Correction Of Misalignment Errors For Large Aperture Thin Film Splicing Telescopes

Increasing the aperture of a telescope can improve the image resolution and light collecting ability of a telescope.Ever since Galileo invented the first telescope in 1609,increasing the size of the telescope has been a constant pursuit.Due to the limite of processing capacity,the diameters of the largest space-based telescope and groundbased telescope are respectively 2.4 meters and 8.4 meters currently.Increasing the size of the telescope has become an important problem for further exploration of outer space.The sub-aperture splicing technique has attracted wide attention since it was proposed.By splicing several small-caliber sub-mirrors to form a large-caliber primary mirror,there is no need to consider the difficulty of machining the large-caliber primary mirror.At present,many reflective splicing telescopes have been put into use at home and abroad.However,the increase of aperture makes it difficult to ignore the problems of high density and close tolerance.The thin film diffraction element,taking advantage of loose surface tolerance,small volume as well as light weight,has been used as the main mirror in the telescope system at home and abroad nowadays.At present,the maximum aperture of the thin film diffraction element can not reach the required magnitude of the large aperture telescope,so the thin film splicing telescope combined with the sub-aperture splicing technology is the mainstream of the next generation of large aperture telescopes.The misalignment error is the deviation of the sub-mirror from the ideal position in the process of misalignment.The splicing system cannot achieve the effect of the single-sided mirror with the same caliber as the result of the existence of misalignment error,which make it difficult for the focus of the submirror to coincide and the phase of the focal plane to maintain the same.Therefore,the correction of misalignment error is the key problem to be solved in the application of splicing mirror.According to the literature,the correction process of misalignment error of splicing mirror is mainly as follows: first the wavefront is detected,then the misalignment error is obtained by wavefront detector,and finally the error is corrected by mechanical structure.The correction of misalignment error of splicing thin film is based on the correction method of splicing mirror.The misalignment error of splicing thin film is detected by means of interference or beam autocollimation.At present,the method of misalignment error detection reported in the literature is not suitable for online correction of misalignment error of the splicing mirror with large primary mirror.The blind optimization algorithm does not need to detect the misalignment error,and directly guides the control device to correct the misalignment error of the submirror through the information such as farfield light spot.In order to realize the on-line correction of the misalignment of largediameter film splicing mirrors,the blind optimization algorithm is used to simulate the on-line correction of the misalignment of splicing mirrors.In this paper,the following research was carried out.Firstly,the model of largediameter splicing thin film mirror was established and the effect of misalignment error on the performance of splicing thin film mirror system was analyzed.The random parallel gradient descent algorithm and simulated annealing algorithm are used to carry out online correction simulation research on the splicing error and the advantages and disadvantages of the two algorithms are analyzed.According to the problem of slow correction speed of simulated annealing algorithm,an adaptive simulated annealing algorithm is proposed to improve the speed of correction of misalignment error.A hybrid algorithm based on the improved simulated annealing algorithm and the stochastic parallel gradient descent algorithm can further improve the correction speed of the misalignment error of the large-aperture film mirror system.The main research work of this paper includes:(1)The research background and significance of this article has been introduced.By analyzing the research status of misalignment error detection methods of splicing mirror and splicing film mirror at home and abroad,an online correction scheme for misalignment error of mosaics thin film mirror based on blind optimization algorithm is proposed.(2)The splicing model of Fresnel band with five submirrors is established.The point spread function and modulation transfer function of the splicing film mirror imaging system under different misalignment errors are analyzed.The relation between the optical path difference introduced by different misalignment errors and the F number of the splicing mirror are calculated.(3)In order to verify the feasibility of blind optimization algorithm for online correction of misalignment error of splicing thin film mirror,the stochastic parallel gradient descent algorithm and simulated annealing algorithm are used.Simulation results show that the random parallel gradient descent algorithm can correct the misalignment error with RMS value under 0.72λ quickly as well as accurately.When the RMS value of the wavefront is greater than 0.72λ,the algorithm can easily fall into the local extreme value,and the correction accuracy drops,failing to achieve the imaging effect required by the splicing thin-film mirror system.The simulated annealing algorithm can converge with the wavefront RMS value greater than 0.72λ,but the correction speed is slow.(4)In order to improve the optimization speed of simulated annealing algorithm,an adaptive perturbation simulated annealing algorithm is proposed.According to the established splicing model of thin-film mirror,this algorithm sets up the threshold for determining the misalignment of the sub-mirror.In the process of correcting the misalignment error,the disturbance quantity is changed adapatively by optimizing the size of the index and threshold.The hybrid algorithm based on the improved simulated annealing algorithm and the random parallel gradient descent algorithm has a higher convergence rate.The simulation results show that the convergence rate of the improved simulated annealing algorithm is nearly double that of the traditional simulated annealing algorithm under the same misalignment error and iteration termination condition.Compared with the improved simulated annealing algorithm,the hybrid algorithm can further improve the correction speed of misalignment errors.