Wavefront Sensing Problems Of The Sodium Laser Guide Star Adaptive Optics System

Adaptive optics system(AOS)has been an essential part for large aperture ground based astronomical telescopes.The use of artificial sodium laser guide stars(SLGS)can greatly increase the sky coverage of the astronomical AOS.However,SLGS usually has a low brightness and elongated spot,and has anisoplanatism wavefront error when used for wavefront sensing.Aiming at these problems,combined with the demands of the SLGS AOS on Lijiang 1.8-meter Telescope when using SLGS for wavefront sensing,content of this dissertation is as follows.Based on the physics of the sodium atoms,a numerical simulation method for predicting the coupling efficiency between the laser and sodium atoms is presented.Coupling efficiency for the Technical Institute of Physics and Chemistry(TIPC)sodium laser is simulated.Influences of polarization status,pulse oscillations,laser spectrum,and D2a+D2brepumping on the coupling efficiency are analyzed.Simulation results indicate that for a given sodium laser,there is a best power intensity,where the value of coupling efficiency will be the largest at this power intensity.Also,advises of upgrading the sodium laser are given through the simulation results.A photometry system of SLGS is built on the Lijiang 1.8-meter Telescope,to measure the brightness of the SLGS and coupling efficiency of the laser.With this photometry system,spot size and returned photons of the SLGS are optimized.Also,on sky-test results verified the enhancement of the coupling efficiency while using D2a+D2brepumping to excite the sodium atoms.Anisoplanatism in SLGS wavefront sensing is modeled using several random phase screens.By setting several SLGS and natural guide stars(NGS)at the same time,angular anisoplanatism and height anisoplanatism are simulated simultaneously.Based on the idea of measuring the wavefront of several SLGS and NGS synchronously,an anisoplanatism measurement system is designed,and experiments will be taken in the near future to validate the model.In this dissertation,numerical simulation and experimental results on the brightness of the SLGS and coupling efficiency of the laser are presented.Also,anisoplanatism of the SLGS is analyzed.All the researches above are essential part for wavefront sensing in SLGS AOS.And the work lays the foundation for the first time in China,SLGS AOS obtained high-resolution closeloop images.This work also provides suggestions for the future upgrading of the SLGS AOS.