Research On Real-Time Control Technology For Solar Multi-Conjugate Adaptive Optics System

It is of vital importance for solar physicists to obtain the large field of view(FOV)and high-resolution images of the whole solar activity area and to study the mechanism of solar magnetic field activity.Conventional adaptive optics(CAO)can only improve the image quality in a small FOV,and cannot meet the needs of large FOV and high-resolution astronomical observation.Multi-conjugate adaptive optics(MCAO)is one of the research focus in recent years from adaptive optical field.Its basic principle is to divide the atmospheric turbulence into several layers,and then measure and correct the wavefront distortion of each layer,eliminate the influence of atmospheric turbulence on the imaging system,and finally achieve the large FOV and high-resolution images.The real-time controller is the computing core of the solar MCAO system.Compared with the CAO system,the number of subapertures of the Shack-Hartmann wavefront sensor and the elements of the deformable mirror in the solar MCAO system are even more.Compared with the night astronomical MCAO system,the wavefront slope detection algorithm is more complicated.Therefore,the biggest difficulty lies in the guarantee of the real-time under the large amount of computing.In order to dynamically compensate for the change of atmospheric turbulence,the frame rate is usually up to hundreds or even thousands of frames per second,and the corresponding calculation must be completed within a frame,so the real-time requirements are very high.In this paper,the real-time control technology of the solar MCAO system is studied,and the parallel algorithm is implemented based on the hardware platform of FPGA+ multi-core DSP.First of all,the paper briefly introduces the temporal and spatial statistical characteristics of the atmosphere,the basic theory of the solar MCAO and the development of the domestic and international,and puts forward higher requirements for the design of the real-time controller of the solar MCAO system.On this basis,according to the characteristics of MCAO real time processing algorithm,the demand of computation and real-time is further analyzed.Secondly,according to the analysis of the requirement of computation and real-time,we compare the advantages and disadvantages of several parallel computing platforms.General multi-core CPU has the advantages of flexible programming and rich in resources.However,due to the system scheduling or interrupt response,it will cause the time jitter and affect the performance of the system.DSP arrays have good real-time performance,but often need large number of DSPs for the Shack-Hartmann wavefront sensor with a large number of subaperture and subfield images,that will usually cause the complicated peripheral circuit design.FPGA often use the hardware description language to define the hardware,and can flexibly process a large number of subaperture and subfield images.At the same time,multi-core DSP is fit for floating point operations,especially the parallel optimization of large matrices.Finally,a heterogeneous platform based on FPGA and multi-core DSP is proposed,and the mapping of the corresponding algorithm is completed.Then,according to the wavefront detection,a variety of parallel combinations are used to accelerate the computations in the FPGA to achieve 750 times hardware acceleration;for the wavefront reconstruction and wavefront control,the corresponding code are completed with the parallel optimization in the multi-core DSP and to achieve a speed up of 8 times.The overall computation delay was greatly reduced and can meet the real time requirement.Finally,according to the two-conjugate adaptive optics system,a MD-WFS which contains 37 subapertures(5 subfields in each subaperture)is used to wavefront detection and two DMs(151 actuators and 37 actuators)are conjugated to 0 km and 2-5 km respectively.We developed an experiment in door with 5 laser point sources and outside the door with the Sun.The real-time controller was successfully applied and the images of the indoor point source correction MCAO and the solar ground layer adaptive optics(GLAO)were obtained.The real-time controller achieves a calculated delay of 116.2 ?s at the sampling frequency of 800 Hz,and the 0dB error bandwidth of the system is up to 30 Hz.The real-time controller is the first set of solar MCAO real-time controller based on FPGA and multi-core DSP,can run in three work modes including CAO,GLAO,and MCAO,free switching,and it has better scalability and compatibility,upgrade is also very convenient.The research of this subject has very important research value and practical engineering significance for the realization of solar MCAO system.