Weijie Deng

Novel method for optimizing polishing tool-path in CCOS based on weighted-iterative algorithm

In Computer Controlled Optical Surfacing (CCOS), polishing tool-path is the base of solving other control parameters such as dwell time. In order to improve the fabrication results of polishing off-axis aspheric, a novel method to optimize the tool-path is discussed in this paper. The optimizing method named weighted-iterative algorithm is according to the balance principle of the particle system. The power factor of each dwell point represents the requirement of dwell density. Considering the factors which influence the polishing result, the power factors cosist of three elements include constant, error distribution and dwell distance of workpiece edge. The tool-path is solved by numerical iterative method. In the end, an error data is simulated with actual parameters using the matrix-based algorithm with two different tool-paths. The one is X-Y uniform spacing model and the other one is to optimize it based on the first. The comparison shows that the results of the optimized one are much better than traditional one, especially the rms convergence rate. Theory of the algorithm is simple and exercisable, and it satisfies practical requirement as well.

Annular sub-aperture stitching interferometry for testing of large aspherical surfaces

Annular subaperture stitching interferometric technology can test large-aperture, high numerical aperture aspheric surfaces with high resolution, low cost and high efficiency without auxiliary null optics. In this paper, the basic principle and theory of the stitching method are introduced, the reasonable mathematical model and effective splicing algorithm are established based on simultaneous least-squares method and Zernike polynomial fitting. The translation errors are eliminated from each subaperture through the synthetical optimization stitching mode, it keeps the error from transmitting and accumulating. The numerical simulations have been carried on by this method. As results, the surface map of the full aperture after stitching is consistent to the input surface map, the difference of PV error and RMS error between them is -0.0074 λ and -0.00052 λ (λ is 632.8nm), respectively; the relative error of PV and RMS is -0.53% and -0.31%; and the PV and RMS of residual error of the full aperture phase distribution is 0.027 λ and 0.0023 λ, respectively. The results conclude that this splicing model and algorithm are accurate and feasible. So it provides another quantitive measurement for test aspheric surfaces especially for large aperture aspheres besides null-compensation.

Positive dwell time algorithm with minimum equal extra material removal in deterministic optical surfacing technology

In deterministic computer-controlled optical surfacing, accurate dwell time execution by computer numeric control machines is crucial in guaranteeing a high-convergence ratio for the optical surface error. It is necessary to consider the machine dynamics limitations in the numerical dwell time algorithms. In this paper, these constraints on dwell time distribution are analyzed, and a model of the equal extra material removal is established. A positive dwell time algorithm with minimum equal extra material removal is developed. Results of simulations based on deterministic magnetorheological finishing demonstrate the necessity of considering machine dynamics performance and illustrate the validity of the proposed algorithm. Indeed, the algorithm effectively facilitates the determinacy of sub-aperture optical surfacing processes.

Ion beam figuring of highly steep mirrors with a 5-axis hybrid machine tool

Ion beam figuring (IBF) is an advanced and deterministic method for optical mirror surface processing. The removal function of IBF varies with the different incident angles of ion beam. Therefore, for the curved surface especially the highly steep one, the Ion Beam Source (IBS) should be equipped with 5-axis machining capability to remove the material along the normal direction of the mirror surface, so as to ensure the stability of the removal function. Based on the 3-RPS parallel mechanism and two dimensional displacement platform, a new type of 5-axis hybrid machine tool for IBF is presented. With the hybrid machine tool, the figuring process of a highly steep fused silica spherical mirror is introduced. The R/# of the mirror is 0.96 and the aperture is 104mm. The figuring result shows that, PV value of the mirror surface error is converged from 121.1nm to32.3nm, and RMS value 23.6nm to 3.4nm.

Nano-scale finishing of off-axis aspheric mirror by ion beam figuring

The high precision off-axis asphere mirrors are quite usefull in the modern sapce optical system.in this paper, the ion beam figuring is researched to accomplish the final surface accuracy. Firstly, The IBF machine we used is introduced simplely. The correction method for the distortion in off-axis mirror interference measurement is studied. Beacasue of the curve shape of the off-axis mirror, the changing of the removal function was analyzed simply. In order to testify the reaserch, a 1100mm×800mm off-axis aspheric mirror is finished by ion beam figuring. After 2 times polishing, the surface error was corrected to RMS 9nm in full aperture and achieved 6.3nm in sub- aperture.

Magnetic medium assistant polishing technology and experiments

The high precision optical mirrors are quite important for the modern optical system. In this paper, a novel Magnetic Medium Assistant Polishing technology and device is researched for optical finishing. The key element of the device is a designed magnetic wheel and accessorial magnetic component. The solid magnetic powder arranged with the magnetic field distribution and became a flexible polishing brush tool. The solid magnetic medium tool working with a high rolling speed and the polishing liquid with abrasive was injected to contact region of the optical surface. The magnetic powder tool is reforming with the magnetic field all the time, and this feature made the removal rate and distribution of the removal function invariable. The device is connected to the polishing machine. The fabrication experiments were completed. The distribution of the removal function is like a raindrop and asymmetry in one orientation. The removal stability of the tool was quite good and achieves 95% sameness of peak value in one hour. The primary experiments shown that the technology can be used in the practical fabrication.

The removal function of edge effect and amending with dwell time function

Computer Controlled Optical Surfacing (CCOS) is widely used for making optical aspheric mirrors. In the practical fabrication, edge effect is an important problem which restricts the fabrication efficiency and accuracy seriously. In this paper, the edge effect is solved by working out the edge removal function and compensate with dwell time function. Skin Model is used to describe the pressure distribution when the tool hangs over the work-piece. The calculation model of edge removal function is derived from Skin Model theoretically. A removal function experiment is completed. The difference between the theoretical model and the experiment results is less than 5%. It means that the calculation model is suit for the practical fabrication. Than the dwell time is solved with edge effect compensation by matrix-based algorithm. In the end, actual experiment was done to validate the edge effect compensation method.

Test of an off-axis asphere by subaperture stitching interferometry

A new method combined subaperture stitching with interferometry(SSI) is introduced. It can test large and off-axis aspheric surfaces without the aid of other null optics. In this paper the basic principle and theory of the technique are analyzed. The synthetical optimization stitching model and effective stitching algorithm are established based on homogeneous coordinates transformation and simultaneous least-squares method. The software of SSI is devised and the prototype for testing of large aspheres by SSI is designed and developed. An off-axis asphere with the aperture of 376mm×188mm is tested by this method. For comparison and validation, the asphere is also tested by null compensation.The synthesized surface profile is consistent to that ofthe entire surface from null test; and the difference of PV and RMS error between them is 0.047 λ and 0.006 λ, respectively. So it provides another quantitative measurement for testing large aspheric surfaces and off-axis aspheres besides null-compensation.