Deterministic polishing is critical to the fabrication of ultra-precision aspheric optics, where precise dwell time algorithms are used to remove surface material. However, the sub-aperture polish removal function introduces surface ripple characteristics and degrades optical performance due to its narrow full width at half maximum and mismatched spatial wavelengths, reducing volume removal rate and introducing high and medium spatial frequency errors on the surface. Therefore, there is an urgent need to introduce pre-processing and post-processing to improve flexibility control in deterministic small tool polishing, enhance the adaptability of the tool to the workpiece surface with changing curvature, and obtain a stable Gaussian-like tool influence function. This paper explores the surface finishing removal mechanism based on the compliant figuring process and verifies the robustness and machining accuracy of the tool influence function for plane and variable curvature Nickel-phosphorous alloy machined surfaces. Firstly, based on the removal mechanism of the figuring process, the relative velocity and contact pressure models were established. Secondly, the influence of the radius of curvature and the amount of offset on the removal function is discussed. Finally, the high frequency band and middle frequency band of the one-dimensional power spectral density curve are analyzed in detail, and the reliability and modification ability of the compliant figuring process are verified according to the amplitude spectrum image.
The removal function in machining is not an ideal impulse function, and the material removal at any point on the workpiece is a superposition of the removal at that point from the distribution of removal at surrounding points. When the residence time suddenly jumped to zero at the edge, the removal function and the residence time convolution results will gradually decrease at the edge, the removal matrix at the edge of the formation of the "collapse", after many convolution iterations, the edge of the face shape of the error convergence rate is lower than the other positions, resulting in edge effects. This paper firstly put forward an iterative algorithm based on the residence time of the cylindrical projection, and then analyze the edge effect in the iterative algorithm, and put forward two methods to suppress the edge effect, and select different removal functions and the form error on the algorithm to carry on the simulation calculation. Finally, the results of machining of mold #54 verifies the effective suppression of edge effect error by the method in this paper.
EP satellite is a scientific exploration satellite for time-domain astronomy and high-energy astrophysics, its function is to carry out high-energy transitory celestial body survey with the highest sensitivity in the soft X-ray band. EP will be equipped with two groups of x-rays focusing mirrors (each group contains 54 Wolter-I mirrors). An X-ray focusing mirror is used to collect X-ray radiation in the universe, and is the core component of an X-ray astronomical satellite observation payload. The current X-ray focusing mirrors are derived from the Wolter-I reflector, and the light is focused through the inner surface. The inner reflection surface is an optical free-form surface connected by parabolic and hyperbolic surfaces. Many conventional measurement methods are not applicable and difficult to measure. High precision contact measurement is easy to scratch the mirror surface, Therefore, noncontact measurement must be used. In this paper, we develop two non-contact surface precision measurement devices are developed to solve the problem of difficult measurement of the inner reflection surface of the X-ray focusing mirror. One is a rapid measurement device based on the principle of optical star measurement method, The other is to measure the mirror inner surface by a single point scan measurement, the optical triangulation method is selected as the principle of the measurement device. We then verified the measurement accuracy of the mirrors with a precision visible light test system. In order to further verify the measurement accuracy of the focusing mirrors, X-ray testing of the focusing mirror was carried out at the 100-meter vacuum X-ray calibration facility of the Institute of High Energy Physics, Chinese Academy of Sciences. The X-ray measurement results are better than the project index requirements.
Based on chemical-mechanical polishing and combining mechanical and tribochemical polishing techniques, a precision lapping method suitable for parabolic polishing specified by grazing incident X-rays is introduced. Various factors affecting the polishing process are analyzed, and the results show that the proposed method is consistent with Preston's equation and Hertz contact principle. Therefore, this paper proposes a general material removal model based on the above two methods. The mid and high-spatial frequency errors are demanded to reach the requirements with an angular resolution consistently < 6 arcsec HEW and a roughness of 0.3 nm rms (between 1 mm and 0.002 mm spatial frequency range). To achieve the conformal ultra-smooth polishing of focusing mirrors, the process of full-aperture super-smooth pitch polishing is investigated. The influences of key polishing parameters are revealed. The evolution of the surface topology has been studied. A polishing setup is established to carry out experimental polishing to verify the optimum processing parameters obtained by simulations and previous polishing tests. Besides, the effect of abrasive particle size on the roughness is also verified. The roughness of the polished mandrel is measured at different positions, and the optimum roughness reaches Ra 0.359 nm. The polishing approach can significantly reduce the surface roughness of the replication mandrel, satisfying the low energy band focusing requirement of grazing incidence X-ray mirrors.
In order to achieve replication of the ultra-thin metal x-ray focusing mirrors with high accuracy and efficiency, which is the key component of the EP satellite independently developed and manufactured in China, the simulation and experimental research on the demolding process of the x-ray focusing mirrors are carried out in this paper. The temperature and stress fields of the entire mandrel (aluminum) and the shell (nickel) during cooling process is simulated by finite element analysis, and the evolution of the interface stress during the demolding process is analyzed. When the temperature of the mandrel and shell decreases from 45℃ to 10℃, the equivalent stress at the interface between the mandrel and the mirror reaches 5.5MPa, which is larger than the adhesion strength between Au film and mandrel. Due to the difference in material thermal expansion coefficient between the mandrel and the mirror, it can be used to release the x-ray focusing mirror shell from the mandrel by cooling according to the experimental validation. Furthermore, the shell could separate from the mandrel by means of high precision demolding automatic device. After demolding, the angular resolution of the mirror is 25.1 "HPD (Half Power Diameter) by the x-ray testing, which meets the requirements of the project. The reliability and advancement of the technology are verified.
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