Fabrication Of Large Aspherics Using Stressed Lap With Orbital Tool Motion

Lots of large apture telescopes is developed for high resolution and energy collect ability. the diameter of the primary mirror is up to 8m in the largest telescopes in the world and to 2m and heading to 4m in our country. Comparing with small aspherics, much more materials need to be removed and the ordinary CCOS (Computer controlled optical surfacing) technology is held back by its low material removing rate and cannot address the need in the production of large aspherics. A solution is put forward in this paper by applying the CCOS (Computer controlled optical surfacing) removing strategy into the stressed lap technology, employing the stressed lap as the removing tool but moving the lap in the CCOS fashion, naming the stressed lap with orbital motion. Based on the pioneer work done by my workmate, my work are in the following few regions.1) Processing and deflection character analysis of the stressed lap with orbital tool motion. The working function of the stressed lap with orbital motion is analyzed comparing to the original stressed lap with the spinning motion and turn out to be better with a nearly Gaussian distribution. The grinding and polishing process of the CCOS with stressed lap is also simulated and the removing rate is about 16~17 times higher than the original CCOS. The effects of the stressed lap’s change in movement is also evaluated, the speed of the stressed lap’s deflection in the new way is only about 1/6 of the original stressed lap, which means it is dramatically relax the requirements of the lap’s respond or can move faster to get a higher material moving rate.2) FEA analysis of the deformation accuracy of the stressed lap. When polishing an aspheric with a stressed lap, a few microns of fitting accuracy is needed, so the ability of the mechanical structure of the stressed lap to fit the aspheric shape of the mirror is critical for the polishing accuracy and the surface quality of the aspheric mirrors. The article focuses on the effect of the shapes of the footprint of the tubes on the lap on the deformation of the lap. A few feasible mechanical structures is simulated by FEA models and evaluated by MATLAB routines. The results show that the one with the least residual error can reach a fitting accuracy of a few 0.1 microns, faithfully fulfilling the accuracy needed by the polishing process.3) deformation accuracy experiment with a prototype stressed lap model. To verify the deform accuracy of the stressed lap and the high order residual error distribution, a prototype stressed lap whose shape could be manually adjusted is build out and tested with a CMM whose accuracy is within 1μm. The results show that most of error is in the outer annual area where those tubes are assembled and the deform accuracy for 100μm power and 100μm astigmatism and 20μm coma and the repeatability of the lap is within 1μm RMS, better than the experimental standard of 3μm given by the University of Arizona, which is very important for the actual usage of the lap in the future.4) Experiment of the processing of large mirrors with large tools with orbital tool motion.For the technology of stressed lap with orbital tool motion upgrade the efficiency by enlarging the lap size naming large tools with orbital tool motion. The edge effect that common for large laps is simulated and compensated by first order approximation and weight functions for high order distribution to improve the convergence rate. A round square 1100mm×800mm SiC mirror is grinded with this method. After 22 runs of 55 hours in all, the overall surface error is 122μm PV before and 5.9μm PV after the process, comparing with previous small lap technology, the efficiency is at least improved by a factor of 2.5) A CNC machine for the processing of aspherics whit 1.5m capacity. both small tools and stressed lap with orbital tool motion can be used on this machine and the characters different from the previous FSGJ asphercis processer are: the machine could afford 3000N force to polishing the mirrors which is suitable for the usage of large tools. the machine is designed for the offline calibration and deforming control of the stressed lap when working on a mirror. and the rotating table whose diameter is 1.6m is much larger and stronger than the former FSGJ machines. the spindle is attached on a socle girder and could be moved out of the processing area which make it convenient to use. the machine work well in the experiment and could process any asphercs whose size is within 1.5m in diameter.6) A high speed and high accuracy swing-arm profilometer. To upgrade the testing accuracy of traditional three coordinate CMM, A high speed and high accuracy swing-arm profilometer is constructed for high accuracy and high testing speedfor large mirrors testing. According to analysis, because the arm rotating run-out is the only error source, high accuracy up 1μm PV could be reached withinΦ1.5m. By applying a non-contact linear sensor, high sample rate was achieved which could decrease testing time to minimize the thermal effects on profilometry. A mirror of 1100×800 mm sphere was tested by the swing-arm profilometer, and it took only 30 minutes for more than 1200,000 sample points. Comparing to a Shark-Hartmann test, the difference of two testing results is 0.02μm RMS and 0.5μm PV respectively, with very good consistency.