The wide field adaptive optics system for solar observations had been developed and put in routine operation at the 1-meter New Vacuum Solar Telescope (NVST) in Fuxian Lake, Yunnan Observatory. This system adopts the configuration based on the high order ground layer correction loop and the low order high altitude correction loop. Three correction modes, namely the conventional adaptive optics (CAO), ground layer adaptive optics (GLAO), and multi-conjugate adaptive optics (MCAO) with three layers correction, can be chosen with the science requirement. The DM for ground layer correction is inherited from the 151-element AO system, and two new DMs are added and conjugated to 2~5km and 7~10km, respectively. The opto-mechanical design allows for changing the conjugate height of the two DMs independently. Two multi-direction Shack-Hartmann wavefront sensors are employed for sensing the aberration from ground layer and high-altitudes turbulence. The control system is based on multi-core CPUs platform, which is flexible for testing various control approaches. We summarize the design and report the observational results of different correction modes.
There are more than 40 years history of adaptive optics (AO) in Institute of Optics and Electronics (IOE), Chinese Academy of Sciences since 1980. The research concern all the aspects including the theories study, devices manufacture, and system development. The recent advances on astronomical AO are reported in this presentation. The recent AO systems developments for 4-meter night-time optical telescope, 1.8-meter solar telescope CLST and the 1-m New Vacuum Solar Telescope at Fuxian Lake Solar Observatory are presented respectively. The Deformable Secondary Mirror advancement is also introduced.
The Educational Adaptive-optics Solar Telescope is the most advanced solar telescope available for scientific education currently. However, in some situations, strong atmospheric turbulence or equipment failure cause the solar adaptive optics system to fail, resulting in high-resolution imaging systems only being able to record open loop solar images with low quality and a lack of detailed texture information. Traditional image interpolation algorithms have limited expressive capabilities, however deep-learning methods can be trained for generating high-quality images from paired data sets. Our research is based on Real-ESRGAN, a powerful deep neural network that is utilized to solve real-world single image super-resolution reconstruction problems. We select 130 close loop TiO band (7057Å) images recorded on 2022 August 4 and another 203 on 2022 September 22 as the training datasets. For two representative examples, we evaluate the effect of image reconstruction from open loop images, and the peak signal-to-noise ratio and the structural similarity index measurement values of the reconstructed images generated by Real-ESRGAN are higher than those obtained using open loop data. As a result, some reconstruction image generated by Real-ESRGAN from the open loop solar image can be used some supplementary data.
KEYWORDS: Solar telescopes, Mirrors, Control systems, Solar processes, Telescopes, Observatories, Prototyping, Astronomical telescopes, Optical instrument design, Imaging systems
For a better understanding of small-scale solar activities, the Chinese Large Solar Telescope (CLST) with a 1.8-m aperture was proposed in 2011. As the first open solar telescope in China, it has some technical challenges that need to be addressed (e.g., thermal controlling for the primary mirror, cooling for the heat stop, system assembly, etc.). To support the design of CLST, a prototype of an open solar telescope (POST) with a 600-mm aperture was designed and fabricated from 2014 to 2017. A series of experiments for technical verifications were carried out based on the POST. The design, integration, and experiments done with the POST are reviewed. The solar observation results during its first commissioning phase are also presented.
Common Hartmann wavefront sensor bases on planar reference wave which prevent it measure the aberration of spherical wave directly. When adjusting an optical system with a Hartmann wavefront sensor, the optical path should be collimated firstly. It will make the measurement more complicated and may bring new aberration. To solve the problem, a Hartmann wavefront sensor which bases on spherical reference wave is proposed. In this paper, we analyze the principles of the Hartmann wavefront sensor and verify its feasibility by numerical simulation. The simulation result shows that, on the condition of RMS=1λ(λ=0.6328μm), the relative RMS of the residual error of the reconstructed wavefront is under 8% when the detected wavefront is the 2~15th order Zernike aberration.
KEYWORDS: Adaptive optics, Solar telescopes, Wavefront sensors, Digital signal processing, Telescopes, Observatories, Turbulence, Field programmable gate arrays, Wavefronts, Image resolution
Multi-conjugate adaptive optics (MCAO) has been proved to obtain the high resolution images with a large field of view in solar observation. A solar MCAO experiment system had been successfully developed and tested at the 1-meter New Vacuum Solar Telescope (NVST) of Fuxian Solar Observatory. It consists of two deformable mirrors (DMs), a multidirection Shack-Hartmann wavefront sensor (MD-WFS), and a real-time controller. In order to command the two DMs, five guide regions were selected from the MD-WFS to retrieve a three-dimensional measurement of the turbulent volume based on atmospheric tomography. This system saw the first light in October, 2017, and a series of MCAO-corrected high resolution sunspots images were acquired. In this presentation, the MCAO experiment system is introduced, and the observation results are presented. Furthermore, a new MCAO system based on our proposed MCAO configuration with a high order ground layer adaptive optics and low order high altitude correction will be developed for the NVST as a regularly operating instrument for scientific observations of the sun.
The mechanism of high power intra-channel crosstalk attack is analyzed theoretically and the conclusion that power of attack signal and crosstalk coefficient of optical switch are the main factors for which high power intra-channel have destructive effect on quality of legitimate signals is drawn. Effects of high power intra-channel crosstalk attack on quality of legitimate signals and its capability of attack propagation are investigated quantitatively by building the simulation system in VPI software. The results show that legitimate signals through the first and the second stage optical switch are affected by attack and legitimate signal through the third stage optical switch is almost unaffected by attack when power of original attack signal (OAS) is above 20dB more than that of legitimate signals and crosstalk coefficient of optical switch is -20dB at optical cross connect 1 (OXC1). High power intra-channel crosstalk attack has a certain capability of attack propagation. Attack capability of OAS can be propagated to OXC3 when power of OAS is 27dB more than that of legitimate signals and crosstalk coefficient of optical switch is -20dB. We also find that the secondary attack signal (SAS) does not have capability of attack propagation.
Correlating Shack-Hartmann wavefront sensor is widely used in solar adaptive optics in which the relative shift between
different subapertures by correlation algorithm is computed, and then the control voltage by wavefront
reconstruction can be estimated to use for correcting the wavefront distortion induced by atmospheric turbulence. In this
paper, several different correlation algorithms including Cross-Correlation Coefficient, Absolute Difference Function,
Absolute Difference Function-Squared and Square Difference Function are used to estimate relative shift in correlating
Shack-Hartmann wave-front sensor with the different observed solar structure such as sunspot, solar pore and solar
granulation. The measurement noise RMS error is computed to compare the performance of the correlation algorithms.
The results show the correlation algorithm precision is directly related to the solar structure. The measurement noise is
relatively small with the relatively high contrast target, and vice versa. At the same time, the size of reference image also
could influence the measurement noise, the larger size of the reference image, the smaller the measurement noise is.
Solar observation with high resolution in large field of view (FoV) is required for some solar active regions with the typical sizes of 1’ to 3’. Conventional adaptive optics (AO) could not satisfy this demand because of the atmospheric anisoplanatism. Through compensating the turbulence in different heights, multi-conjugate adaptive optics (MCAO) has been proved to obtain a larger corrected FoV. A MCAO experimental system including a conventional 151-element AO system and a 37-element MCAO part is being developed. The MCAO part contains a 37-element deformable mirror conjugated into the 2km to 5km height and a multi-direction Shack-Hartmann wavefront sensor (MD-SHWFS) with 7×7 subaperture array and 60 arcsec FoV, the frame rate of the MD-SHWFS is up to 840Hz. Three-dimensional (3-D) wavefront sensing utilizing atmospheric tomography had been validated by solar observation. Based on these results, a ground layer adaptive optics (GLAO) experimental system including a 151-element deformable mirror and the MD-SHWFS has been built at the 1m New Vacuum Solar Telescope (NVST). In this paper, the MCAO experimental system will be introduced. The preliminary experimental results of three-dimensional wavefront sensing and GLAO on the NVST of Full-shine Lake Solar Observatory are presented.
KEYWORDS: Adaptive optics, Solar telescopes, Observatories, Free space optics, Actuators, Digital signal processing, Telescopes, Imaging systems, Cameras, Mirrors
Adaptive Optics (AO) has become the requisite equipment of the ground-based large solar telescope to correct the wavefront aberration induced by the atmospheric turbulence. Two generation solar AO systems, one is the 37-element loworder AO system with 2100Hz frame rate and the other is 151-element high-order AO system with 3500Hz frame rate, were successfully developed in 2013 and 2015 respectively. In this presentation, the development of the two AO systems for 1-m New Vacuum Solar Telescope (NVST) at Fuxian Solar Observatory (FSO) will be introduced and the solar high resolution observational results are presented.
The AO progresses for astronomy in the Key Laboratory of Adaptive Optics, Chinese Academy of Sciences are reported in this presentation. For night-time astronomical observations, the recent AO technological developments, such as Laser Guide Star, Pyramid Sensor and Deformable Secondary Mirror, are introduced. The solar AO researches are also presented for day-time astronomical observations. Furthermore, we will show the on-sky high resolution observational results in the 1.8m telescope at Gaomeigu site, Yunnan Observatory and the 1-m New Vacuum Solar Telescope (NVST) at Fuxian Lake Solar Observatory respectively.
A 3mm narrow interval deformable mirror (DM) with tip-tilt stage has been developed for astronomical instruments.
Benefiting from its compact design, the adaptive optics system can be built with simple structure and smaller optical
elements. First, a 37-elements prototype mirror has been developed for our 1.8-meter telescope, which interval space is
3mm, maximum tilt is ±10’, and maximum deformation is ±2μm. Based on this mirror, a simple adaptive optics system has been set up and its performance was tested in the laboratory especially the closed-loop correction ability. This
adaptive optics subsystem is scheduled to be mounted at one folded Cassegrain focus of the 1.8-meter telescope this
year, and comparison test for star compensation observation using this compact system and conventional adaptive optics
system will also be carried out at the same time.
Using Hartmann-Shack (H-S) wave-front sensor to test lenses with high numerical aperture, the reference spherical
wave-front from pinhole is used to calibrate the Hartmann sensor to improve the precision of calibration, but intensity
uniformity of the reference spherical wave-front affects the precision of Hartmann sensor’s calibration. Based on the
vector diffraction theory, intensity uniformity is calculated with finite-difference time-domain method in case of a
converging Gaussian incident visible light on pinhole. In order to proof the correctness of the intensity model of pinhole
vector diffraction, experimentation of intensity is performed in visible-light. When the pinhole is the material aluminum
with thickness 200nm and pinhole diameter 500nm, the absolute error of intensity uniformity is about 2.57% and 2.31%
within 0.75 NA and 0.5 NA of diffracted wave-front by comparing experiment result with simulation result, so the
intensity model is accurate.
Piezo-electric actuators with advantage of fast responsiveness, large force output, low power consumption, negligible
friction and no backlash are widely used in precision positioning, adaptive optics and vibration conduction. However its inherent hysteresis brings difficulty to high precision positioning. To describe the hysteresis, a mathematical model based on experimental data is used. And the inverse of the model is connected to the piezo-electric actuator as a controller to compensating the hysteresis. In this paper KP operator is used to model the hysteresis of piezo-electric actuators and a numerical algorithm is proposed to compute the inverse. Experiments data of major hysteresis loop and minor loop collected on a nano-positioning stage are used to identify the model Γ based on which the inverse model Γ-1 is developed. Experiments show that given a voltage series the model Γ can give displacement prediction which has an error of 6% relative to experimental results and Γ-1 can give voltage prediction with the error of 5% relative to the experimental data.
KEYWORDS: Adaptive optics, Digital signal processing, Real-time computing, Wavefronts, Field programmable gate arrays, Lawrencium, Wavefront sensors, Signal processing, Solar telescopes, Detection and tracking algorithms
A low-order solar adaptive optics (AO) system had been successfully built and installed at 1m New Vacuum Solar
Telescope (NVST) of Full-shine Lake Solar Observatory. The real time controller (RTC) of the AO system, which
consists of a correlation tracker and a high-order wavefront correction controller, was developed. In this system, the
absolute difference algorithm is used to detect wavefront gradients. A new architecture with field-programmable gate
array (FPGA) and digital signal processor (DSP) for the real-time controller based on systolic array and pipeline was
designed. The controller was integrated into the AO system and saw the first light on February 24th, 2011, using solar
granulation as the beacon. Later, the AO-corrected high resolution sunspots images were obtained using sunspots as the
beacon. The observational results show that the contrast and resolution of the solar images are improved evidently after
the correction by the AO system. The design of the RTC and the observational results will be presented.
KEYWORDS: Adaptive optics, Wavefront sensors, Solar telescopes, Observatories, Wavefronts, Digital signal processing, Mirrors, Telescopes, Multichannel imaging systems, Imaging systems
A low-order solar adaptive optics (AO) system, which consists of a fine tracking loop with a tip/tilt mirror and a
correlation tracker, and a high-order correction loop with a 37-element deformable mirror, a correlating Shack-Hartmann
wavefront sensor and a high-order wavefront correction controller, had been successfully developed and installed at 1-m
New Vacuum Solar Telescope of Full-shine Lake (also called Fuxian Lake) Solar Observatory. This system is an update
of the 37-element solar AO system designed for the 26-cm Solar Fine Structure Telescope at Yunnan Astronomical
Observatory in 2009. The arrangement of subapertures of the Shack-Hartmann wavefront sensor was changed from
square to hexagon to achieve better compensation performance. Moreover, the imaging channel of the updated system
was designed to observe the Sun at 710nm and 1555nm simultaneously. The AO system was integrated into the solar
telescope in 2011, and AO-corrected high resolution sunspots and granulation images were obtained. The observational
results show that the contrast and resolution of the solar images are improved evidently after the correction by the AO
system.
A corneal topography based on Hartmann-Shack Sensor is presented in this paper. In the system, the focus of an
objective lens is precisely positioned on cornea's curve center. Wave-front of the reflecting beam can be measured by the
Hartmann-Shack sensor which is conjugate to the cornea plane. If the corneal surface is a perfect sphere, wave-front
detected by the Hartmann-Shack sensor is a plane. As a result, data measured by Hartmann-Shacks sensor is the
deviation between the sphere and the real cornea surface. This paper describes a methodology for designing instrument
based on Hartmann-Shack sensor. Then, applying this method, an instrument is developed for accurate measurement of
corneal topography. In addition, measuring principle of Hartmann-Shack sensor which determined system
parameters is also introduced. Repeatability is demonstrated by a series of data. The instrument was able to
accurately measure simulative cornea's reflective aberrations, from which corneal topography and corneal refractive
aberrations were derived.
The optical design of a confocal scanning laser ophthalmoscope based on adaptive optics (AOSLO) is described in
this paper. Spherical mirrors are used for pupil relay and astigmatism compensation. The main optical system
achieves diffraction-limited performance through the entire scanning angle (6mm pupil, 3 degree on pupil plane).
Images of retinal cell were collected in 60 male normal volunteers with normal vision using adaptive optic system. All have their right eye checked. Study was focused on the imaging of the outer layer cell at the fovea. And another, 12 specimen of human retina were studied under the differential interference contrast microscopy, focus was placed on different layer of the photoreceptor at the fovea. High-resolution images of the retinal cells were obtained by adaptive
optic system in all the volunteers. In pathological study, we also get clear imaged of different layer of the photoreceptor. Based on histopathological support and others, it is concluded that the outer layer cell taken by the adaptive optic system is the outer and inner segment of the cone.
In 1999, the first Hartmann-Shack wave-front sensor for the human eye aberration measurement in China was established. The H-S sensor was successfully improved and applied to the clinic diagnosis. In this paper, the principle and the method of measuring wave aberrations of the human eye are given. The accuracy of the Hartman-Shack sensor is measured and analyzed. The measurement results of the wave-front aberrations of the real eyes using the sensor are demonstrated.
In 1980, the first laboratory on Adaptive Optics in China was established in Institute of Optics and Electronics, Chinese Academy of Sciences. Several adaptive optical systems had been set up and applied in Inertial Confinement Fusion (ICF) and retinal high-resolution imaging. In 1985, the first adaptive optical system for ICF equipment was set up in the world. Another 45 element adaptive optical system was first built for correcting the static and dynamic wavefront aberrations existed in the large-aperture Nd: glass laser for inertial confinement fusion in 2001. Two set adaptive optical system with 19-element and 37-element deformable mirror had been developed for human retina imaging in 2000 and 2002 respectively. In this paper, the function and performance of these adaptive optical systems are described and the experiment results are presented.
Hartmann-Shack wavefront sensors[1] are widely used in adaptive optical systems. It can measure the spatial-temporal errors of dynamic wavefront. Not only the phase but also the amplitude of a wavefront can be measured. Unlike an interferometer, it is not necessary to have a real-time reference beam, so it can work in a disturbing environment. Besides used in adaptive optics systems, Hartmann-Shack wavefront sensors also become a powerful tool in two fields: light beam diagnosis and optical testing of optical components and systems. We have developed a serial of Hartmann-Shack wavefront sensors used in these two fields. In this presentation, various applications of Hartmann-Shack wavefront sensors in these fields will be reported.
A wave aberration compensation method for optical system and human eyes has been studied. The wave distortion of the human eye can be measured using a Shack-Hartmann sensor and described in an eight-order Zernike polynomial. By picking up the higher order’s terms of the wave aberration from the measured polynomial, characteristics of the higher order wave aberration were analyzed and concluded. The conjugate phase plates were designed based on the distribution of the single and the overall wave aberration of the eye. The thermal-filtered halftone mask technique is developed to manufacture such an abnormity micro-optical structure. The compensators for the single or the overall wave aberration were fabricated respectively. The relief profiles were evaluated using both three-dimensional profiler and interferometer. A primary experiment for compensating the higher-order wave aberration of an artificial eye was demonstrated.
At the base of the early reported 19 element adaptive optical system for human retina imaging, a new adaptive optical system has been developed. The wavefront correction element is a newly developed 37 element deformable mirror. Some modifications have been adopted for easy operation. Experiments for different imaging wavelengths and axial positions were conducted. Mosaic pictures of photoreceptors and capillaries were obtained. This would be the most detailed image of capillary distribution cover ±3° by ±3° field around the fovea ever reported. Normal and abnormal eyes of different ages have been inspected. Some preliminary very early diagnosis experiment has been tried in laboratory. This system is being planned to move to the hospital for clinic experiments.
Two generations of adaptive optical system for human retina imaging have been developed. The wavefront correcting elements are small PZT 19 and 37 element deformable mirrors (DM) with novel structure. The diameters of these DMs are 24 and 50mm respectively. By using these DMs, the size of whole optical system are rather small and can be fit on table. These systems are successfully used to correct the aberrations of living human eye. High-resolution images of microscopic structure in the scale of single photo-receptor cell and capillary in the human retina have been obtained by real-time correction of adaptive optical systems.
Known the wave-fronts of the ICF amplifiers is very important for improving the design and adjustment of the amplifiers and designing the adaptive optical system that can be used to shape the beam or clean it up. Because the pulse of ICF laser is ns scale, the wave-front can not be measured with common methods. In this paper, a method with a Hartmann-Shack wave- front sensor based on the progressive scan CCD camera is introduced. The test results show that this method is effective.
When optical beam propagates through the atmosphere, the wavefront is distributed by the turbulence. The wavefront error can be measured by Hartmann-Shack wavefront sensor (HSWFS) as arrival angle fluctuations in subapertures. Statistic characteristics of atmospheric turbulence can be calculated from these data. To make these characteristics more accurate, the spatial and temporal resolution should be high, and the measurement time should be long enough. A high resolution measuring and data processing system has been built for this usage. A 37-element HSWFS and a high frame rate (800 Hz) CCD image sensor are used in this system. Image data from CCD are processed by a multi-processor system in real time to extract the wavefront slope data of each subaperture. These slope data are sent into the extension memory of a personal computer, which is the host computer of the system. Data of about 200,000 frames can be stored continuously for one measurement period. After this period, these data are saved in hard disk, and the characteristics of turbulence can be obtained from these data.
Deformable mirror is the key element for adaptive optical wavefront correction. The number of actuators decides the complexity and cost of adaptive optical system. In this paper computer simulations of wavefront error for fitting different Zernike terms by deformable mirror with different number of actuators are presented. The arrangement of actuator and the influence function of mirror are discussed in respect of fitting error. The minimum number of actuators for fitting different Zernike orders of wavefront are given. Some optical experiments of fitting capability have been done with 19 and 37-element deformable mirrors and a Zygo interferometer.
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