Study On Computer Aided Alignment Technology For Off-axis Optical System

With the development of optical design and optical manufacture technology，thereflective optical system style have been development from axial symmetry tooff-axis or eccentric. The assemble of off-axis optical system is absolutelynecessarily to achieve the diffraction-limited system image quality, and thehigh-powered mount off-axis system are more and more depended on high tech.Computer–aided alignment (CAA) technology has been developed to solve thedifficulties of high quality optical system alignment in recent years, which play thekey role in the process of system development, and the development of CAA willpromote the optical design and optical manufacture.The theory of CAA technology and some issues about its application in off-axissystem are studied in this paper, the main research contents are as follows:1. The misalignment induced aberrations of symmetry optical systemUsing the vector aberration theory, we analyzed the third order aberration fieldsof misaligned symmetry optical system, the relationship between perturbed systemwavefront error Zernike coefficients and system field have been found. Themounting process of symmetry RC and TMA is analyzed by vector aberration theory,the characteristic of3rdspherical aberration and3rdcoma and3rdastigmatism ofmisaligned RC TMA is obtained, the reason that the alignment of optical systemcannot be accomplished using on-axis performance data alone is analyzed. Thedifferent element misaligned induced aberration compensate is studied and theconclusion guided the process of optical design.2. The misalignment induced aberrations of off-axis optical systemBy introducing the coordinates transform, the relationship between axialsymmetry system and off-axis system wavefront error Zernike coefficients have beenfound, and using a transformed pupil vector into the aberration expansions of ansymmetry optical system, the vector aberration expansions through third order ofoff-axis optical system are obtained. The result show that the off-axis and misalignedoff-axis optical system’s aberrations are still make up of3rdspherical aberration and3rdcoma and3rdastigmatism, different aberration have different field dependencies.By the vector aberration expansions of off-axis system, the characteristic of3rd spherical aberration and3rdcoma and3rdastigmatism of misaligned off-axis RC andoff-axis TMA is obtained. Based on the coordinates transform, the influence ofsurface-profile error of larger mirror on aberrations characteristics of optical systemis gained. The aberrations compensate relation between element misaligned inducedand surface-profile error induced is studied and the conclusion guided the process ofoptical design based on free form surface.3. The analysis misalignment of off-axis optical system’s elementAccording to the demand for the alignment of off-axis three-mirror reflectiveoptical system in an engineering project, the misaligned model of co-axis opticalsystem and off-axis optical system is build by vector aberration theory. Thealignment process is simulate at different misaligned magnitude, the result show thatthe adjust time increased with the misaligned magnitude. It may be getting a worseassemble result by CAA arithmetic at biggish misalignment, the reason is analyzedby the vector aberration theory.4. The calculation misalignment of a real off-axis optical system’s elementA singular value decomposition method is applied to the CAA arithmetic, thecausation of poor solution of the alignment is analyzed by the matrix conditionnumber and the singular value, the threshold of singular value is proposed to achievethe correct result. The coarse alignment process is introduced, the stimulantcalculation is processed to an off-axis three-mirror reflective optical system underthe different signal noise ratio and the result shows that the method in this paper isrational and effective and steady, and the off-axis three-mirror reflective opticalsystem is well aligned by this method, the off-axis system produced a measuredwavefront error less than0.09λ RMS at λ equal to632.8nm across all image plane.