| Adaptive optic system is indispensable for high resolution imaging of large-aperture ground-based telescope.It can perform high-precision correction of optical wavefront distortion caused by atmospheric turbulence in real time,thus restoring the telescope's high resolution capability.In adaptive optics,high-precision wavefront detection is one of the core technologies that ensures its corrective performance.In traditional astronomical imaging,it is common to use brightness-measuring star targets as beacons for wavefront measurements to guide high-resolution imaging observations of dim targets in the isobaric range around them.This dependence on the bright target makes the traditional system only adaptively observe less than 1% of the sky area during astronomical observations.Thus,Foy and Labeyrie proposed the concept of laser guide stars in 1985.That is,using a laser to form a man-made beacon in the atmosphere of the target attachment to be measured for turbulence distortion measurements,thereby escaping severe reliance on natural bright targets.There are two techniques for the formation of laser guide stars: Rayleigh scattering off air molecules at moderate altitudes(10km~20km),and resonance scattering from sodium atoms at an altitude of about 90 km.Since the laser guide star is an artificial beacon,there is a big difference from the traditional guide star of the infinity.At present,laser guide stars also have the problems of low wavefront detection accuracy and low GLAO wavefront reconstruction accuracy.The low-precision detection of the laser guide star wavefront is mainly due to the fact that the laser guide star has a certain elongation,and there is high-order aberration in the Hartmann sub-aperture during the detection.The low reconstruction accuracy of the GLAO wavefront is mainly due to the fact that the laser guide star is located at a limited distance and with cone effect,making the traditional average method reconstruction information missing.In response to the above-mentioned issues,this paper carried out a series of innovative research work and achieved a series of research results.Aiming at the problem of laser guide star wave detection accuracy,a centroid detection algorithm based on maximum likelihood estimation method is proposed.While reducing the photon noise,the effects of high-order distortion in the sub-aperture are partially eliminated,and the accuracy of the centroid detection is improved.The advantage of the proposed algorithm over the traditional algorithm is more obvious with the increase of the laser guide star spot elongation and the signal-to-noise ratio.This conclusion has passed indoor verification experiments.This paper simulates a 20 m aperture telescope with SNR=20.In the case of side launched laser,40×40 subaperture Hartmann wavefront detectors are used to detect the accuracy of four algorithms.In the single Gaussian model,the RMS of residual wavefront in this paper is 67.12 nm,which is 12% lower than that of the center of gravity.Compared with the weighted center of gravity,the algorithm is reduced by 46% and 31% compared with the correlation algorithm.In the bi-model profile,the RMS of residual wavefront is 70 nm,a 10% reduction compared to the center of gravity,a 52% reduction compared to the weighted center of gravity,and a 38% reduction compared to the related algorithm.The proposed method would be highly beneficial for ground-based astronomical AO systems intended to provide moderate to high Strehl ratios at near infrared wavelengths on larger than 10m-aperture telescopes.In order to obtain the laser guide star spot,the algorithm proposed in this paper was supposed to be validated in the outside experiment.A laser guide star launch and receive system was designed.The launching system of the laser guide star is also affected by the atmospheric turbulence.After analyzing this effect,it is concluded that the optimal launch aperture of the laser guide star launch system is 3.8 times the atmospheric coherence constant.In order to ensure the light output capability of the launching system,the launch aperture is 3 times the radius of the laser spot at the exit pupil;at the same time,after considering the effect of atmospheric turbulence,the laser guide star size is approximately 1.67 times the size of the laser guide star without turbulence.The launching system of the laser guide star was finally designed.The waist height of the laser guide star was 10 km.After tolerance analysis,the launching system produced a guide star spot radius of 0.6arcsec and less than 1arcsec in 80% of cases.In order to ensure the height of the Rayleigh laser guide star and the sampling thickness,the pulse delayer is used as the system clock.After delaying 66.7 us after the laser pulse,the Pockels box is controlled to open the switch,and the opening time is 6.7 us.For the problem of missing information of multiple laser guide star wavefront reconstruction in GLAO,a weighted reconstruction algorithm was used.The multiple laser guide star weighted reconstruction algorithm allocates different weight factors for each laser guide star based on the position of the laser guide star,which compensates for the lack of probe information brought by the laser guide star cone effect.The weight factor in the GLAO mode of the large field of view is the average of the weighting factors in each direction within the field of view.The wavefront reconstruction accuracy of the weighting algorithm is improved compared to the traditional averaging method.The system Strehl ratio(SR)obtained by the weighting algorithm decreases with the increase of the field of view,and compared with the average method,the SR is improved about 0.2 at the center of the field of view.When the field of view is 2arcmin,the weighting algorithm has higher reconstruction accuracy than the average method.In order to verify the multi-guided weighted reconstruction algorithm,a multiple guide star adaptive optical system was designed.The experimental results show that the SR in the central field of view system is approximately 0.42,which is approximately 0.22 compared to the conventional average method.The system SR at the center field of view has increased by 0.2,which is consistent with the simulation results.For the first time,the weighted wavefront reconstruction method effectively decreases the reconstruction error in multi-LGSs AO system.The simulation and experimental result show that weighted method is also promising to improve the imaging resolution of LGS adaptive optics systems in astronomical observations.The laser guide star wavefront detection and reconstrution algorithm proposed in this paper and the above conclusions solve the problems,encountered in the application process of laser guide star adaptive optics.It will play a role in promoting the development of laser guide star technology. |
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