Spot Centroid Research Articles

The Study on Reducing Mixed Image Noise of Beacon Spot in Satellite Optical Communication

In satellite optical communication system, the spot centroid locating of a beacon plays an important role in establishment and maintenance of laser links. In this paper, we analyze the type and source of noises in a laser spot, and the principles of some denoising methods are summarized. To improve spot locating accuracy, four different methods are used to process the mixed noise. The simulation results show that denoising effect of Butterworth low-pass filtering is relatively good. This work provides guidance for spot locating in satellite optical communication system.

Intrapixel effects of CCD and CMOS detectors

Intrapixel nonuniformity is known to exist in CCD and CMOS image sensors, though the effects in backside illuminated (BSI) CCDs are too small to be a concern for most astronomical observations. However, projects like the Large Synoptic Survey Telescope require precise knowledge of the detector characteristics, and intrapixel effects may need more attention. By scanning CCD and CMOS cameras with a small light spot (unresolved by the optics), we find in the images that the spot's flux, centroid displacement, and ellipticity vary periodically on the pixel scale in most cases. The amplitude of variation depends on not only the detector but also how well the spot is sampled by the pixels. With a spot radius of 2 pixels (encircling 80% energy) as measured, the flux and the ellipticity extracted from the BSI CCD camera vary by 0.2–0.3% (rms) and 0.005 (rms), respectively, while the deviation of the centroid position (rms ∼ 0.01 pixel) is not correlated with the pixels. The effects are more pronounced for the BSI CMOS camera and even worse for the frontside illuminated CMOS camera. The results suggest that a closer examination of the intrapixel effects is needed for precision astronomy.

Angles-centroids fitting calibration and the centroid algorithm applied to reverse Hartmann test.

In this paper, we develop an angles-centroids fitting (ACF) system and the centroid algorithm to calibrate the reverse Hartmann test (RHT) with sufficient precision. The essence of ACF calibration is to establish the relationship between ray angles and detector coordinates. Centroids computation is used to find correspondences between the rays of datum marks and detector pixels. Here, the point spread function of RHT is classified as circle of confusion (CoC), and the fitting of a CoC spot with 2D Gaussian profile to identify the centroid forms the basis of the centroid algorithm. Theoretical and experimental results of centroids computation demonstrate that the Gaussian fitting method has a less centroid shift or the shift grows at a slower pace when the quality of the image is reduced. In ACF tests, the optical instrumental alignments reach an overall accuracy of 0.1 pixel with the application of laser spot centroids tracking program. Locating the crystal at different positions, the feasibility and accuracy of ACF calibration are further validated to 10-6-10-4 rad root-mean-square error of the calibrations differences.

提高夏克-哈特曼波前传感器光斑质心的定位精度

提高夏克-哈特曼波前传感器光斑质心的定位精度

Displacement monitoring technique using a smartphone based on the laser projection-sensing method

In this paper, one displacement monitoring technique using a smartphone based on the laser projection sensing method is proposed and developed. By means of a common laser device and a projection plate, structural displacement can be monitored using a smartphone. First, a laser spot centroid recognition method was utilized for displacement recognition. Next, one displacement monitoring APP (D-Viewer) based on the android platform was developed. To verify the feasibility of the proposed method, static and dynamic comparison experiments using two android smartphones were conducted. Finally, the mid span displacement of one suspension bridge model was tested using the proposed technique. The results show that this technique is a convenient, fast, low-cost structural displacement monitoring method with reasonable precision.

基于序列图像提高光斑质心定位精度

基于序列图像提高光斑质心定位精度

Precision enhancement method for multiplexing image detector-based sun sensor with varying and coded apertures.

The multiplexing image detector-based sun sensor has an extremely high accuracy and a large field of view (FOV) due to its large focal length, hundreds of apertures, and tens of sub-FOVs. Because of the optical interference effect, the diffraction spots of the sun on the image detector will be greatly influenced by the incident sun angles and the sizes of apertures, which affect the extraction precision of the sun spot centroid to a great extent. In this work, according to the Huygens-Fresnel diffraction integral formula and the aperture numerical simulations at different incident sun angles, we present a novel proposal for the mask with varying aperture sizes in different sub-FOVs. We encoded the aperture arrays with distance information for sub-FOV distinction. The laboratory test results indicated that, compared with the same aperture pattern design, the extraction precision of the sun spots with the varying apertures pattern design was better in a larger angle and more stable in the whole FOV, and the precision of the sun sensor could be improved to 1.32" (1σ) from 4.52" (1σ) at a 50° incident sun angle.

Technical requirements and uncertainty of far field laser spot centroid measurement using array detection method

Array detection method is widely used for measuring the far field laser centroids. The influence of array detector's aperture and dynamic range on the far field laser power was analyzed on the basis of the measurement method and measurement process for the laser spot centroid. The effect of array detector's spatial resolution and spatial duty ratio on the far field laser spot centroid was simulated, and the results were verified by performing experiments, enabling the determination of the technology requirements for the array detector. The expression for the laser spot centroid uncertainty was deduced on the basis of the uncertainty theory and laser spot centroid expression, and the sources of uncertainty were analyzed. The results will be of great importance for measuring the far field laser spot centroids and designing array detectors.

Optimization of spatial acquisition systems for low-light-level robustness in space optical communications.

The channel establishment in space optical communications relies on the Acquisition, Tracking, and Pointing (ATP) systems to initially acquire and then stably track the beacon beam. However, insufficient optical power may lead to unstable acquisition or even acquisition failure. In this Letter, we describe the mechanisms causing the instability, and then propose an approach to constrain the acquisition velocity. The approach is based on velocity prediction obtained from the light spot centroids and angle measurement data. Theoretical and experimental results show that the acceptable minimum optical power for acquisition decreases by 5.5dB after optimization, which effectively enhances the acquisition system's robustness under low-light-level conditions. This approach improves the adaptability of satellite-ground optical communications and also has practical value for deep-space optical communications.

Centroid shift analysis of microlens array detector in interference imaging system

Most CCD imaging detectors integrated microlens arrays (MLAs) to increase fill factor and sensitivity. However, they also introduce spot calibration issues with the inconsistency of spot geometry center and intensity distribution center. We setup theoretical and experimental models to research the problem of centroid shifting. According to the Seidel and Zernike coefficients of the optical model, we analyze main aberrations of microlens. In “Chief Ray” and “Centroid” reference frames, centroid shift numerical value is calculated with Geometric Ensquared Energy (GEE). Based on pentaprism test for 8.4m mirror segment, we conduct spot imaging experiment in interference system. Spots images are obtained, and two-dimensional centroid algorithm processing is performed on them to get the analog experiment values of centroid movements. The results show that the MLA placed in KAI‐16000 imaging detector causes the spot centroid to move. When there is a 14° (or −14°) angle of incident ray, the shifting values are about 1.46μm in simulation and 2.18μm in experiment. Our research makes a contribution to the compensation of calibrated error in metrology technology. We also prove that a significant portion of the shift comes from the low order aberration of microlens.

Spot centroid sensitivity to angle of intensity on image detector with lenslet array.

Lenslet array was introduced to an image detector to compensate for low sensitivity. These lenses deviate the light from different incident angles and potentially introduce errors when subpixel accuracy is needed. We investigated the spot centroid position because the angle of incidence changes on a Kodak KAI-16000 image detector with lenslet array. In our experiment, we noticed that there is a cubic dependency on the incident angle. The experimental results show that dependence on the angle of incidence is related to the lenslet array in the Kodak detector used for the pentaprism test. This situation caused an error in spherical aberration on the test surface after integration. The magnitude of the cubic component at incident angle of 14° (equivalent to F/2) is 11.6μm, which corresponds to a 48nm rms spherical aberration for the test surface and brings the scanning pentaprism test closer to the principal test while there is a 56nm rms discrepancy. The discrepancy in spherical aberration between the two tests reduced to 8nm after this calibration. It also showed the contrast measurement results for the Kodak detector and PointGrey detector. We performed experiments with two different detectors to quantify this effect.

Event localization in bulk scintillator crystals using coded apertures

The localization of radiation interactions in bulk scintillators is generally limited by the size of the light distribution at the readout surface of the crystal/light-pipe system. By finding the centroid of the light spot, which is typically of order centimeters across, practical single-event localization is limited to ~2mm/cm of crystal thickness. Similar resolution can also be achieved for the depth of interaction by measuring the size of the light spot. Through the use of near-field coded-aperture techniques applied to the scintillation light, light transport simulations show that for 3-cm-thick crystals, more than a five-fold improvement (millimeter spatial resolution) can be achieved both laterally and in event depth. At the core of the technique is the requirement to resolve the shadow from an optical mask placed in the scintillation light path between the crystal and the readout. In this paper, experimental results are presented that demonstrate the overall concept using a 1D shadow mask, a thin-scintillator crystal and a light pipe of varying thickness to emulate a 2.2-cm-thick crystal. Spatial resolutions of ~1mm in both depth and transverse to the readout face are obtained over most of the crystal depth.

Testing a class of non-Kerr metrics with hot spots orbiting SgrA*

SgrA*, the supermassive black hole candidate at the Galactic Center, exhibits flares in the X-ray, NIR, and sub-mm bands that may be interpreted within a hot spot model. Light curves and images of hot spots orbiting a black hole are affected by a number of special and general relativistic effects, and they can be potentially used to check whether the object is a Kerr black hole of general relativity. However, in a previous study we have shown that the relativistic features are usually subdominant with respect to the background noise and the model-dependent properties of the hot spot, and eventually it is at most possible to estimate the frequency of the innermost stable circular orbit. In this case, tests of the Kerr metric are only possible in combination with other measurements. In the present work, we consider a class of non-Kerr spacetimes in which the hot spot orbit may be outside the equatorial plane. These metrics are difficult to constrain from the study of accretion disks and indeed current X-ray observations of stellar-mass and supermassive black hole candidates cannot put interesting bounds. Here we show that near future observations of SgrA* may do it. If the hot spot is sufficiently close to the massive object, the image affected by Doppler blueshift is brighter than the other one and this provides a specific observational signature in the hot spot's centroid track. We conclude that accurate astrometric observations of SgrA* with an instrument like GRAVITY should be able to test this class of metrics, except in the more unlikely case of a small viewing angle.

Micrometer accuracy method for small-scale laser focal spot centroid measurement

Abstract The measurement of the centroid of small-scale laser focal spot is one of the most promising technologies for small-scale laser focal spot precise positioning. A method of two-dimensional scanning with CCD has been conducted to precisely measure the centroid of the small-scale laser focal spot (diameter in microns) by obtaining the change curve for gray value. The theoretical analysis is consistent with the experimental results.

Compensation of heliostat drift by seasonal sampling

Heliostat image drift is a common phenomenon in central receiver solar power plants. Several geometrical errors produce drift of the heliostat solar spot at receiver surface, increasing radiation spillage. A heuristic drift compensation method is proposed, based on a polynomial approximation to the drift trajectories. Results of the practical implementation of the proposed method for the control of 10 heliostats in a solar tower facility are presented. A substantial improvement of heliostat tracking is observed on the experimental tests. Because heliostat drift experimental monitoring is a time consuming task, a numerical analysis of the yearly behavior of the compensation method, based on simulations of heliostat drift, was carried out. In these simulations, the behavior of the daily RMS deviation of the concentrated solar spot centroid is evaluated for a whole year, as the polynomial correction is applied. The simulations serve also to test the effectiveness of the proposal polynomial method in a wider range of conditions. Thus, heliostats with a variety of primary error values are simulated. Random wind induced vibrations are introduced in the simulation to evaluate the effectiveness of the calibration method under noise conditions. It is found that a very effective calibration can be achieved with a few sampling events of the heliostat behavior during the year, taking only a few minutes. The RMS deviation can be reduced to values of the order of the wind induced noise level. The proposed polynomial compensation looks like a promising alternative to be implemented in heliostat fields.

Fast star centroid extraction algorithm with sub-pixel accuracy based on FPGA

Spacecraft's attitude information plays an important role in celestial navigation. The attitude is mainly determined by matching the star's centroid in the obtained image with its corresponding information in star catalog. Generally, the star image can be regarded as a spot with a diameter <5 pixels. Therefore, it is very difficult to extract the star centroid with sub-pixel accuracy, especially in the hardware system, such as FPGAs. The existing spot centroid extraction methods with high accuracy require plenty of pixels to realize the complex computations. Limited to the star's diameter and hardware requirements, such methods are not suitable for star centroid extraction in hardware system. To solve the problem, a two-step extraction method for star centroid with sub-pixel accuracy is proposed. The maximum pixel-level center can be located through zero crossing of the first derivative in a small region. Taking the pixel-level center as the middle of the window with fixed size, the sub-pixel offsets to the sub-pixel center can be calculated using fixed window weighted centroid method. The sub-pixel center of the star is then obtained by adding the offsets to the pixel-level center. This method can be implemented in hardware to increase processing speed, using Verilog hardware description languages. A simulation is performed on computer and FPGA. Experimental results show the excellent performance in accuracy and processing speed of two-step method. In addition, two-step method has strong ability of resisting noise and good robustness compared to other methods.

Shack-Hartmann spot dislocation map determination using an optical flow method

We present a robust, dense, and accurate Shack-Hartmann spot dislocation map determination method based on a regularized optical flow algorithm that does not require obtaining the spot centroids. The method is capable to measure in presence of strong noise, background illumination and spot modulating signals, which are typical limiting factors of traditional centroid detection algorithms. Moreover, the proposed approach is able to face cases where some of the reference beam spots have not a corresponding one in the distorted Hartmann diagram, and it can expand the dynamic range of the Shack-Hartmann sensor unwrapping the obtained dense dislocation maps. We have tested the algorithm with both simulations and experimental data obtaining satisfactory results. A complete MATLAB package that can reproduce all the results can be downloaded from [http://goo.gl/XbZVOr].

A New Approach to Determine the Centroid of Star Spot for Star Tracker Based on MEMS-Gyro

Along with the development of MEMS technology, the precision of MEMS-Gyro is increasing and the range of application of MEMS-Gyro is expanding. The accuracy of centroid determination of the spot directly relates to the precision of star tracker, but, when the exposure time is shorter or the satellite is in an unstable state, the noise of detector limits the accuracy greatly in the traditional approach of centroid determination. In view of these issues, this paper presents an approach which is based on high-precision MEMS-Gyro to determine the centroid of spot. This approach references EKF to estimate the position of centroid optimally. So, when the SNR is reduced because of shorter exposure and unstable state, this approach can help to increase the accuracy of centroid determination by introducing MEMS-Gyro. Further, it improves the precision and dynamic performance of the star tracker. In the end of this article, the feasibility of the approach is verified by the numerical simulation.

Experiment and analysis of the effect of fine tracking system on the unstable platform in laser communication

Atmospheric turbulence and platform vibration in the space optical communication can cause the offset and jitter of beam, which further result in the fluctuations of received optical power. To resist this effect, a communication system with fine tracking systems in the receiver and transmitter is designed. The system is used in the experiment of laser communication between high-rise buildings over a distance of 3.5 km. After adding a vibration source to the transmitter, the centroids of spots captured by the camera of the transmitter and the optical power of receiver are recorded for the purpose of analysis. When the vibration source works at the designated frequency, a peak appears at the corresponding frequency in the spectrum of the spot centroids and the optical power of receiver. Then the peak disappears once the fine tracking system begins to work. Compared with the condition without the fine tracking system, the minimum value of the optical power of receiver is increased by 5 dB, and the standard deviation is decreased by 30%.

Adaptive centroid-finding algorithm for freeform surface measurements

Wavefront sensing systems measure the slope or curvature of a surface by calculating the centroid displacement of two focal spot images. Accurately finding the centroid of each focal spot determines the measurement results. This paper studied several widely used centroid-finding techniques and observed that thresholding is the most critical factor affecting the centroid-finding accuracy. Since the focal spot image of a freeform surface usually suffers from various types of image degradation, it is difficult and sometimes impossible to set a best threshold value for the whole image. We propose an adaptive centroid-finding algorithm to tackle this problem and have experimentally proven its effectiveness in measuring freeform surfaces.