Deformation verification and surface improvement of active stressed lap for 4  m-class primary mirror fabrication

Abstract

The surface shape accuracy of the active stressed lap impacts the performance of grinding and polishing in the fabrication of large mirrors. We introduce a model of active stressed lap for the fabrication of a 4 m f/1.5 mirror based on finite element analysis (FEA), and the lap surface accuracy achieves RMS<1.8 μm in the FEA method. Using the lap surface measurement system, experimental verification is put forward, and the RMS of the measured lap surface is within 2 μm in practice. A general improvement in lap surface accuracy using the Zernike polynomial is shown. After compensating the calculation errors, the lap surface accuracy is improved by 8%-23%, and achieves RMS<1.5 μm, which is appropriate for practical grinding and polishing.

Manufacturing technologies toward extreme precision

“…As early as 1984, et al [154] proposed that the polishing tool should be actively deformed in order to reproduce the subaperture shape of the aspheric mirror corresponding to the pad position on the mirror surface, as shown in figure 17 [156]. Based on this concept, several stressed laps were designed to change their shape in-process to coincide with the mirror surface during polishing [157][158][159][160]. Figure 18 shows one design of a stressed lap in which the deformation of the pad surface is achieved by drawing steel wire using a servo motor [155].…”