| Adaptive optics is currently an indispensable technology to solve the interference of atmospheric turbulence and restore the optical diffraction limit resolution imaging of ground-based telescopes.However,the core of the system,the deformable mirror wavefront corrector,is limited by the mechanical manufacturing process,and the number of drive units is difficult to exceed 200.Once it exceeds,the cost will increase nonlinearly,which will make the wavefront adaptive correction imaging of large-aperture telescopes above 2 meters in the visible light band a very difficult problem.In recent years,the liquid crystal wavefront corrector for electronic components using high-density pixels as the driving unit can easily achieve the wavefront correction density of a thousand-unit deformable mirror,and the cost is lower than the cost of a 200-unit deformable mirror.It becomes adaptive optics A promising new star in the field.However,due to the slow response speed of the liquid crystal wavefront corrector in the long infrared waveband and the serious dispersion in the short waveband of visible light,its working waveband is very narrow,generally in the 700nm-950 nm waveband,which is difficult to meet the needs of multi-band adaptive correction imaging.This study is based on the rule that atmospheric turbulence is severe in the short waveband and greatly reduced in the long waveband,combined with the advantages of the deformable mirror and the liquid crystal corrector to form a dual corrector cascade adaptive system.The former corrects the low-order wavefront distortion in the long waveband,and the latter corrects the high-order wavefront distortion in the short waveband.It can solve the problem of wide-band adaptive correction imaging for large-aperture telescopes over 2 meters and the wavelength after 700 nm,and the cost is low.More importantly,the cascade system can shift the working short-wavelength limit of the deformable mirror by 250 nm,and the energy of solar spectrum is much stronger than the energy of its infrared spectrum,which is an extremely important imaging detection band.At present,there are two main problems in the system: 1.Limited by the structural parameters of the deformable mirror itself,it cannot accurately fit the Zernike mode,which affects the accuracy of low-order aberration correction;2.The cascade system is in a dynamic turbulent flow.The work effect is not reported.In response to the above problems,this research conducted an in-depth study on the working mode of the cascade system and its performance in dynamic turbulence.First,a deformable mirror-liquid crystal corrector cascaded adaptive optics experimental system for a 2m telescope was designed and built: the incident light interfered by the turbulence simulator is a broad-spectrum beam in the 400-1700 nm band,and a 145-unit continuous surface deformable mirror and The 256x256 pixel liquid crystal corrector works in series,the working wavelength of the Hartmann wavefront detector is 400-700 nm,three cameras can form three-channel spectral imaging in the 700-1700 nm waveband;the deformable mirror is located at the front end of the optical path,and the Hartmann wave The front detectors are connected in series to form a closed-loop control system;the liquid crystal corrector is located in the rear optical path and parallel to the Hartmann wavefront detector optical path to form an open-loop control system;the incident full-wavelength beam first passes through a deformable mirror to correct low-order aberrations to make 950-The wavefront aberration of the 1700 nm infrared waveband is basically eliminated,and then the beam is split through the 700 nm high-pass dichroic plate,and the 400-700 nm waveband beam with the most comprehensive aberration information enters the Hartmann wavefront detector to detect the deformed mirror after correction Residual low-order distortion residuals and high-order distortion aberrations in the wavefront,and these two signals are fed back to the deformable mirror in the front optical path,and the liquid crystal corrector in the rear optical path is fed forward;in addition,with the Hartmann wavefront The 700-1700 nm band beam separated by the optical path of the detector is divided into two wavelength bands 700-950 nm and 950-1700 nm by a 950 nm high-pass dichroic.Among them,the 950-1700 nm long wavelength band,which has basically no aberration,directly enters the two infrared The cameras image separately,and the 700-950 nm short-wavelength band enters the liquid crystal corrector,and the remaining high-order aberrations are corrected by the feed-forward signal of the Hartmann wavefront detector.This waveband is exactly the non-liquid crystal corrector.Dispersion band,in view of the high-precision correction advantage of the liquid crystal corrector,even if it forms an open-loop control system with the Hartmann wavefront detector,the 700-950 nm short-wavelength beam after the secondary correction can still show the high diffraction limit in the camera.Resolution imaging.The above-mentioned optical path involves the reasonable allocation of high and low-order aberrations between the deformable mirror and the liquid crystal corrector.Using the Zernike mode to divide the high and low-order aberrations is the simplest and most feasible method.However,the simulation analysis of this research shows that the deformable mirror is effective for Zernike.The model has a fitting error,and the higher the order,the larger the fitting error,which will affect the imaging quality of the infrared band,and will also deteriorate the image after the secondary correction of the liquid crystal corrector.To solve this problem,the eigenmode is constructed according to the response matrix of the deformable mirror,and the eigenmode is used to decompose the distorted wavefront and sort the high and low-order aberrations.The low-order aberration is performed by the deformable mirror using the eigenmode method.Correction,the remaining high-order aberrations are reconstructed in Zernike mode and then handed over to the liquid crystal corrector for correction.Because the response matrix of the liquid crystal corrector is difficult to construct using the intrinsic mode of the deformable mirror,the liquid crystal corrector in this system is still Work with Zernike model method.The distribution of high and low order aberrations according to the above method does not require decoupling operation,which greatly simplifies the dual corrector control problem in the cascade system.After using the eigenmodes of the deformable mirror to sort the high and low-order aberrations of the distortion wavefront,the number of eigenmodes allocated to the deformable mirror needs to be strictly demonstrated.In this study,by simulating the correction process of the deformable mirror under different conditions,the relationship between the deformable mirror correction residual RMS and the number of eigenmode correction items N and the turbulence intensity D/r0 were obtained: residual RMS and N,D/r0 Respectively,they are-7/9(when N>10)and 5/6 exponential relationship,and the correctness of this relationship is verified through experiments.According to the relationship between the wavefront residual and the imaging resolution,it is finally concluded that when D is 2 meters in diameter and the atmospheric coherence length r0=10cm@?=550nm,the deformable mirror needs to correct at least 55 eigenmodes to obtain the 950-1700 nm band Diffraction-limited resolution imaging.In order to verify the correction imaging performance of the dynamic turbulence correction of the deformable mirror-liquid crystal corrector cascade adaptive system,a cascaded adaptive system was built,and the USAF-1951 standard resolution plate was used as the imaging object;the deformable mirror and liquid crystal correction in the system were measured The error suppression-3d B bandwidth of the detector is 89 Hz and 77 Hz respectively;the incident beam is passed through the turbulence simulator of the Near-Index-Match TM phase plate,and the diameter D=2m and r0=10cm@?=550nm.The experimental results of adaptive optics imaging show that under static turbulence,after the 55 eigenmodes are corrected by the deformable mirror,the infrared 950-1500 nm and 1500-1700 nm bands reach 1.1 times and 1 times the diffraction limit resolution imaging,700-950 nm short-wavelength band,under the coordinated correction of the deformable mirror and the liquid crystal corrector,also obtains 1x diffraction limit resolution imaging,indicating that the basic design and the allocation of high and low order mode aberrations are reasonable;under dynamic turbulence,this level The connected system can cope with the turbulence change speed of f G=45Hz in the 950-1500 nm and 1500-1700 nm bands,and can cope with the turbulence change speed of Greenwood frequency f G=37Hz in the 700-950 nm band.This is a result that has not been reported in the world.Through the above research,the cooperative wavefront correction capabilities of the two correctors have been brought into play,laying a foundation for the engineering application of the cascade adaptive system of deformable mirrors and liquid crystal correctors.The direction of this research is the pioneering work of the research laboratory.So far,no other research team has reported similar work in the world.It is hoped that the results of this research can effectively promote the development of low-cost adaptive optics systems. |
PDF Full Text Request