Adaptive Threshold Wave Front Sensing for Portable Adaptive Optics System Driven by Hybrid Parallel Technique

Our Portable Adaptive Optics(PAO)system designed for high-contrast imaging of exoplanets with current 2-4 m class telescopes achieves a correction speed of nearly 1000 Hz,utilizing a Shack-Hartmann Wave Front Sensor(WFS)in a 9×9 sub-aperture configuration.As we look towards adapting the PAO system for larger telescopes,an increase in the number of sub-apertures in the WFS and enhanced precision in wave front detection are imperative.Originally programmed in LabVIEW,our initial PAO software is based on a traditional centroid calculation module for nighttime wave front sensing and lacks adaptive processing of background noise.To address these limitations and to boost the PAO system's performance and accuracy in wave front detection,we propose a compressive neural network(Th-Net)combined with a specialized hybrid parallel programming approach for wave front detection.Our experimental results indicate that this hybrid parallel technique and Th-Net significantly enhance the PAO system's operational speed and wave front detection precision under uneven background noise.This work paves the way so that a duplicable and low-cost PAO system can be used for direct imaging of exoplanets with large telescopes.

Solar adaptive optics systems for the New Vacuum Solar Telescope at the Fuxian Lake Solar Observatory

Adaptive optics(AO)is essential for high-quality ground-based observations with large telescopes because it counters the impact of wavefront aberrations caused by atmospheric turbulence.The new vacuum solar telescope(NVST)is one of the most important high-resolution solar observation instruments in the world.Three sets of solar adaptive optics systems have been developed and installed on this telescope:conventional adaptive optics,ground layer adaptive optics,and multi-conjugate adaptive optics.These have been in operation from 2018 to 2023.This paper details the development and application of solar adaptive optics on the NVST and discusses the newest instrumentation.

Preliminary investigations of a potential optics system for wideband X-ray telescopes

We present preliminary investigations of a potential optics system for wideband X-ray telescopes.The optical design adopts the conical approximation of the Wolter-I configuration and a combination of multilayer coatings and silicon pore optics.The total number of mirror modules is 79,distributed in 8 rows with the radii at the intersection plane between 250 mm and 500 mm.The optimization of the total effective area using the figure of merits method suggests that the focal length is 30 m and the mirror coating is a combination of the W/Si and Pt/C multilayers.This fulfills the on-axis effective area requirements of 2000 cm^(2) at 10 keV and 300 cm^(2) at 60 keV and provides a broad energy response between 3 keV and 78.4 keV.With the current geometry and coating compositions,we implement a mass modeling of the telescope in Geant4 to predict mirror performances via the ray-tracing algorithm,including the angular resolution and effective area.With the presumed metrological data as input,this can provide precision and finishing requirements for the manufacture of optics.This work demonstrates the feasibility of combining multilayer coatings and silicon pore optics for potential use in wideband X-ray telescopes and advances the development and progress of such missions.

Adaptive Optics System and Its Application Predictions at 1.2m Telescope of Yunnan Observatory

A 61 element adaptive optical system has been preliminary tested in the Coudé path of the 1 2m telescope at the Yunnan observatory this year. The whole system will be fully operated next year. This paper describes the AO system performances and its first experiment results, and the possible astronomical research topics.

A slope-based decoupling algorithm to simultaneously control dual deformable mirrors in a woofer–tweeter adaptive optics system

We propose a slope-based decoupling algorithm to simultaneously control the dual deformable mirrors （DMs） in a woofer-tweeter adaptive optics system. This algorithm can directly use the woofer＇s response matrix measured from a Shack-Hartmann wave-front sensor to construct a slope-based orthogonal basis, and then selectively distribute the large- amplitude low-order aberration to woofer DM and the remaining aberration to tweeter DM through the slope-based orthogonal basis. At the same moment, in order to avoid the two DMs generating opposite compensation, a constraint matrix used to reset tweeter control vector is convenient to be calculated with the slope-based orthogonal basis. Numeral simulation demonstrates that this algorithm has a good performance to control the adaptive optics system with dual DMs simultaneously. Compared with the typical decoupling algorithm, this algorithm can take full use of the compensation ability of woofer DM and release the stroke of tweeter DM to compensate high-order aberration. More importantly, it does not need to measure the accurate shape of tweeter＇s influence function and keeps better performance of restraining the coupling error with the continuous-dynamic aberration.

First light for the sodium laser guide star adaptive optics system on the Lijiang 1.8 m telescope

A first generation sodium Laser Guide Star Adaptive Optics System （LGS-AOS） was developed and integrated into the Lijiang 1.8 m telescope in 2013. The LGS-AOS has three sub-systems： （1） a 20W long pulsed sodium laser, （2） a 300-millimeter-diameter laser launch telescope, and （3） a 37-element com- pact adaptive optics system. On 2014 January 25, we obtained high resolution images of an my 8.18 star, HIP 43963, during the first light of the LGS-AOS. In this paper, the sodium laser, the laser launch telescope, the compact adaptive optics system and the first light results will be presented.

Comparison between iterative wavefront control algorithm and direct gradient wavefront control algorithm for adaptive optics system

Among all kinds of wavefront control algorithms in adaptive optics systems, the direct gradient wavefront control algorithm is the most widespread and common method. This control algorithm obtains the actuator voltages directly from wavefront slopes through pre-measuring the relational matrix between deformable mirror actuators and Hartmann wavefront sensor with perfect real-time characteristic and stability. However, with increasing the number of sub-apertures in wavefront sensor and deformable mirror actuators of adaptive optics systems, the matrix operation in direct gradient algorithm takes too much time, which becomes a major factor influencing control effect of adaptive optics systems. In this paper we apply an iterative wavefront control algorithm to high-resolution adaptive optics systems, in which the voltages of each actuator are obtained through iteration arithmetic, which gains great advantage in calculation and storage. For AO system with thousands of actuators, the computational complexity estimate is about O(n2) ～ O(n3) in direct gradient wavefront control algorithm, while the computational complexity estimate in iterative wavefront control algorithm is about O(n) ～(O(n)3/2), in which n is the number of actuators of AO system. And the more the numbers of sub-apertures and deformable mirror actuators, the more significant advantage the iterative wavefront control algorithm exhibits.

First generation solar adaptive optics system for 1-m New Vacuum Solar Telescope at Fuxian Solar Observatory

The first generation solar adaptive optics （AO） system, which consists of a fine tracking loop with a tip-tilt mirror （TTM） and a correlation tracker, and a high-order correction loop with a 37-element deformable mirror （DM）, a correlating Shack-Hartmann （SH） wavefront sensor （WFS） based on the ab- solute difference algorithm and a real time controller （RTC）, has been developed and installed at the 1-m New Vacuum Solar Telescope （NVST） that is part of Fuxian Solar Observatory （FSO）. Compared with the 37-element solar AO system developed for the 26-cm Solar Fine Structure Telescope, administered by Yunnan Astronomical Observatories, this AO system has two updates： one is the subaperture arrangement of the WFS changed from square to hexagon; the other is the high speed camera of the WFS and the corre- sponding real time controller. The WFS can be operated at a frame rate of 2100 Hz and the error correction bandwidth can exceed 100 Hz. After AO correction, the averaged residual image motion and the averaged RMS wavefront error are reduced to 0.06＂ and 45 nm, respectively. The results of on-sky testing obser- vations demonstrate better contrast and finer structures of the images taken with AO than those without AO.

Optics System Design of Microwave Imaging Reflectometry for the EAST Tokamak

A front-end optics system has been developed for the EAST microwave imaging reflectometry for 2D density fluctuation measurement.Via the transmitter optics system,a combination of eight transmitter beams with independent frequencies is employed to illuminate wide poloidal regions on eight distinct cutoff layers.The receiver optics collect the reflected wavefront and project them onto the vertical detector array with 12 antennas.Utilizing optimized Field Curvature adjustment lenses in the receiver optics,the front-end optics system provides a flexible and perfect matching between the image plane and a specified cutoff layer in the plasma,which ensures the correct data interpretation of density fluctuation measurement.

The Perturbed Riemann Problem for a Geometrical Optics System

The perturbed Riemann problem for a hyperbolic system of conservation laws arising in geometrical optics with three constant initial states is solved.By studying the interactions among of the delta-shock,vacuum,and contact discontinuity,fourteen kinds of structures of Riemann solutions are obtained.The compound wave solutions consisting of delta-shocks,vacuums,and contact discontinuities are found.The single and double closed vacuum cavitations develop in solutions.Furthermore,it is shown that the solutions of the Riemann problem for the geometrical optics system are stable under certain perturbation of the initial data.Finally,the numerical results completely coinciding with theoretical analysis are presented.

Simulation of a ground-layer adaptive optics system for the Kunlun Dark Universe Survey Telescope

Ground Layer Adaptive Optics （GLAO） is a recently developed technique extensively applied to ground-based telescopes, which mainly compensates for the wavefront errors induced by ground-layer turbulence to get an appropriate point spread function in a wide field of view. The compensation results mainly depend on the turbu-lence distribution. The atmospheric turbulence at Dome A in the Antarctic is mainly distributed below 15 meters, which is an ideal site for applications of GLAO. The GLAO system has been simulated for the Kunlun Dark Universe Survey Telescope, which will be set up at Dome A, and uses a rotating mirror to generate several laser guide stars and a wavefront sensor with a wide field of view to sequentially measure the wavefronts from different laser guide stars. The system is simulated on a computer and parameters of the system are given, which provide detailed information about the design of a practical GLAO system.

First light on an adaptive optics system using a non-modulation pyramid wavefront sensor for a 1.8 m telescope

Our adaptive optics system based on a non-modulation pyramid wavefront sensor is integrated into a 1.8 m astronomical telescope installed at the Yunnan Observatory in LiJiang, and the first light with high-resolution imaging of an astronomical star is successfully achieved. In this Letter, the structure and performance of this system are introduced briefly, and then the observation results of star imaging are reported to show that the angular resolution of an adaptive optics system using a non-modulation pyramid wavefront sensor can approach the diffraction limit quality of a 1.8 m telescope.

Objective visual performance evaluation with visual evoked potential measurements based on an adaptive optics system

An objective visual performance evaluation with visual evoked potential （VEP） measurements was first inte- grated into an adaptive optics （AO） system. The optical and neural limits to vision can be bypassed through this system. Visual performance can be measured electrophysiologically with VEP, which reflects the objective func- tion from the retina to the primary visual cortex. The VEP ts without and with AO correction were preliminarily carried out using this system, demonstrating the great potential of this system in the objective visual performance evaluation. The new system will provide the necessary technique and equipment support for the further study of human visual function.

Real-time measurement of atmospheric parameters for the 127-element adaptive optics system of 1.8-m telescope

A real-time method for measuring atmospheric parameters based on co-processor field-programmable gate array （FPGA） and main processor digital signal processing （DSP） is proposed for ground-based telescopes with adaptive optics （AO） systems. Coherence length, outer scale, average wind speed, and coherence time are estimated according to closed-loop data on the residual slopes and the corrected voltages of AO systems. This letter introduces the principle and architecture design of the proposed method, which is successfully applied in the 127-element AO system of the 1.8-m telescope of Yunnan Astronomical Observatory. The method enables real-time atmospheric observations with the same object and path of the AO system. This method is also applicable to extended objects.

Combinational-deformable-mirror adaptive optics system for compensation of high-order modes of wavefront

A new kind of adaptive optics （AO） system, in which several low spatial frequency deformable mirrors （DMs） with optical conjugation relationship are combined to correct high-order aberrations, is proposed. The phase compensation principle and the control method of the combinational AO system are introduced. The numerical simulations for the AO system with two 60-element DMs are presented. The results indicate that the combinational DM in the AO system can correct different aberrations effectively as one single DM with more actuators, and there is no change of control method. This technique can be applied to a large telescope AO system to improve the spatial compensation capability for wavefront by using current DM.

Optimization of the transmitted wavefront for the infrared adaptive optics system

The adaptive optics system for the second-generation Very Large Telescope-interferometer(VLTI)instrument GRAVITY consists of a novel cryogenic near-infrared wavefront sensor to be installed at each of the four unit telescopes of the Very Large Telescope(VLT).Feeding the GRAVITY wavefront sensor with light in the 1.4–2.4μm band,while suppressing laser light originating from the GRAVITY metrology system requires custom-built optical componets.In this paper,we present the development of a quantitative near-infraredpoint diffraction interferometric characterization technique,which allows measuring the transmitted wavefront error of the silicon entrance windows of the wavefront sensor cryostat.The technique can be readily applied to quantitative phase measurements in the near-infrared regime.Moreover,by employing a slightly off-axis optical setup,the proposed method can optimize the required spatial resolution and enable real time measurement capabilities.The feasibility of the proposed setup is demonstrated,followed by a theoretical analysis and experimental results.Our experimental results show that the phase error repeatability in the nanometer regime can be achieved.

Zonal decoupling algorithm for dual deformable mirror adaptive optics system

A zonal decoupling algorithm used to control a dual deformable mirror （DM） is proposed. One of the two DMs is characterized with a large stroke （woofer）, while the other one is characterized by a high spatial frequency （tweeter）. A numerical model is used to compare the zonal decoupling algorithm with some traditional zonal decoupling algorithms. The simulation results indicate that the algorithm presented in this Letter improves the performance in suppressing the coupling error. An experimental system is built to prove the effectiveness of this algorithm. The experiments demonstrate that the phase aberrations could be effectively compensated and that the coupling error could also be suppressed.

Performance modeling of the adaptive optics system on the 2.16 m telescope

The adaptive optics system on the 2.16 m telescope is modeled and simulated using an ends-to-ends Monte Carlo method in this paper.Under the optimized parameters,the Strehl ratio can be increased from about 0.02 to more than 0.2 in the J,H and K bands with a classical adaptive optics system.Based on the simulation results,the practical system design is also considered and discussed.

Wave-front correction of high-intensity fs laser beams by using closed-loop adaptive optics system

We developed an adaptive optics system to correct the wave-front distortion of an intense fs laser beam from our multi-TW laser system, Jiguang II. In this paper, the instruments of the adaptive optical system are described and the experimental results of the closed-loop wave-front correction are presented. A distorted laser wave-front of 20 wavelengths of P-V values was corrected to 0.15 wavelength of P-V values. The beam quality of the laser system varies from 3.5 diffraction limit to 1.5 diffraction limit.

Performance measurement of adaptive optics system based on Strehl ratio

In this article, a method for measuring the performance of adaptive optics （AO） systems is designed and validated by experiments. The Strehl ratio （SR） which is based on the target images is used to evaluate the performance quantitatively because it relates to the effect of AO correction directly. In the calculation of the SR, to avoid energy scaling in the diffraction-limited point spread function, an algorithm based on the integral of the optical transfer function （OTF） is proposed. Then, a 97-element AO system is established to validate this method, and a white-light fiber source is used as a point-like target. To simulate the practical conditions which influence the performance of the AO system, targets of different brightness are simulated in terms of different signal-to-noise ratios （SNRs） of the Shack-Hartmann （SH）, and atmospheric turbulence is simulated in terms of the Fried＇s coherence length and the Greenwood＇s frequency. Finally, two experiments are conducted in which the SR of different simulated conditions are measured. The results of the experiments show that for a moderate SNR of SH the experimenting AO system is capable of closed-loop wavefront correction when the Fried＇s coherence length is larger than 5cm and the Greenwood＇s frequency is lower than 60 Hz. The results also show that the performance of AO is susceptible to the SNR of SH. The experiments validates the effectiveness of this method.