Correlation-based Shack-Hartmann wavefront slope sensing in strong turbulence

An innovative experimental method for studying the influence of supersonic flow on laser beam transmitting has been proposed. The wavefront phase properties and the dynamic wavefront change of laser beam traversing supersonic flow can be detected by Hartmann-Shack (H-S) wavefront sensor. Beam aberrations are constructed by adopting Zernike mode-reconstruction theory. The centroid change of laser beam and the far-field properties can be obtained by further analyzing the experimental data. In the paper, the experimental principle is presented, and wavefront aberrations with several different flow parameters are given. Some beam-quality criteria such as Zernike aberration coefficient, PV and RMS value, circle energy, Strehl ratio, and centroid change of beam, are also calculated. Experimental results show that the stage of flow establishing, stabilizing and ending can be detected by using H-S wavefront sensor. By comparing the result of no flow with that of a given-parameter flow, defocus and astigmatism aberration increase obviously. At the same time, the Strehl ratio is reduced and the beam quality is degraded. The same conclusion can be obtained by comparing no test-model with the given-structural test-mode in same flow. Moreover, the wavefront aberration of laser beam in the condition of different flow parameters can also be measured and compared accurately by using the test method.

This paper compares the closed-loop, adaptive-optics-system performance obtained with a Shack-Hartmann wavefront sensor (WFS) to that of a fixed-pyramid WFS, assuming a point-source beacon. To quantify performance, the analysis uses wave-optics simulations and calculates the peak Strehl in weak to moderately deep turbulence conditions. Initial results show that the Shack-Hartmann WFS outperforms the fixed-pyramid WFS.

The Shack–Hartmann wavefront sensor (SHWFS) has been widely used for measuring aberrations in adaptive optics systems. However, its traditional wavefront reconstruction method usually has limited precision under field conditions because the weight-of-center calculation is affected by many factors, such as low signal-to-noise-ratio objects, strong turbulence, and so on. In this paper, we present a ResNet50+ network that reconstructs the wavefront with high precision from the spot pattern of the SHWFS. In this method, a nonlinear relationship is built between the spot pattern and the corresponding Zernike coefficients without using a traditional weight-of-center calculation. The results indicate that the root-mean-square (RMS) value of the residual wavefront is 0.0128 μm, which is 0.79% of the original wavefront RMS. Additionally, we can reconstruct the wavefront under atmospheric conditions, if the ratio between the telescope aperture’s diameter D and the coherent length r 0 is 20 or if a natural guide star of the ninth magnitude is available, with an RMS reconstruction error of less than 0.1 μm. The method presented is effective in the measurement of wavefronts disturbed by atmospheric turbulence for the observation of weak astronomical objects.

Several telescopes include large Deformable Mirrors (DM) located directly inside the telescope. These adaptive telescopes trigger new constraints for the calibration of the Adaptive Optics (AO) systems as they usually offer no access to an artificial calibration source for the interaction matrix measurement. Moreover, the optical propagation between the DM and the Wave-Front Sensor (WFS) may evolve during the operation, resulting in mis-registrations that highly affect the AO performance and thus the scientific observation. They have to be measured and compensated, for instance by updating the calibration. A new strategy consists of estimating the mis-registrations and injecting them into synthetic models to generate noise-free interaction matrices. This pseudo-synthetic approach is the baseline for the Adaptive Optics Facility working with a Shack-Hartmann WFS and seems particularly suited for the future Extremely Large Telescope as the calibration will have to be regularly updated, for a large numbers of actuators. In this paper, the feasibility of a pseudo synthetic calibration with Pyramid WFS at the Large Binocular Telescope (LBT) is investigated. A synthetic model of the LBT AO systems is developed, and the procedure to adjust the mis-registrations parameters is introduced, extracting them from an experimental interaction matrix. We successfully tested an interaction matrix generated from the model on the real system in high-order AO mode. We recorded a slightly better performance with respect to the experimental one. This work demonstrates that a high accuracy calibration can be obtained using the pseudo synthetic approach with pyramid WFS.

We report on a test bed to compare the performance of three different wavefront sensors, the Shack-Hartmann Wavefront Sensor (SHWFS), the Pyramid Wavefront Sensor (PWFS), and the non-linear Curvature Wavefront Sensor (nlCWFS). No single wavefront sensor easily allows for sensing all aspects of atmospheric turbulence. For instance the SHWFS has a large dynamic range and a linear response to input phase aberrations but is not sensitive to low order modes. The PWFS uses the full spatial resolution of the pupil which gives it increased sensitivity to low order modes, however it still treads the line between achieving high dynamic range and high sensitivity. The nlCWFS is the only wavefront sensor designed to sense low and high, spatial frequencies, however this leads to a complex algorithm. We discuss the reconstruction algorithm for each WFS along with simulated comparisons, we present the optical design for the WFS comparison tes tbed, and outline the adaptive optics controls system.

The performance of a ground-based optical imaging system is severely degraded from the diffraction limit by the random effects of the atmosphere. Adaptive optics techniques have been used to compensate for atmospheric turbulence effects. A critical component in the adaptive optics system is the wavefront sensor. Presently, two types of sensors are common: the Hartmann-Shack wavefront sensor and the shearing interferometer wavefront sensor. In this paper we make a direct performance comparison of these two sensors. The performance calculations are restricted to common configurations of these two sensors and the fundamental limits imposed by shot noise and atmospheric effects. These two effects encompass the effects of extended reference beacons and sensor subaperture spacings larger than the Fried parameter, r<SUB>o</SUB>. Our results indicate comparable performance for good seeing conditions, and small beacons. However, for poor seeing conditions and extended beacons, the Hartmann sensor has lower error levels than the Shearing interferometer.

Adaptive optics have been used to compensate the detrimental effects of aberrations in a range of high resolution microscopes. Aberration measurement has been implemented in various way, using direct wave front sensing or indirect optimisation methods. We investigate how backscattered laser illumination can be used as the source for direct wave front sensing using a pinhole filtered Shack Hartmann wave front sensor. It is found that the sensor produces linear response to input aberrations for a given specimen. The gradient of this response is dependent upon experimental configuration and specimen structure. The double pass nature of the microscope system leads to lower sensitivity to odd-symmetry aberration modes.

Working on low-contrast, extended, time-varying objects such as the solar granulation, solar adaptive optics (AO) system uses correlation algorithms to detect image shift of the Shack-Hartmann (SH) wavefront sensor (WFS) instead of centroid algorithm in night-time adaptive optics system. An real-time image shift detection processor, which consists of a Xilinx FPGA and a TI DSP, has been developed for a low-order solar AO system based on cross correlation coefficient algorithm. Image shift of integer pixels can be calculated in the FPGA and DSP is responsible for parabolic interpolation to obtain subpixel accuracy. The experimental results show that the processor can obtain correct image shift and satisfy the time latency requirement of the AO system.

This article examines the use of a Shack–Hartmann wave front sensor to accurately measure the line-integrated electron density gradient formed in laser-produced and Z-pinch plasma experiments. The minimum discernable line-integrated density gradient is derived for the Shack–Hartmann wave front sensor, as well as its range of applicability. A laboratory comparison between a Shack–Hartmann wave front sensor and a Twyman–Green interferometer is also presented. For this comparison, a liquid-crystal spatial-light modulator is used to introduce a spatially varying phase onto both of the wave front sensors, simulating a phase profile that could occur when a probe passes through a plasma. The phase change measured by the Shack–Hartmann sensor is then compared directly with the Twyman–Green interferometer. In this article, the merits associated with the use of a Shack–Hartmann sensor are discussed. These include a wide dynamic range, high optical efficiency, broadband or low coherence length light capability, experimental simplicity, two-dimensional gradient determination, and multiplexing capability.

The recent 2020 Decadal Survey of Astronomy and Astrophysics listed habitable exoplanet imaging with future extreme adaptive optics (AO) on 30m-class telescopes as a key priority in the coming decade. However, there is a current 100x contrast gap between the best systems today and what is needed to enable this goal. Astronomical AO is a required approach to enable ground-based diffraction-limited imaging of exoplanets on future extremely large telescopes. Time lag between the end of an exposure and the application of deformable mirror commands is a major contributor to the error budget in many AO systems, and detector read time is often a large component of this lag. We present two designs for a modified Shack Hartmann wavefront sensor (SHWFS), named Focal plane Actualized Shifted Technique Realized for a SHWFS (fastrSHWFS), to reduce the time lag component. This design steers the spot pattern at the focal plane into a rectangular or linear array with a custom aspect ratio, reducing readout time. The mask with focus yields aberrated results while the mask with tip/tilt only yields some defined spots. This essay outlines the current SHWFS concept, our fastrSHWFS theoretical solution to addressing time lag, reflection and quality analysis of printed mask designs, and results from testing both masks on the High Contrast Testbed at Lawrence Livermore National Lab. This work follows the test of a previous fastrSHWFS design.

Atmospheric disturbed wavefront with Kolmogoroff spectrum is represented by the Zernike polynomials suggested by R. No!! in 1976. From the irradiance distribution in two defocal planes , one is in the front of focal plane and the other is in the behind focal plane , the response elements for each term of Zernike polynomialswith a given detector array and successively the whole response matrix R have been obtained. For an incoming atmospheric disturbed random wavefront WR , with its irradiance distribution detected and the matrix R , it iseasy to solve the corresponding coefficients for each term of Zernike polynomials and reconstruct the detected wavefront.KEYWORDS : wavefront sensor , adaptive optics , computer simulation , Zernike polynomials , irradiance distri- bution1. INTRODUCTION Wavefront sensor is one of the most important parts in adaptive optics system (AOS). Currently, lateralshearing interferometer and Hartmann-Shack wavefront sensor are commonused wavefront sensors2. In order

The European Extremely Large Telescope (ELT) is a 39m large, ground-based optical and near- to mid-infrared telescope under construction in the Chilean Atacama desert. Operation is planned to start around the middle of the next decade. All first light instruments will come with wavefront sensing devices that allow control of the ELT’s intrinsic M4 and M5 wavefront correction units, thus building an adaptive optics (AO) system. To take advantage of the ELT’s optical performance, full diffraction-limited operation is required and only a high performance AO system can deliver this. Further technically challenging requirements for the AO come from the exoplanet research field, where the task to resolve the very small angular separations between host star and planet, has also to take into account the high-contrast ratio between the two objects. We present in detail the results of our simulations and their impact on high-contrast imaging in order to find the optimal wavefront sensing device for the METIS instrument. METIS is the mid-infrared imager and spectrograph for the ELT with specialised high-contrast, coronagraphic imaging capabilities, whose performance strongly depends on the AO residual wavefront errors. We examined the sky and target sample coverage of a generic wavefront sensor in two spectral regimes, visible and near-infrared, to pre-select the spectral range for the more detailed wavefront sensor type analysis. We find that the near-infrared regime is the most suitable for METIS. We then analysed the performance of Shack-Hartmann and pyramid wavefront sensors under realistic conditions at the ELT, did a balancing with our scientific requirements, and concluded that a pyramid wavefront sensor is the best choice for METIS. For this choice we additionally examined the impact of non-common path aberrations, of vibrations, and the long-term stability of the SCAO system including high-contrast imaging performance.

Observations have been made at the Steward Observatory 1.55 m telescope of a four-star asterism in the constellation Serpens Cauda, using a Shack-Hartmann wavefront sensor. The stars are all within a 2 arcminute field, and range in apparent brightness from m v of 9.4 to 10.6. The instrument placed a 5 × 5 array of square subapertures across the pupil of the telescope, and had sufficient field of view to allow wavefront data to be recorded from all four stars simultaneously. Snapshots at 1/30 s exposure time were recorded, with no temporal coherence between exposures. We have reconstructed the first 20 Zernike modes from the slope data for each star. In a preliminary analysis, we show that the wavefront aberration in each star can be roughly halved by subtracting the average of the wavefronts from the other three stars. The averages represent estimates of the aberration introduced by the lowest few hundred meters of the atmosphere, so the result provides an early indication of the potential for image sharpening by compensation of boundary layer turbulence. Keywords: Wavefront sensors, adaptive optics, ground layer adaptive optics

This article describes the design, implementation and characterization of a novel optical tweezer system. The system utilizes a deformable mirror, wavefront sensor and controller to manipulate an optically trapped micro-particle within a small chamber. This method for optical trapping employs technology adopted from astronomical instrumentation; in particular, adaptive optics. A deformable mirror is employed to control the wavefront phase of a laser beam before it is imaged into a chamber by a high numerical aperture microscope objective lens. The wavefront phase is measured by a Shack–Hartman wavefront sensor and the particle's position monitored by a video camera. The goals of the work presented here are to trap particles ranging in size from 1 μm to 10 μm; create a suitable controller for moving trapped particles in three dimensions; image the trapped particle; determine the prototype system's performance specifications; and determine the trap stiffness.

This paper presents the design of an Adaptive Optics (AO) system for retinal imaging applications. The development of retinal imaging systems allows for early diagnosis of eye diseases. Such systems can increase the quality of life of patients as well as curtail increasing health care costs through early eye disease detection and treatment. Until recently, AO systems have been prohibitively expensive and cumbersome. This has been mainly due to the size and cost of flexible membrane mirrors normally used as the aberration correction device. Recent developments in the technology of Microelectromechanical System (MEMS) based actuators allow the implementation of AO systems which would have been difficult to implement a few years ago due to exorbitant costs. The aim of this paper is to present the design of a compact and flexible low cost AO system using off the shelf components to measure and compensate for the aberrations of the eye. The design is based around the system’s main components which include a 52 channel magnetically actuated deformable membrane mirror, a Shack Hartmann wavefront sensor and a control system which runs on a single processor personal computer. All the components are commercially available. The use of the MEMS-based magnetically actuated mirror allows for increased resolution and force compared to conventional membrane mirrors designed mainly for use in astronomical applications. The performance of the closed-loop system is evaluated through experiments. Although designed as a diagnostic tool for eye diseases, such a system will find a number of applications in basic research in the visual sciences, including the study of microscopic structures in the living retina that could not be seen before. Optometrists, retinal surgeons, and ophthalmologists will also benefit from using such a system, through potential improvements on commonly used instruments such as phoropters and fundus cameras.