Research On The Shack-hartmann Wavefront Sensor For Dynamic Property Improvement

As an efficient wavefront sensor,Shack-Hartmann wavefront sensor(SHWFS)has been widely used in Optical Testing and Adaptive Optics.Different research areas propose different requirements for SHWFS.For the satellites tracking and imaging or the near ground extended targets high precision imaging through the adaptive optics systems,the brightness of the satellites is very weak and fast changing or the atmospheric turbulence may be strong,or the sensing target is extended that the centroid method is unavailable.In order to improve the dynamic property of SHWFS,different analyses are conducted in this study:1)Research on SHWFS sensed with the brightness changing dim targets.The Electronic Multiplication CCD(EMCCD)is applied in SHWFS and highly improves the capability to sense dim targets,for that the sensing signal can be multiplied and the readout noise can be decreased by setting a proper EM gain.When the brightness of the practical targets changes fast and in a great range during the sensing,setting a fixed EM gain is not the optimum.Auto-gain-control method of the EMCCD suited to SHWFS is designed in the study.The key work is to find a suitable control value,and the average of the maximum signal of each spot in the output spot array is chosen,which is confirmed to be stable under detecting noise and the influence of the disturbed wavefront,and in a linear relation with the brightness of the target and the set EM gain,basing on the exposure function of CCD.The control method is designed based on the linear relation,in which a goal control value is applied.The performance of the SHWFS with EMCCD is highly improved by the auto-gain-control method,with which the sensing Signal-to-Noise ratio is kept at the highest level under the sensing brightness,and the self-protection process of the EMCCD is avoided during the sensing process.2)Research on the method to sense more modes in each sub-aperture of the SHWFS.The property of the centroid based SHWFS is limited by its plane wavefront assumption,because the sampling sub-aperture should be small,resulting that only a little energy is collected in the focal spot and only two slopes can be sensed in each sub-aperture,which limits the sensing sensitivity and wavefront reconstruction precision.The modified Phase Diversity(PD)technique is applied in the sub-aperture as the wavefront estimation method.Through modification and optimization,large sub-aperture design can be realized and five modes are sensed in each sub-aperture.Four × four or six × six pixels are set in a sub-aperture,as it's confirmed that fewer pixels used,larger linearity range achieved,which is valuable for the real-time working SHWFS,and the linearity range for Z2 and Z3 modes is 3.0 RMS rad and the linearity range for the modes Z4 to Z6 is 2.0 RMS rad through optimizing the introduces focus.Totally,with PD applied in the sub-aperture,the sensing precision is improved and the reconstruction precision is improved,and only half sub-apertures of the traditional design are needed that the design of SHWFS is no more limited by the size of the sub-aperture.3)Research on the method of sensing with the extended scene target by SHWFS.In order to figure out the dislocation of the images of the extended target in the sub-apertures of SHWFS,the correlation method is applied and the dislocation is the position of the maximum of the correlation function of the testing image and the reference image.Both the sub-aperture parameters and the image content have effect on the sensing precision of the correlation method.The sub-aperture parameters are optimized in the simulation and the pixel number in the sub-aperture is set as 32×32 and the airy spot of the lenslet is set to be sampled by 2×2 pixels.The Fast Fourier Transform(FFT)is used to compute the correlation function and the obtained precision is high enough.Imaging process methods are applied to weak the influence of the imaging content,and the intensity transform algorithm are applied,which can weak the background of the imaging and increase the procession of the correlation method.4)Research on the correlation algorithm to calculate centroids for the dim spot targets.The correlation algorithm can work a modified centroid method in the ordinary spot based SHWFS,and is confirmed in the experiment to be of high precision and stable under detecting noise and disturbed wavefront.A correction and imaging experiments of a dim target of 5.5 magnitude through the adaptive optics system is conducted,and better performance is obtained than that by the centroid method.Besides,to realize real-time processing,Multi-CPUs procession is realized to speed up the correlation calculation.In conclusion,an auto-gain control method of the EMCCD in SHWFS is proposed for the brightness changing target sensing,and five modes sensing in each sub-aperture is realized to recover the limit of the small sub-aperture,and the correlation algorithm is applied to calculate the dislocation of the sub-images when extended targets are sensed.As the SHWFS is applied in different research areas,proper analysis methods and efficient modification methods are proposed for analysis in the study,and the dynamic property of the SHWFS is highly improved.