KEYWORDS: Target detection, Signal to noise ratio, Image processing, Image filtering, Digital filtering, Detection and tracking algorithms, Adaptive optics, Wavefronts, Sensors, Control systems
For large astronomical telescope system based on adaptive optics(AO), the correcting ability of the wavefront processor in real time is the critical factor to the target observation. However, when we are observing the faint-target, it will lead to the uneven energy distribution of the spot in the Hartmann sensor, even no spots in some of the sub-apertures. However, whether to control the adaptive optical system to close loop is mainly dependent upon the distribution of spot in the microlens of Hartmann sensor, so it is necessary to detect the existence of the sub-spot in the Hartmann sub-aperture. This paper presents an improved approach based on template matching and threshold processing to detect the sub-spot, it could judge whether there is spot in sub-aperture self-adaptively, then, we provide an intelligence foundation for adaptive optical system. In this paper, not only do we give the detailed implementation of the detection algorithm, but also test it with the reality observed images. The experimental results have shown that the proposed approach in the paper could accurately detect whether there is spot in Hartmann sub-aperture.
During 2014-2016, the Laser guide star (LGS) adaptive optics (AO) system observation campaign has been carried out on Lijiang 1.8 meter telescope. During the campaign, two generation LGS AO systems have been developed and installed. In 2014, a long-pulsed solid Sodium prototype laser with 20W@400Hz, a beam transfer optical (BTO) system, and a laser launch telescope (LLT) with 300mm diameter were mounted onto the telescope and moved with telescope azimuth journal. At the same time, a 37-elements compact LGS AO system had been mounted on the Bent-Cassegrain focus and got its first light on observing HIP43963 (mV= 8.18mv) and reached Sr=0.27 in J Band after LGS AO compensation. In 2016, the solid Sodium laser has been upgrade to stable 32W@800Hz while D2a plus D2b repumping is used to increase the photon return, and a totally new LGS AO system with 164-elements Deformable Mirror, Linux Real Time Controller, inner closed loop Tip/tilt mirror, Multiple-PMT tracking detector is established and installed on the telescope. And the throughput for the BTO/LLT is improved nearly 20%. The campaign process, the performance of the two LGS AO systems especially the latter one, the characteristics of the BTO/LLT system and the result are present in this paper.
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.
An adaptive optics system (AOS), which consists of a 73-element piezoelectric deformable secondary mirror (DSM), a 9x9 Shack-Hartmann wavefront sensor and a real time controller has been integrated on the 1.8m telescope at the Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. Compared to the traditional AOS on Coude focus, the DSM AOS adopts much less reflections and consequently restrains the thermal noise and increases the energy transmitting to the system. Before the first on-sky test, this system has been demonstrated in the laboratory by compensating the simulated atmospheric turbulence generated by a rotating phase screen. A new multichannel-modulation calibration method which is used to measure the DSM based AOS interaction matrix is proposed. After integration on the 1.8m telescope, the closed-loop compensation of the atmospheric turbulence with the DSM based AOS is achieved, and the first light results from the on-sky experiment are reported.
In order to test the working status of adaptive optics systems, it is necessary to design a disturbance signal module. Disturbance signal module based on DDS (Direct Digital frequency Synthesis) is used to generate single-frequency disturbance signal to test the working conditions of deformable mirror and adaptive optics systems. But DDS is a periodic sampling sequence and will inevitably lead to the introduction of periodic noise which makes the disturbance signal scattering. This paper uses two methods to reduce the scattering of the single-frequency signal generated by DDS technology. The first method is the compression ROM table. In the case of the same ROM capacity, it is equivalent to extend the compressed ROM table with 256 points to ROM table with 1024 points. In this process, Oversampling is introduced to improve spectral purity to reduce the scattering of the single-frequency signal. The second method is the random phase jitter technology. It introduces m sequence as DDS sampling output random phase jitter unit. The purpose is to generate some random number added at the end of the phase accumulator. As a result, the output does not always push back than ideal, but randomly in advance, thus breaking its periodicity. This method changes the original uniform look-up sampling into a random non-uniform look-up sampling, making DDS output spectrum white. It can also improve spectral purity of the DDS output, thereby reducing the scatting of the single-frequency signal generated by DDS technology.
KEYWORDS: Signal generators, Adaptive optics, Signal processing, Interference (communication), Field programmable gate arrays, Oscillators, Data storage, Clocks, Materials science, Optoelectronics
In order to test the working state of adaptive optics system, it is necessary to design an online sweep-frequency circuit module to test the frequency response of the adaptive system. Sweep-frequency signal generator based on Direct Digital frequency Synthesis (DDS) is one of the core components. But the classic DDS technology also has some drawbacks: the truncation error of phase, the truncation error of magnitude (caused by memory FWL) and high occupancy of ROM. These are also the optimization directions in this paper. This paper presents a FPGA-based DDS sweep-frequency signal generator suitable in adaptive optics. It has a low occupancy rate with ROM. And in the case of low-ROM, the paper reduces the noise generated by the truncation error of phase and the truncation error of magnitude of DDS sweepfrequency signal generator by method of linear interpolation. The results show that, when the reference frequency is 100 MHz, the frequency resolution can be as low as 0.025 Hz. It only takes up 0.5 KB ROM with the ROM compression ratio of 64:1 in the optimized scheme in the paper and has higher precision due to the method of linear interpolation than the unoptimized scheme, which can meet the engineering needs. Compared with other schemes, the scheme in the paper improves signal accuracy in the case of reducing the truncation error of phase, the truncation error of magnitude and the occupancy rate with ROM, but only adds a multiplication and division circuit, which is a practical solution.
KEYWORDS: Digital signal processing, Wavefront reconstruction, Wavefronts, Actuators, Reconstruction algorithms, Adaptive optics, Error analysis, Data communications, Control systems, Parallel processing
In an Adaptive Optics system, the Real Time Processor is as important as the human brain. Processing latency is a key index of Real Time Proceesors . In this paper, we propose a new processing method that significantly reduce the processing latency, which combined the design idea of multi-core parallel processing on space and time. In addition, by comparing the operating speed of CPU and the I/O speed of memory, we propose a reasonable memory allocation scheme. The experimental results show that the processing latency is 59.7us per frame using multi-core DSP TMS320C6678 as processing platform. The experiment is conducted on a system with 968 sub-apertures and 913 actuators.
In this paper, a real-time on-line performance evaluation processor based on graphic processing unit (GPU) for adaptive
optics (AO) system is presented, aiming to monitor the 127-element AO system during its close-loop work by
quantifying its correction results, which can provide reference to improve the performance of the system. Since there is a
contradiction between the heavy computation burden and the real-time processing requirement, we modified operations
and algorithms to fit the CPU-GPU heterogeneous environment, in which GPU is used to handle the complex
computation but simple logicality, and CPU is assigned to undertake data transportation between internal storage and
video memory,as well as some small-scale computations. In the real-time processor, performance parameters to be
computed include peak-valley (PV) and root-mean-square (RMS) of near-field wavefront phase, point spread function
(PSF), full width half maximum (FWHM) of far-field image,modulation transfer function (MTF) and Strehl ratio (SR).
And the inputs are residual slopes obtained from Hartmann wavefront sensor of 127-element AO system. By
computation 4096 frames of parameters, the average rate by single precision is 4.11ms/frame.
In 2009, A 127-element adaptive system had been manufactured and installed at the Coude room of the 1.8-meter
telescope at the Gaomeigu site of Yunnan Astronomical Observatory, Chinese Academy of Sciences. A set of new
adaptive optical system based on a 73-element deformable secondary mirror is being developed and will be integrated
into the 1.8-meter telescope. The 73-element deformable secondary mirror with convex reflecting surface is designed to
be compatible with the Cassegrain focus of the 1.8-meter telescope. Comparing with the AO system of Coude focus, the
AO system on the deformable secondary mirror adopts much less reflections and consequently restrains the thermal
noise and increases the energy transmitting to the system. The design and simulation results of this system will be
described in this paper. Furthermore, the preliminary test result of the deformable secondary mirror in the lab is also presented.
We are developing a sodium guide star adaptive optics system for the 1.8 meter telescope, which consists of three
main parts: (i) 20W microsecond pulsed laser system, (ii) Φ200mm laser launch telescope and (iii) 37-elements adaptive
optics system. All of these three parts are mounted on the 1.8 meter telescope which is located in Gaomeigu site of
Yunnan Astronomical Observatory, Chinese Academy of Sciences. The pulsed laser system and the launch telescope are
rotated with the azimuthal base of the telescope. A miniaturized 37-elements low-order adaptive optics system including
a 37-elelment deformable mirror and a 6x6 array Hartmann-Shack wavefront sensor is mounted at the Cassegrain focus
taking account of the pulsed laser mode. A separate tip-tilt correction loop is also integrated into the system. This paper
describes the details of this system, the simulation result and the test result in the lab. After the indoor test, the whole
system will be shipped to 1.8 meter telescope. The latest commissioning status and results is presented also in this paper.
The 127-element adaptive optical system for the 1.8m astronomical telescope is being developed. In this system, the
wavefront correction loop consists of a 127-element deformable mirror, a Hartmann-Shack (H-S) wavefront sensor, and
a high-speed digital wavefront processor. The tracking system consists of a tip-tilt mirror, a tracking sensor and a
tracking processor. The wavelength for the H-S wavefront sensor ranges from 400-700nm. The imaging observation
wavelengths range from 700-1000nm and 1000-1700nm respectively. In this paper, the optical configuration of 1.8m
telescope will be briefly introduced. The 127-element adaptive optical system is described in detailed. Furthermore, the
preliminary performances and test results on the 127-element adaptive optical system is reported.
The 61-element upgraded adaptive optical system for the 1.2m telescope of Yannan Observatory for astronomical observation had been in operation since May 2004. In this paper, the 61-element upgraded adaptive optical system for 1.2m telescope of Yunnan Observatory will be briefly described. The performance on the 61-element upgraded adaptive optical system is analyzed. Furthermore, the observational results for the stars will be presented.
The capability of real time wave-front reconstruction is important for an adaptive optics (AO) system. The bandwidth of system and the real-time processing ability of the wave-front processor is mainly affected by the speed of calculation. The system requires enough number of subapertures and high sampling frequency to compensate atmospheric turbulence. The number of reconstruction operation is increased accordingly. Since the performance of AO system improves with the decrease of calculation latency, it is necessary to study how to increase the speed of wavefront reconstruction. There are
two methods to improve the real time of the reconstruction. One is to convert the wavefront reconstruction matrix, such as by wavelet or FFT. The other is enhancing the performance of the processing element. Analysis shows that the latency cutting is performed with the cost of reconstruction precision by the former method. In this article, the latter method is adopted. From the characteristic of the wavefront reconstruction algorithm, a systolic array by FPGA is properly designed to implement real-time wavefront reconstruction. The system delay is reduced greatly by the utilization of pipeline and parallel processing. The minimum latency of reconstruction is the reconstruction calculation of one subaperture.
Latency of wavefront processor is an important factor of closed loop adaptive optical systems. For an adaptive optical system using Shack-Hartmann wave-front sensing and point beam, by ways of task queue, subtask arithmetic decomposition and subtask structure design, a multi-processors structure based on moder parallelism theory is built to realize a pipeline of wavefront gradient, wavefront reconstruction and wavefront control. By traits of field programmable gate array(FPGA) and digital signal processor(DSP), a pipeline wavefront processor based on FPGA+DSP structure is built with highly real-time performance. Clocks of FPGA and DSP, “age” of correctors are primary sources of this wavefront processor’s latency. For a 61-element adaptive optical system whose sampling frequency is 2900HZ, latency of this wavefront processor is less than 100us.
The 61-element adaptive optical system built for the 1.2m telescope of Yannan Observatory for astronomical observation is being upgraded. The Hartmann-Shack wavefront sensor, the tracking system, and the imaging system have been manufactured newly. The wavelengths for the Hartmann-Shack wavefront sensor and the imaging observation range from 400-700nm and 700-1000nm respectively. The arrangement of subapertures is hexagon matched with triangle arrangement of actuators. The detector of Hartmann-Shack sensor is a high-quantum-efficiency CCD with variable frame rate. The tracking system consists of two cascade control loops in order to improve the low-frequency compensation performance. In this paper, the upgrade on 61-element adaptive optical system for 1.2m telescope of Yunnan Observatory will be shown. The preliminary results of the upgraded 61-element adaptive optical system will be presented.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.