4m口径SiC反射镜原位检测用静压支撑系统

Hydrostatic support system for in-situ optical testing of a 4 m aperture SiC mirror

研制了一套用于4m SiC反射镜原位检测的静压支撑系统,以降低超大口径SiC反射镜离线检测的风险,提高其制造效率。首先,推导了单元刚度的解析式,确定了其中关键因素;然后,对支撑单元进行抽样测试,结合解析式预测了支撑群组中单元的工作刚度。最后,通过密封性测试和反射镜原位检测,验证了支撑系统的稳定性;通过有限元模拟,计算了系统的重力卸载面形精度。结果表明:5个单元连组时,单元刚度约为1.9kN/mm,刚度值分布在±3%误差区间;独立单元刚度可高至15kN/mm;3种分组单元刚度预测值分别为1.7,1.1和0.8kN/mm。支撑系统空载时管路压强变化缓慢,表明密封性良好;用该系统支撑4m反射镜时,11天内高度绝对变化量小于50μm,相对变化量小于20μm。54个单元刚度随机分布时,镜面面形高阶残差(RMS)为20nm。提出的系统基本满足原位检测的稳定性和精度要求。

A set of hydrostatic support system for a 4m SiC mirror in-situ testing was designed to reduce the risk of ultra-large mirror off-line testing and to improve fabrication efficiency.Firstly,the analytical formula for calculating support stiffness was derived,and its main factor was found.Then,some support samples were tested,and the support stiffness of a single support unit in different groups was predicted by combining the sample results and analytical formula.Finally,the stability of the support system was evaluated by pressurization test and in-situ testing,and the mirror surface precision with its gravity offloaded by the support system was calculated by finite element simulation.Results show that the average stiffness is about 1.9kN/mm with a relative difference among support units about 3% when 5 units are linked together,the stiffness for a single unit isolated is as high as 15kN/mm,and 3 kinds of unit stiffness in the support system are 1.7,1.1 and 0.8kN/mm respectively.Moreover,pressurization test shows a slow pressure change,which indicates that the system is well sealed.When the hydrostatic system was used for a 4m mirror,the height changes rise up and down within 50μm in 11 days,and the relative change is less than 20μm.When the mirror is supported on 54 units with stiffness difference by 3% randomly,the surface RMS is less than 20 nm.The proposed system meets the requirement of in-situ optical testing for precision and stability.