targets

Abstract

The application discloses a regional enhancement wave front detection method for a low-contrast extended target, which is based on a sub-aperture array image obtained by a Hartmann wave front sensor, and a sub-image array with higher contrast is obtained by image superposition of joint adjacent sub-images; then, displacement information of the sub-image is obtained by using the sub-image array with enhanced contrast, and approximate overall wave front information is represented. The method can detect the wave front information of the low-contrast extended target without adding extra hardware devices, greatly reduces the requirement for the contrast of the target, and does not increase the complexity of the whole system. The application has strong adaptability, and can even supplement the wave front information of a blocked aperture.

Description

Technical Field

[0001] This invention belongs to the field of wavefront detection in adaptive optics, specifically relating to a region-enhanced wavefront detection method for low-contrast extended targets. Background Technology

[0002] Adaptive optics systems were initially designed primarily for observing nighttime astronomical activities. However, as their applications have expanded, these systems have needed to adapt to different observation environments and conditions. For example, adaptive optics is used in high-resolution optical microscopy to observe living biological tissues and structures, and in solar telescopes to observe solar targets such as sunspots and solar microparticles. These targets exhibit very low contrast, and beacon light cannot be used to guide wavefront sensors to characterize wavefront distortion information. Therefore, high-resolution imaging observation becomes a significant challenge under these conditions.

[0003] Wavefront detection based on extended targets primarily relies on cross-correlation algorithms. These algorithms select any sub-aperture as a reference aperture and use it to calculate the relative displacements of other apertures, thereby determining the tilt information of the entire wavefront. In wavefront detection applications with various extended targets, it is inevitable to need to reference low-contrast targets, such as solar grain structures. When the contrast of the sub-aperture images is low, the content of the images cannot be well distinguished, and the cross-correlation algorithm cannot accurately measure the similarity between two images. This leads to a decrease in the accuracy of the calculated image displacement, ultimately resulting in a reduction in the accuracy of the wavefront information calculation.

[0004] Currently, there are also related post-processing algorithms, such as gamma transform, to preprocess sub-aperture images to enhance their contrast, but this increases the computational load and processing time, resulting in significant time delay errors. Summary of the Invention

[0005] The technical problem this invention aims to solve is that, for wavefront detection of low-contrast extended targets, the cross-correlation algorithm cannot accurately calculate the sub-aperture image displacement, leading to reduced wavefront detection accuracy. This invention proposes an innovative calculation method.

[0006] The technical solution adopted in this invention is as follows: a method for regional enhancement wavefront detection of low-contrast extended targets. After the Hartmann wavefront sensor obtains a sub-image array, a window is divided at a fixed position within the sub-aperture. The window contains the feature information of the sub-image. By combining the gray values ​​of neighboring sub-aperture images within a defined pixel range, a sub-image array with higher contrast is obtained. The wavefront information of the target under test is then obtained using the contrast-enhanced sub-image array.

[0007] The process for selecting joint neighboring sub-aperture images is described below:

[0008] Step 1: The measured atmospheric coherence length is r0, and the effective diameters of the two furthest sub-apertures that need to be superimposed in the microlens array are d and the center distance between the sub-apertures is r;

[0009] Step 2: By statistically analyzing the variance of the difference in the angle of arrival between the two sub-apertures, the probability density formula for the angle of arrival is obtained: in Let X and Y be the components of the variance of the difference angle of arrival in the x and y directions, respectively; where X and Y are the components of the difference angle of arrival in the x and y directions, respectively.

[0010] Step 3: Determine the probability by integrating the probability density over a circle with radius equal to the Rayleigh criterion. Use the probability result and the acceptable error of the item to ensure enhanced contrast in the superimposed sub-image array. This determines the distance *r* between the two furthest superimposed sub-apertures; all apertures with a distance less than *r* can be superimposed.

[0011] Furthermore, regarding the overlay process of edge sub-apertures, the overlay process combines neighboring sub-apertures and overlays the gray values ​​of all images within the window according to pixel position, thereby obtaining a contrast-enhanced overlay sub-image array. Similarly, for edge sub-apertures, this method utilizes surrounding sub-apertures with clear image information for overlay, thereby supplementing the image information of edge sub-apertures that are missing sub-image information and have image distortion.

[0012] Furthermore, both the extraction and reconstruction of wavefront information utilize the contrast-enhanced sub-image array. A sub-aperture image is selected from the sub-image array as a reference sub-aperture. Correlation operations are performed between the other sub-aperture images and the reference sub-aperture image, and the relative tilt information of the wavefront is extracted. Then, the wavefront tilt information of the sub-aperture and the Zernike mode wavefront reconstruction method are used to reconstruct the wavefront.

[0013] The principle of this invention is:

[0014] The low-contrast extended target adaptive optics wavefront detection method adds an aperture stacking process to the conventional Hartmann wavefront detection method, such as... Figure 1 As shown, after the Hartmann wavefront sensor obtains the sub-image array, it combines neighboring sub-aperture images and performs grayscale superposition within a defined pixel range to obtain a sub-image array with higher contrast; then, the contrast-enhanced sub-image array is used to obtain the image displacement information.

[0015] The number of superimposed sub-apertures is determined by an aperture correlation model based on the degree of wavefront distortion. The optimal number and range of superimposed sub-apertures need to be further determined based on specific observation conditions (atmospheric coherence length) and the number of sub-apertures in the pupil of the microlens array. The criterion is that the sub-image displacement difference of the superimposed apertures does not exceed the Rayleigh criterion, i.e., the images cannot be completely separated.

[0016] By overlaying the image array, all sub-apertures with missing image information within the overlay range can be supplemented using information from other apertures, thus completing the sub-aperture image gaps caused by strong turbulence, weak targets, or telescope structural obstruction. Then, a sub-aperture image is selected as a reference sub-aperture from the contrast-enhanced sub-image array, and correlation operations are performed between the other sub-aperture images and the reference sub-aperture image to extract relative wavefront tilt information. Finally, wavefront reconstruction is performed using the wavefront tilt information of the sub-apertures and the Zernike mode wavefront reconstruction method.

[0017] The main principle of this invention is based on the small phase difference in wavefront arrival angles between adjacent (or closely spaced) apertures in a Shaker-Hartmann wavefront sensor. The grayscale information of the superimposed image is enhanced, thus increasing the root mean square (RMS) of the overall grayscale values, i.e., the image contrast. Figure 2 As shown.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] (1) The present invention adopts the superimposed sub-aperture method, and the superimposed sub-aperture sub-image array has higher contrast, so the method can significantly reduce the contrast requirement of the extended target.

[0020] (2) Compared with existing contrast enhancement algorithms for sub-image preprocessing, the present invention has faster processing and is more suitable for application in scenarios that require fast response, such as closed-loop control of solar target observation.

[0021] (3) The present invention has stronger adaptability, such as being more suitable for strong turbulence observation conditions and wavefront detection of faint targets; it can even make up for the lack of aperture image information caused by these reasons or the obstruction of the telescope structure. Attached Figure Description

[0022] Figure 1 The principle and flowchart of the sub-aperture image overlay method;

[0023] Figure 2 The image shows a comparison of the sub-image contrast before and after the detector aperture is superimposed; where (a) is the sub-image array before superposition and (b) is the sub-image array after superposition. Detailed Implementation

[0024] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0025] like Figure 1 As shown, this invention provides a region-enhanced wavefront detection method for low-contrast extended targets. The specific implementation of this method is as follows:

[0026] Step 1: Assuming the measured atmospheric coherence length is r0 = 10 cm, and the microlens used is a 12×12 sub-aperture array, such as... Figure 2 As shown in (a) Figure 2 (For comparing the contrast of sub-images before and after detector aperture stacking), assuming the telescope's incident aperture is 1m and the equivalent diameter of the sub-aperture is d = 0.083m; assuming the sub-apertures to be stacked are the top, bottom, left, and right sides of one sub-aperture, for a total of five sub-apertures, then the equivalent center distance between the two furthest stacked sub-apertures is r = 0.167m. The actual microlens array sub-aperture size used is d0 = 154μm, the focal length is f = 5.2mm, and the calculated incident light wavelength is λ = 0.532μm. The Rayleigh criterion 1.22λ / d0 translates to a displacement distance of 21.92μm on the detector (calculated as 1.22 × 0.532μm × 5.2mm × 1000 / 154μm = 21.92μm).

[0027] Step 2: Calculate the difference between the two farthest sub-apertures using the following theoretical formula to obtain the components of the angular variance in the x and y directions. and

[0028]

[0029]

[0030] Here, d and r represent the equivalent diameter of the sub-aperture in the telescope and the equivalent center distance between the two sub-apertures, respectively. The calculated variance is then substituted into the probability density formula of a bivariate Gaussian distribution. The integration is performed within a circular region of radius 1.22λ / d0. Projecting this onto the detector implies the probability that the displacement difference of the superimposed sub-aperture images does not exceed 21.92 μm.

[0031] Step 3: The calculated probability is P≤95.5%. The error is within an acceptable range. Therefore, the maximum distance for stackable apertures under this condition can be determined to be r=0.167m. All apertures within this range can be stacked to enhance the image contrast of the sub-apertures.

[0032] Step 4: Therefore, under these conditions, by superimposing the sub-aperture images of five sub-apertures (top, bottom, left, and right) from a single sub-aperture, the image contrast will definitely be enhanced. The superimposed sub-aperture image is as follows: Figure 2As shown in (b), the contrast of the superimposed image is significantly improved. This is highly advantageous for wavefront detection of low-contrast extended targets. Notably, the sub-image information missing from the original sub-aperture center before superposition is filled in by the surrounding image information after superposition. Thus, we obtained a contrast-enhanced sub-image array based on the traditional Hartmann sub-image.

[0033] Step 5: Then, select a sub-aperture image from the contrast-enhanced sub-image array as a reference sub-aperture, and perform correlation operations and extract the relative tilt information of the wavefronts from the other sub-aperture images and the reference sub-aperture image. Then, use the wavefront tilt information of the sub-apertures and the Zernike mode wavefront reconstruction method to perform wavefront reconstruction.

Claims

1. A method for region-enhanced wavefront detection of low-contrast extended targets, characterized in that: After the Hartmann wavefront sensor obtains the sub-image array, it divides a window at a fixed position within the sub-aperture. The window contains the feature information of the sub-image. By combining the gray values ​​of neighboring sub-aperture images within the defined pixel range, a sub-image array with higher contrast is obtained. The wavefront information of the target under test is then obtained using the contrast-enhanced sub-image array. The process for selecting joint neighboring sub-aperture images is described below: Step 1: The measured atmospheric coherence length is r 0, the equivalent diameter of the two furthest sub-apertures that need to be superimposed in the microlens array is 0. d The center distance between the apertures is r ; Step 2: By statistically analyzing the variance of the difference in the angle of arrival between the two sub-apertures, the probability density formula for the angle of arrival is obtained: ,in , The variances of the difference angles of arrival are respectively in x and y Components in direction; where X , Y The differential angle of arrival for the two sub-apertures is respectively at x and y Components in direction; Step 3: Determine the probability by integrating the probability density over a circle with a radius equal to the Rayleigh criterion. Use the probability result and the acceptable error of the item to ensure enhanced contrast in the superimposed sub-image array, thereby determining the distance between the two furthest superimposed sub-apertures. r At a distance of r All apertures within the range can be stacked.

2. The method for region-enhanced wavefront detection of low-contrast extended targets according to claim 1, characterized in that: For the overlay process of edge sub-apertures, the overlay process combines the gray values ​​of all images within the window according to pixel position, thereby obtaining a contrast-enhanced overlay sub-image array. Similarly, for edge sub-apertures, this method uses surrounding sub-apertures with clear image information to overlay, thereby supplementing the image information of edge sub-apertures that are missing sub-image information and have image distortion.

3. The method for region-enhanced wavefront detection of low-contrast extended targets according to claim 1, characterized in that: Both wavefront information extraction and reconstruction utilize contrast-enhanced sub-image arrays. A sub-aperture image is selected from the sub-image array as a reference sub-aperture. Correlation operations are performed between other sub-aperture images and the reference sub-aperture image, and the relative tilt information of the wavefront is extracted. Then, wavefront reconstruction is performed using the wavefront tilt information of the sub-apertures and the Zernike mode wavefront reconstruction method.

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