Using Multi-field Measurements To Eliminate Alignment Degeneracies Of Secondary Mirror

Large optical system with a segmented primary mirror,whether it is a ground-based telescope or a remote space sensing system,because of the influence of the ground-alignment or external environmental factors(such as vibration,temperature,gravity and air pressure),the secondary mirror of the system will be deviated from the original positon,leading to the deterioration of the imaging quality,so the alignment degeneracies of secondary mirror need to be detected and corrected.At present,sensitivity matrix inversion method or wavefront sensorless adaptive optical correction method are used to correct the secondary mirror position errors.However,both of them will fail when they are applied in the case where the primary mirror has not a center mirror used as the base mirror,in order to settle this problem,in this paper,a method of calculating the position misalignment of the secondary mirror based on multi-field wavefront sensor information is present,which is of great significance and provides a new research idea to eliminate alignment degeneracies of secondary mirror.First of all,the traditional sensitivity matrix inversion method and wavefront sensorless adaptive optical correction method are introduced.The applicable conditions of the two methods are analyzed and the limitations of their uses are pointed out.Secondly,we take the 36-segmented primary mirror optical system without a center mirror as an example and simulate the influence of secondary mirror misalignment on system image quality.And then we present the theory of multi-field measurement,which is the theoretical support for the follow-up research work.Thirdly,based on ZEMAX and MATLAB,we choose different field points to build a mathematical model to obtain the misalignment of secondary mirror by means of the field-dependent errors which can be got by multi-field wavefront sensing and study the detection accuracy and dynamic range of multi-field measurement method.The results show that the detection range of the misalignment of secondary mirror on XY-translation and XY-tilt are 0-1.5 mm and 0-0.03o respectively.And the detection accuracy of the misalignment of secondary mirror on XY-translation and XY-tilt reach30 nm and 15" respectively.And a lot of simulations based on the real space-to-ground optical remote sensing system are done to prove the feasibility of the proposed method.Finally,we analyze the influence of wavefront sensing error,the number of field points and the number of primary mirror segments on the detection accuracy The results show that when the wavefront sensing error is ?/40(?=632.8 nm),the detection accuracy of multi-field measurement is basically unaffected,while the wavefront sensing error is ?/2(?=632.8 nm),the detection accuracy of the misalignment of secondary mirror on XY-translation and XY-tilt are decreased by65 nm and 1".The more the number of field points,the higher the detection accuracy of multi-field measurement.When the number of field points increases from 7 to 12,the detection accuracy of the misalignment of secondary mirror on XY-translational and XY-tilt are improved by about 10 nm and 1" respectively.At the same time,the segment number of primary mirror will affect the detection accuracy of multi-field measurement.For the same aperture diameter,when the segment number is increased from 6 to 36,the detection accuracy of the misalignment of secondary mirror on XY-translation and XY-tilt are improved by about 80 nm and 1 ".