Research On Removal Model And Technology For Ion Beam Figuring Large Aspherical Mirror

As the development of modern optical system, higher revolution and larger view angle is needed. The requirement of optical components for the optical system is much more critical too. Aspherical mirror has advantages in the performance of the system, weight and size. Especially for the space camera, aspherical mirror is becoming one of the key factors. Optical fabrication methods were proposed to meet the higher requirement of large asphere. Contiguous methods based on CCOS(Computer controlled optical surfacing) such as ordinary lap and stress lap promote the efficiency of large asphere fabrication. But during the figuring process the lap can’t fit the asphere as well as it fits the sphere, especially for the large steep ones. The material removal distribution is not stable again. Besides this, edge effect and stress effect will influence the mirror at the same time.For much higher figuring precision of large aspherical mirror, new techniques are brought into this field. Including IBF( ion beam figuring), MRF and so on. During the process of IBF, ion beam with settled energy and space distribution bombs the target surface to remove the material accurately and uncontiguously. Comparing with the conventional figuring methods, IBF has more stable removal function, no stress effect and no edge effect, which is much more suitable for fabrication of large aspherical mirror. In this paper, systematical research on removal model and applied technology of ion beam figuring large aspherical mirror is proposed. The main work is in the following aspect.1. removal model of ion beam figuring processHigh precise removal model is the key factor to calculate the removal function and removal rate of the IBF process. Based on Sigmund sputtering theory, material removal rate is directly related to the energy distribution beneath the target surface. Different energy leads to different removal rate. Removal function in Gaussian shape is got when the ion beam is vertical to the flat surface. Working parameters of ion beam source is optimized to get the most suitable removal function for some given surface error.2. On line test of removal function for IBF processPrecise information of removal function during the IBF process is signality for high accurate figuring. The conventional way to measure removal functions are footprint experiment, which can get removal function on the chosen flat mirror for a group of working parameters. It will cost lots of time and materials to calculate the information. In this paper, removal functions are measured on line by Faraday cup. According to removal model of ion beam figuring, removal functions are determined by energy an d ion distribution. Calculate the measured results of Faraday cup to get the removal functions. This method can cost much less time and material than the traditional ways. And removal functions can be optimized real time.3. Revise removal functions by matrixRemoval functions change a lot during the process of ion beam figuring steep curved surface, which is different from the situation of figuring flat mirror. Ion beam distribution, ion density and property of incident ion are three key factors which determine the change of removal function. So the removal functions have to be revised to fit the real value. Revision matrix is calculated from the geometry information of the mirror and ion property. Combining the matrix and standard removal function, removal function on each dwell point can be got accurately, which can improve the figuring efficiency and precision.4. Improve surface roughness by IBFRoughness of optical surface will influence the quality of the system, so improving the roughness will promote the system performance. Simulations and experiments are done to study the metrology during the ion beam figuring process. Two different strategies are applied for different space frequency error. The results show that the roughness can be improved by ion beam figuring.5. Applying of IBF for large off-axis aspherical mirrorBased on the research above, figuring processes are done for several large off-axis aspherical mirrors. For instance, Φ1450mm aspherical mirror, RMS value from 0.057λto 0.012λ, which prove the feasibility of the research.