Kinematic model for material removal distribution and surface figure in full-aperture polishing

Monitoring of the glazing state of the pitch polishing lap by an image texture analysis method in full-aperture continuous polishing

Full-aperture continuous polishing using a pitch lap is one of the key processes in finishing large flat optical elements. The pitch lap has relief lines cutting into the surface to increase its surface roughness and to improve the polishing results. During polishing, the pitch lap surface is glazed due to the transverse rheologic flow and creep of the pitch lap by the element and conditioner, which has significant influence on the polishing results. In this study, an image texture analysis method is proposed to monitor the glazing state of the pitch lap. The images of the pitch lap surface are captured online, and the image textures are analyzed with the gray level co-occurrence matrix (GLCM) method. The experiments revealed that the GLCM eigenvalues of the surface image are strongly correlative with the glazing state of the pitch lap, which has a linear impact on the material removal rate and material removal uniformity. To the best of our knowledge, this study provides a novel and useful method to monitor the surface glazing state of the pitch lap.

Surface Shape Evolution of Optical Elements during Continuous Polishing of Fused Quartz

Continuous polishing is the first choice for machining optical elements with a large aperture. The lubrication in the continuous polishing is an important factor affecting the surface quality of the optical elements. In this study, the lubrication system between the optic element and polishing lap was analyzed firstly and then was verified by the measurement experiment of the friction coefficient. In addition, the numerical simulation model of the mixture lubrication was established. The polishing pressure distribution and material removal distribution can be obtained by the model. The influences of the rotating speed, optical element load, and surface roughness of the polishing lap on polishing pressure were also analyzed. Finally, the influence rules of the lubrication on the surface shape of optical elements were revealed by the polishing experiments.

Method to improve the surface shape of BK7 glass in full-aperture polishing

Full aperture polishing is a significant process to fabricate optical workpieces with nano figure precision. It has the characteristics of excellent full spectrum uniform removal ability, fast convergence speed, high machining accuracy, and low production cost, which makes it the first choice for the processing of large-aperture optical elements. However, with BK7 glass it is difficult to achieve deterministic processing due to its large thermal expansion coefficient and other factors, and the surface shape accuracy is difficult to improve. In this paper, the thermal deformation of BK7 glass is analyzed first, and then the temperature distribution in the element is measured during full-aperture polishing. The influence of the temperature field in the optics on the accuracy of the final surface shape is analyzed, and the quantitative relationship between the temperature difference and the surface shape is established. In addition, the methods of deterministically controlling the surface shape of optics such as polishing liquid temperature control and immersion polishing are proposed. Finally, the model of quantitative control of the surface shape of optics is verified by experiments, which improve the surface shape accuracy of 600 mm BK7 optics to 0.2λ.