Study Of Reflective Telescopes Alignment Containing Freeform Surfaces

Freeform optical surface, which is non-rotationally symmetric optical surface, isnow more widely used in the field of optical engineering. Expressed by differentforms, it can extend the optical design freedom to achieve the design of wide field ofview or large relative aperture optical system or other purposes. However, due to thetechnical problems of freeform surfaces in the design, machining, testing andalignment processes, now it’s not yet widely used in high-resolution reflectivetelescopes.Because of the high-precision request, currently complex reflective telescopealignment usually takes use of computer-aided alignment. Computer-aided alignmentmeans mathematical connections will be established between the imaging patternparameters and disalignment amount of the optical system, by measuring theimaging pattern parameters of the disaligned optical system to get the disalignmentamount, and then adjust the system to achieve the purpose of alignment. Thetraditional computer-aided alignment method is based on mathematicalapproximation or iterative algorithm to get the exact solution of the disalignmentamount, due to the special optical properties of freeform surfaces, it is difficult toresolve aberration formula to get disalignment amount from a complex opticalsystem which containing freeform surfaces by building and solving equations, then,proposing a new alignment method will be a necessary technological breakthroughto apply freeform optical surfaces in reflective telescopes. Based on the above, this issue focuses on the optical properties of freeformoptical surface and computer-aided alignment technique, aims at avoiding theuncertainties, inefficiency and blindness in alignment of freeform reflectivetelescopes and other issues, thus to compensate for the lack of computer-aidedalignment method, and improve the practicability of freeform optical surfaces. Themain contents are summarized as follows:(1)The imaging properties of freeform optical surface were analyzed in detail，and the characteristics of various expressions of freeform surfaces were discussed;(2)A discussion of the third-order wavefront aberration characteristics ofoff-axis optical system was presented, which focused on spherical aberration, comaand astigmatism distribution throughout the entire field of view;(3)Based on artificial neural networks, a new computer-aided alignmentalgorithm was proposed, mathematical structure of this algorithm was designed, theapplied program was compiled, the practical alignment case is simulated, and thus anactual large field optical system containing freeform surfaces was aligned;(4)Data link between design and alignment of freeform optical surface was built,and the technical problems in practical processes were analyzed.