Study On Scene-based Correlating Shack-hartmann Wavefront Sensing Method

Shack-Hartmann wavefront sensor,as a wavefront phase measurement device,is widely used in adaptive optics,active optics,image restoration and other fields due to its superior performance.In 1998,the introduction and successful application of the correlating Shack-Hartmann wavefront sensor made the solar adaptive optics system practical,and promoted the wavefront sensing of ground scenery or space extended targets.For the aberrated wavefront detection based on extended scene,the contrast of extended scene,the feature location of the extended scene will affect the wavefront sensing accuracy of the correlating Shack-Hartmann wavefront sensor.In order to improve the performance of the correlating Shack-Hartmann wavefront sensor in wavefront sensing based on extended scene,the following studies have been carried out and a series of research results have been achieved.The correlating Hartmann wavefront sensor estimates the sub-aperture image shifts by calculating the peak value of the cross-correlation function between the subaperture image and the reference image.When the contrast of the extended target is low,the cross-correlation function value,which based on the product summation of gray values between the sub-aperture images,can no longer accurately reflect the similarity between the images,and the error of shift estimated via the cross-correlation function value is large.To solve this problem,a shift estimation algorithm based on Gamma transform for reference sub-aperture image is proposed.In the simulation analysis,it is shown that the application of gamma transform correlation algorithm with suitable gamma value can greatly improve the subaperture shift estimation accuracy.For RMS=6.86 sunspot-like low contrast extended target,the subaperture shift estimation error can be reduced by 88.12%.The choice of the best gamma value is affected by the sub-aperture resolution.With the decrease of the sub-aperture resolution,the effect of improving the shift estimation accuracy by changing the gamma value is limited.The anti-noise ability of gamma transform correlation algorithm is weak,and for PSNR=43dB subaperture image,the application of it will increase the shift estimation error,and at this time,using high PSNR image as reference can make the wavefront sensing result of the algorithm reach the same accuracy as that of high PSNR.In the wavefront reconstruction experiment,?=1.5 gamma transform correlation algorithm is applied to the sub-aperture image with the brightest feature part and darker background part,and the reconstruction error is reduced by 22.86%,which shows that the shift estimation accuracy can be improved by choosing the appropriate gamma transform correlation algorithm according to the characteristics of the target.The calculation of cross-correlation between subaperture images by FFT is a widely used method at present,but it needs to periodically extend the finite image to infinite image,so that the correlation value reflects the similarity between reference image and periodic extension image.When the feature of extended object is adjacent to the edge of field of view,the shift estimation error will be very large.To solve this problem,a shift estimation algorithm based on gradient cross-correlation is proposed.The simulation results of single sub-aperture image shift estimation and wavefront reconstruction show that the gradient cross-correlation algorithm enabled the shift estimation with high precision,and eliminated the error reconstruction mode caused by the wraparound effect.In the wavefront reconstruction experiment,it is verified that gradient cross-correlation algorithm can improve the wavefront sensing accuracy for extended object which contains feature at edges of field of view,as the residual of wavefront reconstruction can be reduced by 28.1% averagely by using gradient crosscorrelation algorithm.Close loop AO correction also showed considerable improvement in image resolution by employing the gradient cross-correlation algorithm.The results of this study improve the sensing capability of the correlating ShackHartmann wavefront sensor,make the correlating Shack-Hartmann wavefront sensor have excellent performance in the wavefront sensing of a variety of extended targets,and broaden the application scope of the scene-based correlating Shack-Hartmann wavefront sensor.