Large-aperture Aspheric Surface Research Articles

Research on detection method of large-aperture aspheric surface by laser tracker

Research on detection method of large-aperture aspheric surface by laser tracker

Theoretical and experimental analysis of material removal and surface generation in novel fixed abrasive lapping of optical surface

Abstract Fixed abrasive machining, which overcomes the lack of determinacy and efficiency of current loose abrasive polishing or lapping processes, is an alternative technology for fabricating large-aperture aspheric surfaces with high finishing efficiency and high-quality surface finish. A novel fixed abrasive lapping (NFAL) tool, which combines computer-controlled optical surfacing and conventional fixed abrasive lapping process, is introduced in this study to produce off-axis aspheric surfaces efficiently while surface state is controlled and subsurface damage (SSD) is avoided. The removal mechanism analysis shows that fused quartz glass is primarily removed in ductile regime and the SSD depth is 14.7 μm when the tool load is 25 N. The material removal volume is linear with the machining time under the ductile removal mode. The material removal and surface generation models are developed on the basis of the calculation of the spatial distribution of abrasive particles, the pad–particle–workpiece interactions, the single-particle abrasion mechanism, and the linearly cumulative removal of surface generation in the NFAL process. The models are verified through a series of spot and surface lapping experiments. Results show that the theoretical model can be successfully used to predict and optimize the NFAL process. The spot lapping experiments indicate that the material removal volume is linear with the rotation speed, and the maximum depth is limited to the specific value that depends on the stiffness of lapping tool. The surface roughness Ra and Rz values of the measured and simulated surface data decrease with the increase in feed rate and increase with the increase in raster spacing. The power spectral density analysis indicates that high feed rate can distinctly improve the high-frequency errors, and the selection of the raster spacing can principally affect the low-frequency and middle-spatial frequency errors.

Stitching Measurement Technique for Large-aperture Aspheric Surface during Grinding Process

In order to test large-aperture aspheric surface optice element accurately during grinding process, a comprehensive optimization data processing model of stitching measurement is proposed based on coordinate measuring. Firstly, the stitching model for two segments of surface shape contour is built based on multi-body system theory and least square principle; secondly, the rejecting redundant data model is built based on curvature principle and least square fitting of aspheric equation; then the comprehensive optimization processing model for multiple segments stitching is built; three generatrices of a non-axisymmetry aspherical optical element whose aperture is 176 mm is stitching measured by Taylor Hobson profilometer and the data is processed by the model mentioned in the text. The results show that the maximum average error of the stitching measurement is 0.25 μm, and the minimum average error of the stitching measurement is –0.22 μm which is compared with the single measurement. The error is an order of magnitude higher than the surface shape peak–valley value 2.77 μm of the optic element, and matches the measurement requirements. The experimental results prove the correctness and effectiveness of the comprehensive optimization data processing model of stitching measurement, and the model can be applied in engineering.

大口径光学非球面元件拼接优化及精度验证

大口径光学非球面元件拼接优化及精度验证

Research on Eliminating Cumulative Errors for Stitching Algorithm of Large Aperture Aspheric Optical Elements

The sptitching measuring method applied in large-aperture aspheric surface optical elements can expand the range of precision measuring instrument. And the key point is developing the technique for high precision measuring data processing. To deal with the problem of cumulative errors which existed in the process of multi-section stitching, datum-transformation method was proposed, according to the characteristic of stitching measurement. The stitching measuring experiments were carried out based on Talysurf PGI 1240 profilometer of Taylor Hobson Company to verify the purpose for eliminating cumulative errors. The results show that datum-transformation method is feasible with the characteristic such as fast speed, high precision, and good operability.

Optimal strategy for fabrication of large aperture aspheric surfaces

Aspheric surfaces are widely used because of their desirable characteristics. Such a surface can obtain nearly perfect imaging quality with fewer optical elements and reduce the size and mass of optical systems. Various machine systems have been developed based on modern deterministic polishing technologies for large aperture aspheric surfaces. Several factors affect the final precision of large aperture aspheric surfaces, such as the velocity limit of the machine and the path design. Excess velocity, which will be truncated automatically by the computer numerical control system, may cause the dwell time to deviate from the desired time. When a path designed on a two-dimensional surface map with equidistant pitch is projected onto an aspheric surface, the pitch changes as a result of the varied curvature of the aspheric surface. This may affect the removal map and cause some ripple errors. A multiregion distribution strategy, which includes velocity checking, is proposed in this study to avoid exceeding the velocity limits. The strategy can be used to modify local errors and edge effects. A three-dimensional spiral path generation method is also presented using an iterative method to ensure uniformity in the space length of the adjacent circle of the spiral path. This process can reduce the ripple error caused by the overlapping of tool paths. A polishing experiment was conducted, and the results proved the validity of the proposed strategies.

The Research for Rejecting the Redundant Data during Section Splicing Large-Aperture Aspheric Optical Elements

The splicing measuring method applied in large-aperture aspheric surface optical elements, can extend the range of precision measuring instrument and expand measurement capability of the system. In this paper, two methods are proposed: the fitting evaluation method and the curvature comparison method in the light of how to reject the redundant data of the splicing coincidence part, and the theoretical analysis are investigated to calculate the theoretical error. The Talysurf PGI 1240 profilometer of Taylor Hobson Company is used as application object to do experimental research. The results show that the curvature comparison method is feasible and has characteristic such as fast speed, high precision, and good operability.

Shack-Hartmann波前传感器检测大口径圆对称非球面反射镜

As the surface shape of a reflection mirror is different from the target shape at the end stage of the grinding process,and the profilometers and common interferometers can not measure the surface error exactly,this paper proposed a method to measure the large aperture aspheric surface by using a Shack-Hartmann Wavefront Sensor(SHWS) with a large dynamic range and high precision.The principle of the measuring system for SHWS was researched,its measuring errors were analyzed and a corresponding data processing software was compiled.Using a simulation reference file,a hyperboloid reflection mirror with a diameter of 350 mm was tested by the SHWS.Results indicate that the PV and RMS of the surface error are 0.388λ and 0.043λ,respectively(λ=632.8 nm).In order to testify the measurement result,the mirror is also measured by an interferometer with a null compensator.Comparing the two testing results,the PV and RMS of the deviation are 0.014λ and 0.001λ,respectively,which proves the feasibility of measuring large aperture aspheric surfaces by SHWSs.

Quantitative measurement of large-aperture aspheric surfaces by off-axis Ronchi test

An off-axis Ronchi test based on the phase-shifting technique is presented to measure the aspheric mirror quantitatively. In this paper, slopes of deviations from the mirror's ideal aspheric shape are derived when an off-axis point source is used. By integrating these slopes, the deviations of the shape are obtained and then the tested shape can be reconstructed. Compared with the coaxial Ronchi test, the measurement error caused by combination fringes can be avoided, and this system is more flexible. In the experiment, a marked point is used to guide the phase unwrapping procedure, which improves the accuracy of extracting corresponding phase points. In addition, the method has strong anti-noise ability. The validity of the method has been verified by both computer simulations and a preliminary experiment.

用正弦光栅的朗奇法检验大口径非球面面形

用正弦光栅的朗奇法检验大口径非球面面形

用正弦光栅的朗奇法检验大口径非球面面形

用正弦光栅的朗奇法检验大口径非球面面形

用正弦光栅的朗奇法检验大口径非球面面形

用正弦光栅的朗奇法检验大口径非球面面形

Ф630mm F/1.34抛物面的Ronchi光栅定量测量

The Ronchi grating is applied to measure the large-aperture aspheric surfaces in a quantitative way on the basis of self-made software which includes Ronchi null grating design, collection of Ronchi graph, and data processing. The measured concave parabolic mirror has a diameter of 630 mm and an F number of 1.34. The measurement result is approximately the same as that of the ZYGO interferometer. This analysis software and test method establish a good foundation for the quantitative measurement of the big error about the large-aperture aspheric surfaces of the next generation telescope.

龙基光栅在大口径非球面定量测量中的应用

龙基光栅在大口径非球面定量测量中的应用

Testing a large aspheric surface based on analysis of scanning 3D interferogram

A new method based on new reconstruction algorithm and new interferogram fringe analysis was proposed for testing a large aspheric surface. In the testing process, firstly, the interferometer or the tested aspheric surface was shifted relatively for measurement of the whole aperture of the tested aspheric surface to obtain a series of interferograms. Then these interferograms were analyzed to extract the best matching point, where the annular sub-surface was tangent with the corresponding reference sphere of the reference wavefront. Finally, the wavefront aberration of the whole aperture was figured out to describe the deformation information of the tested surface. Without any additional null optical elements, this method can perform the measurement directly. So, this method provides an effective and convenient means of testing the large-aperture aspheric surface. The mathematical model of this method is described, and the results of simulation tests are presented to verify it.