Method Of Segmented Mirror Co-phasing Based On Dispersed Fringe Sensing Technology

For better observing resolution,the developing trend of space imaging optical system is toward long focal length and large aperture size.However,with the existing technologies,the enlarging diameter of single primary mirror telescopes brings great difficulties in the mirror processing and testing,the support structure,and the subsequent operation and maintenance.The proposal of the segmented primary mirror breaks through the traditional concept of full aperture by breaking up the whole optical system into parts.This ensures the imaging quality while minimizing the weight and cost of the telescope effectively.At the same time,it points out a new direction for the manufacture of large and extremely large space telescopes.However,a series of technical challenges come along.And the most difficult problem,which is also most critical,is the measurement of co-phasing error of the segmented primary mirror.It requests that all the sub-mirrors have the same focus and phase in order to make the telescope system imaging quality at or near the diffraction limit.In fact,coarse alignment is easy to achieve,the difficulty lies in co-phasing.Traditional wavefront detection methods such as shear interferometry,SharkHartmann wavefront sensor,curvature sensor,etc,are difficult to be used for measuring the piston error between sub-mirrors due to their complicated structure and insensitivity to piston error.Though phase diversity method can measure the piston error with high accuracy,the detection range is very limited.So it is unsuitable for the detection of large piston error.Different from these standard wavefront detection methods,dispersed fringe sensor(DFS)realizes non-contact relative piston measurement between neighboring segments from the optical energy distribution information of the broadband interference fringes.It is a piston sensor with operating convenience,high automation,small volume,low cost,large range and strong anti-interference capacity,which is especially suitable for space telescope in orbit detection.In this paper,study on method of segmented mirror co-phasing based on dispersed fringe sensing technology is carried out.According to the principle,an optical imaging model of dispersed fringe sensor is introduced,and simulation study for the whole process is carried out on computer system.Two other auxiliary schemes: Dispersed Hartmann Sensing(DHS)and dispersed template matching(DTM)are brought out to solve the problem that DFS loses efficacy under the near phasing condition,and also verified through feasibility simulations.By combining these methods,piston measurement accuracy better than ?10 in visible waveband phasing system in the whole range covering ±60?m is achieved.Besides,the impacts of various factors on the detection accuracy have been analyzed in quantity,including Hartmann lens positioning error,segment aperture orientation,and uncertainty of the signal extraction axis line.Maximum permissible level and resistant capacity to system error can be determined by testing various degrees of error with DFS,DHS and DTM.In particular,solutions such as multi-trace and more accurate wavelength calibration are put forward to solve the problem brought by uncertainty of the signal extraction axis line.And an advanced algorithm is also presented to make the method less insensitive to small calibration errors by using the information of the fitted phase term.Finally,a dispersed fringe sensing system is designed and an optical experimental platform is built to verify the effectiveness of this approach.The results show that the proposed method can detect piston error with high precision and no blind spots in an extremely wide range,and can be widely applied in the coarse phasing calibration and phasing control for space and ground based segmented mirror telescopes.