Bin Fan

Edge effect modeling and experiments on active lap processing.

Edge effect is regarded as one of the most difficult technical issues for fabricating large primary mirrors, especially for large polishing tools. Computer controlled active lap (CCAL) uses a large size pad (e.g., 1/3 to 1/5 workpiece diameters) to grind and polish the primary mirror. Edge effect also exists in the CCAL process in our previous fabrication. In this paper the material removal rules when edge effects happen (i.e. edge tool influence functions (TIFs)) are obtained through experiments, which are carried out on a Φ1090-mm circular flat mirror with a 375-mm-diameter lap. Two methods are proposed to model the edge TIFs for CCAL. One is adopting the pressure distribution which is calculated based on the finite element analysis method. The other is building up a parametric equivalent pressure model to fit the removed material curve directly. Experimental results show that these two methods both effectively model the edge TIF of CCAL.

Influence study of solving correction forces caused by fitting errors for thin meniscus mirror

The actuator influence functions of a thin meniscus mirror can be expanded in Zernike polynomials. And the correctness of influence functions has a great effect on solving the correction forces. The Zernike coefficients are applied as parameters in the all-floating support system. We analyze the main interferential factors when different deformation modes are corrected. The influence caused by fitting errors is studied in this paper. A preferable result can be obtained after eliminating the interferential factors. The method can obtain useful correction forces. Comparing the peak-to-valley and root mean square values among the results calculated by different accuracy influence functions, we find that there is a limited convergence property when the accuracy increases.

Measurement of an off-axis parabolic mirror using coordinates measurement machine and swing arm profilometer during the grinding process

We present the measurement methods for off-axis parabolic mirrors using Coordinates Measurement Machine and Swing Arm Profilometer during the grinding process. An off-axis parabolic mirror with 1200mm in parent focal length, 360mm in off-axis displacement and 410mm in diameter is measured using the methods introduced in this paper. The convergence curves of the surface error are reported, which is reduced from 204.7μm in PV, 55.3μm in rms to 1.1μm in PV, 0.3μm in rms after 39 iterations of grinding. Besides, the measurement methods are able to control the parent focal length and off-axis displacement to less than 0.1mm tolerance. An interferometric result is presented, which validates the measurement methods.

A new method on measuring radius of curvature of a conic aspherical mirror

We present a method to measure the radius of curvature of a concave conic asphere. By analysis the central area of the asphere, we can measure the radius of an arbitrary point in the central area instead of the vertex of asphere. In the procedure, we firstly adjust the interferometer until the interferogram of the central area approach nulls, then put the laser tracker ball at the beam focus of the interferometer and move the tracker ball to touch the central area of the aspherical surface to get the two positions. With these measurement data, we can calculate the radius of curvature of the aspherical vertex and its uncertainty.

Absolute subaperture testing by multiangle averaging and Zernike polynomial fitting method

A method to obtain the absolute shape of a large plane or sphere surface in the subaperture testing with multiangle averaging and Zernike polynomial fitting method is proposed. The subaperture data obtained by the interferometer contain not only the test mirror deviation but also the reference surface deviation. The reference surface deviation has become the main limit of the subaperture testing accuracy. To correct the deviation, the measurements are made so that the reference deviation could be eliminated by rotational asymmetric deviation and the rotational symmetric deviation. This technique can get not only the full absolute shape of the large mirrors, but also the reference surface shape. Experimental results have been given to verify the effectiveness of this method

Grinding and polishing technology by computer controlled active lap for Φ1250mmF/1.5 aspheric mirror

For large aspheric optical elements, Computer Controlled Active Lap(CCAL) manufacturing which developed in IOE (Institute of Optics and Electronics, Chinese Academy of Science), have some advantages such as higher manufacturing efficiency, lower middle-frequency and high-frequency errors comparing the fixed lapping technology and CCOS(Computer Controlled Optical Surface) technology. A paraboloid surface of Φ1250mmF/1.5 was grinded by the active lap bonded with ceramic pills, as well as polished will pitch bonded active lap. During polishing processing a null lens was designed to test the paraboloid surface, the final testing data of RMS with ZYGO interferometer reached to 0.027λ(λ=0.6328μm).

Testing large aspheric surfaces with complementary annular subaperture interferometric method

Annular subaperture interferometric method has provided an alternative solution to testing rotationally symmetric aspheric surfaces with low cost and flexibility. However, some new challenges, particularly in the motion and algorithm components, appear when applied to large aspheric surfaces with large departure in the practical engineering. Based on our previously reported annular subaperture reconstruction algorithm with Zernike annular polynomials and matrix method, and the experimental results for an approximate 130-mm diameter and f/2 parabolic mirror, an experimental investigation by testing an approximate 302-mm diameter and f/1.7 parabolic mirror with the complementary annular subaperture interferometric method is presented. We have focused on full-aperture reconstruction accuracy, and discuss some error effects and limitations of testing larger aspheric surfaces with the annular subaperture method. Some considerations about testing sector segment with complementary sector subapertures are provided.

The grind and polish prediction of computer controlled active lap

Computer Controlled Active Lap(CCAL) manufacture is a new manufacturing technology comparing the classical manufacture and Computer Controlling Optical Surface(CCOS) for large aspheric mirrors manufacturing. The removal function model of active lap is found based on the investigation of active lap structure and working features. After analysis the edge effect, we optimize the founded model through the method of pressure compensation. The mirror surface predicated by the optimized model above basically matches the mirror surface tested by Hartmann-Shack wavefront sensor during the experiments of Φ1300mm(F/2)aspheric mirror manufactured by active lap.

Research advances and key technologies of macrostructure membrane telescope

微结构薄膜望远镜通过表面微纳结构调制光波相位和传播方向，具有轻量化、公差容限大、易于折叠展开的特点，因此成为大口径轻量化空间光学成像技术中的颠覆性技术。本文通过对国内外微纳薄膜望远镜研究进展的调研和分析，概括了薄膜望远镜研制的关键技术和主要技术途径，重点分析了薄膜材料制备、微结构类型研究、系统光学设计理论等内容。微纳薄膜望远镜研制涉及材料、空间环境工程、微纳加工工艺、精密机械和二元光学等众多交叉学科，随着工程化程度要求的提高，会出现新的技术问题，而随着问题的解决很可能获得具有影响力的科技成果。

Solving surface parameters of conic asphere mirror based on computer simulation

Radius of curvature R and conic constant k are important parameters of aspheres.Null testing or CGH are usually used to evaluate the processing quality of aspheric mirrors in fabricating process . When the null compensator emerges a problem, additional method to ensure the accuracy of paraxial radius of curvature and conic constant is required. Based on the equation of conic aspheric, the computing model from which the paraxial radius of curvature R and conic constant k can be obtained was established, and a set of solving algorithm using singular value decomposition (SVD) method was derived. The simulating result of a 1800mm aspheric mirror is presented and the solving precision reaches R=6120±0.026mm, k=-1.0194±0.0008, thus the supplement to null testing of aspheric mirror is achieved effectively .