ABSTRACT
We introduce a method to measure the dynamic surface deformation of an active stressed lap for fabricating a 4 mf/1.5 mirror. Lap surface accuracy working in some typical deformation velocities is put forward. Experimental results indicate that dynamic lap surface accuracy is worse than that of a static surface, and dynamic surface accuracy gets worse if deformation velocity increases, while the difference of lap surface error RMS is less than 1 µm. An optimization of the processing strategy is feasible through changing the deformation velocity of the active stressed lap depending on the processing schedule. After optimizing the grinding and polishing strategy, efficiency is expected to have a significant increase.
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.
ABSTRACT
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.
