Study On High-Low Order Adaptive Optics System Based On Liquid Crystal Wavefront Corrector And Deformable Mirror

Adaptive optics systems(AOSs)are essential equipments for the observation of large aperture telescopes,they can detect and compensate the wavefront distortion caused by the atmospheric turbulence,and improve the imaging resolution of the telescopes.However,with the enlarging of the telescope's aperture,the spatial resolution of the wavefront corrector is also put forward higher requirement.Due to the limitation of the manufacturing process,the actuator number of the deformable mirror(DM)is difficult to be improved.For this reason,most of the deformable mirrors work in the infrared band that the turbulence intensity is weaker.The liquid crystal wavefront corrector(LCWFC)has the unique advantage with high pixel density,which makes it could be applied in the visible band.However,due to the dispersion of the liquid crystal material,the working waveband is difficult to extend to infrared waveband.Therefore,this paper proposes a new AOS that combines the two wavefront correctors to improve the imaging waveband.Firstly,the relationship between the intensity of the atmospheric turbulence and the wavelength is analyzed,it is concluded that the long waveband and the short waveband should have the same wavefront distortion.However,due to the diffraction limit resolution of the long waveband is lower than the short waveband,thus the effects can be ignored when the high frequency distortion of the turbulence aberration is smaller than the diffraction limit of long waveband.A set of high-low order AOS based on LCWFC and DM system is designed for the 2m telescope,a 97 actuator deformable mirror and a 256x256 pixel liquid crystal wavefront corrector are used in the system,the imaging waveband is 700-1700 nm.Firstly,the low order wavefront distortions are corrected by the DM,and the wavefront distortions of 950-1700 nm infrared waveband are eliminated.Then the beam can be splitted by a 950 nm filter,which makes 950-1700 nm waveband pass the filter and focus on the infrared camera,700-950 nm waveband is reflected and goes into the LCWFC,and then is reflected and imaged by a visible camera.The distribution of the Zernike modes for the DM and the LCWFC is analyzed.Due to the coherence length of atmospheric turbulence increases as the 6/5 power of the wavelength,700 nm,1000nm and 1500 nm are selected as the short wavelength of the three wavebands,respectively.The coherence length of atmospheric turbulence at 700 nm is 10.0cm,and the coherence length of atmospheric turbulence of another two wavebands are 15.3cm and 24.9cm,when the RMS value of the wavefront error is equal to 1rad,the number of the required Zernike modes is 77,34 and 14 respectively.According to a 2 meter telescope,the coherence length of atmospheric turbulence at 700 nm is 10.0cm,the K-L mode expansion method is used to generate one hundred random turbulence aberrations of Zernike polynomials.When the first 29 Zernike modes have been corrected by the DM,the RMS value of the residual wavefront is 0.1?m,and the coefficients of the first 29 Zernike modes are less than 0.01?m,the high order Zernike coefficients is about 0.05?m.1500-1700 nm waveband can reach the diffraction limited resolution,and 950-1500 nm waveband can reach 1.2 times diffraction limited resolution.Then the 30-77 Zernike modes are corrected by the LCWFC,the RMS value of the residual wavefront can be reduced to 0.03?m,after the two corrections,the coefficients of the first 77 Zernike modes are less than 0.01?m,and 700-950 nm waveband can reach the diffraction limited resolution.Since the system contains two correctors,in order to simplify the complexity of the optical path and reduce the chromatic aberration,the influence of the wavefront correction on the non conjugate positions has been analyzed.When the tip tilt mirror on the non conjugate position,the errors caused by the spot movement and beam deflection are analyzed.To the 2m telescope system,in order to ensure the jitter of the beam on the Hartmann detector is less than 1/10 subaperture,the maximum deviation distance from the conjugate position is 20 mm.When the DM on the non conjugate positions,the change rule of the wavefront aberration when it propagates in different aperture and the correction error of the off-axis field are discussed.To the 2m telescope system,when the aperture of the DM is 20 mm and the deviation is less than 250 mm,the variation of the wavefront is less than 1/14 wavelength.And when the deviation is less than 160 mm,the correction error of 5 seconds can be ignored.A set of high-low order AOS based on LCWFC and DM is designed by using optical design software for 2 m telescope.DM is operated with closed-loop control,and LCWFC is operated with open-loop control.The system uses 400-700 nm for wavefront detection,and 700-1700 nm for imaging.The system aberration can reach the diffraction limit.According to the designed optical system,the experimental light path is built,the dynamic characteristics of the DM and the LCWFC are tested,the-3dB rejection frequencies of the two correctors are 88 Hz and 75 Hz respectively,which could work at a Greenwood frequency of 50 Hz.A turbulence simulator is used to do the experiment,and to evaluate the effects of the correction,a resolution target is chosen as the object,with the correction,the waveband of 1500-1700 nm could reach the diffraction limited resolution;the waveband of 950-1500 nm could reach 1.2 times diffraction limited resolution;and the waveband of 950-1500 nm could achieve 1.1 times diffraction limited resolution,the experiment results and simulations are in good agreement.This thesis is a pioneering work of the DM/LCWFC AOS,it will promote the development of the AOSs for broad imaging waveband.