Study On Optical Technology Of Wavefront Sensor For Rayleigh Laser Guide Stars

Adaptive optical imaging technique can recover the high resolution imaging capability of the telescope to the spatial object by real-time detection and correction of the wavefront aberration caused by atmospheric turbulence. But the observed target is often too faint to provide the enough energy.for adaptive optics system to detect the wavefront, which limits the image resolution of the telescope, the artificial laser guide star technology is proposed to improve the detection ability of the adaptive optics system. Becasue the Rayleigh guide star is relatively easy to achieve and the laser has been commercially available products, this paper is focused on the study of the adaptive optical technology for Rayleigh laser guide star.The laser launchsystem is projected the pulse laser to direction of the observed target, and laser pulse forms Rayleigh guide star at 10~20km. The principle of Rayleigh guide star adaptive optics system is that the received Rayleigh backscattering light is deteced as wavefront distortion signal for adaptive optics system. Currently, the main problems as following: 1) Rayleigh guide star scattering layer sampling thickness is generally limited to 1~2km, leading to the outside of the scope of this light energy can’t be received, and Rayleigh guide star wavefront detection energy is relatively low; 2) Because Rayleigh laser guide staris placed a finite distance from the telescope leading to forming the cone optical channel unlike the cylindrical optical channel of the infinite observed target at, and the wavefront sensor, a portion of the atmosphere remains unsensed resulting in the larger wavefront sensing error called “focus anisoplanatism”. 3) Rayleigh guide star are not used for sensing overall tip-tilt information of telescope aperture.The dynamic refocus system was design to increase the wavefront detection energy by maintain focus ideally while the laser pulse travels through at the atmophere over an extended height range. The dynamic refocus system can achieve the ideal imaging for the sample thickness range of 10 km, and the brightness of the Rayleigh laser guide star at the height of 13 km reaches 2.4th magnitude. Comaparing to the sample thickness range of 1km, the wavefront detection energy is improved with a factor of 6.5 theoretically.Five laser guide stars were used to reduce focus anisoplanatism errors to improve the wavefront detection accuracy. The focus anisoplanatism error will drop about 34% for a 10-meter telescope using five Rayleigh guide star located at 10 km. Accordingly, the use of a collimator lens array and a telecentric optical system was proposesd to achieve the multi-guide star wavefront detection system.The wavelength band of 400nm~600nm of the observed target was used to detect the tip-tilt of the pupil wavefront. The wavefront tip-tilt residual must be less than 0.0266" for the 1.23 m telescope observation target to realize twice the diffraction limit imaging with the high-order correction residual being 1 rad. Thus the brightness of observation target of 9th magnitude can meet the target detection requirement. It demonstrated that using Rayleigh guide star improves the limit observation magnitude of the adaptive optics system from 5 to 9.Finally, design a Rayleigh guide star adaptive optics system to solve the problem of Rayleigh guide star defocus limited the height of the telescope imaging focus and infinity focus generated imaging. In order to improve the correction accuracy of the system, according to the working mechanism of the liquid crystal wavefront correction(LCWFC), the device errors caused by the limited drive voltages and gray levels were analyzed and optimizated; then, propose a novel method to improve interaction matrix measurement accuracy, by tuning the Zernike polynomial coefficient for specific mode so that the quantization level reached to 10 for each Zernike wavefront mode for the 256×256 pixels of LCWFC; Meanwhile, the least square method was used to eliminate random noise in the measurement process. The method with respect to the conventional method, the power spectrum of the corrected image significantly increased, part of the band increase 2 to 3 times.This investigation is the pioneering work of Rayleigh guide star adaptive optics system.With the improvement of the wavefront detection energy, wavefront detection accuracy and wavefront correction precision, it demonstrates the Rayleigh guide star can be applied onto large-diameter telescope adaptive optics system potentially. The research results of this deissertation make a contribution for the applications of Rayleigh laser guide star adaptive optics system.