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针对非旋转对称像差和旋转对称像差的不同特点,采用不同的方法来获取其精确的测试数据,从而实现对全部重力误差的准确评估:通过变换反射镜组件的方位进行像散等非旋转对称像差的测试,分离重力误差与装配误差;在一定精度的重力卸载下进行对比测试,获取球差等旋转对称像差,避免检测光路对旋转对称像差测试精度的影响。
重力作用下的反射镜面形W为:W=W1+W2+W1g。因为各像差具有正交性,W1g又分解为因重力引起的非旋转对称像差(三叶像差等)和旋转对称像差(Power、球差等):W1g=Woff-axis+Won-axis。
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通过变换反射镜组件的方位进行重力变形测量。任何结构体在分别承受大小相同、方向完全相反的力场时,其变形量应该是大小一致,方向相反。重力翻转测试方法正是基于这一理论进行推演,分别测试光学组件在光轴竖直向上和光轴竖直向下两个状态下的面形变化,并通过两个状态的数据叠加来获得无重力影响的光学测试面形结果[14-15]。Bipod结构是装配关系简单、力学边界条件明确的准静定支撑,因此在进行翻转测试的过程中,因翻转工装引入的误差对测试结果影响比较轻微,光学组件光轴竖直向上和向下的两个状态相比,其重力造成的面形误差在数值上相等,在相位上相反,而加工残留误差和装配误差不变。因此,当光学组件光轴竖直向上时,其面形Wu为:
$$ \begin{array}{c}{{W}}_{{\rm{u}}}={W}_{1}+{W}_{2}+{W}_{{\rm{off-axis}}}+{W}_{{\rm{on-axis}}}\end{array} $$ (1) 当光轴竖直向下时,其面形Wd为:
$$ \begin{array}{c}{{W}}_{\mathrm{d}}={W}_{1}+{W}_{2}-{W}_{{\rm{off-axis}}}-{W}_{{\rm{on-axis}}}\end{array} $$ (2) 剔除Won-axis,此时光轴竖直向上和向下的面形
${{W}_{{\rm{u}}}'}$ 、${W}_{{\rm{d}}}'$ 分别为:${{W}_{{\rm{u}}}' }$ =W1+W2+Woff-axis,${W}_{{\rm{d}}}'$ =W1+W2−Woff-axis。因此,重力误差的非旋转对称像差为:$$ \begin{array}{c}{{W}}_{{\rm{off-axis}}}=\dfrac{{W}_{{\rm{u}}}'-{W}_{{\rm{d}}}'}{2}\end{array} $$ (3) -
通过测试反射镜组件进行重力卸载与不卸载的Power、球差等旋转对称像差,计算其变化量,从而得到因重力引起的旋转对称像差。与大口径反射镜加工阶段的重力卸载相比,组件级的重力卸载只需要对检测精度不足的特定像差进行卸载,不要求对全部重力误差进行卸载,设计和装调的难度都会大大降低,从而更加容易实现较高的精度。针对1.3 m口径、采用背部6点Bipod支撑的反射镜组件,设计18点的重力卸载装置,即可将其面形卸载至0.013λ,其中Power、球差的残余值为0.002λ和0.003λ。卸载后面形残差如图4所示。与加工阶段使用的200点卸载装置相比,点数大幅度降低。同时,卸载力的加载形式由气缸加载改为杠杆加载,大大简化了复杂程度,能够在有限空间内进行布局。
调整好检测光路,先测试无卸载状态下的反射镜面形,仅保留旋转对称像差Won-axis1,再行安装卸载装置,调整各卸载点的位置与卸载力。保持检测光路不变,测试此时的反射镜面形,此时面形包含了卸载引起的非旋转对称像差,予以剔除,仅保留旋转对称像差Won-axis2。卸载前后镜体刚体位移造成的旋转对称像差为W’on-axis。则重力引起的旋转对称像差为:
$$ \begin{array}{c}{{W}}_{{\rm{off-axis}}}={W}_{{\rm{off-axis1}}}-{W}_{{\rm{off-axis2}}}+{W}_{{\rm{off-axis}}}^{\text{'}}\end{array} $$ (4) 大口径反射镜组件的重力误差和零重力面形分别为:
$$ \begin{array}{c}{{W}}_{1g}=\dfrac{{W}_{{\rm{u}}}^{\text{'}}+{W}_{{\rm{d}}}^{\text{'}}}{2}+{W}_{{\rm{off-axis1}}}-{W}_{{\rm{off-axis2}}}+{W}_{{\rm{off-axis}}}^{\text{'}}\end{array} $$ (5) $$ \begin{array}{c}{W}_{0g}={W}_{{\rm{u}}}-{W}_{1g}={W}_{{\rm{d}}}+{W}_{1g}\end{array} $$ (6)
0 g surface figure test of large aperture mirror supported by Bipod
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摘要: 双脚架结构具有静定支撑的特点,可以隔离机械附加载荷,因此成为大口径空间相机反射镜组件的常用支撑形式之一。在地面装调时,采用双脚架支撑的反射镜的面形因重力作用而下降。空间相机入轨后,随着重力变形的释放,反射镜面形会再次发生改变。有限元分析方法评估反射镜组件的重力误差,其精度难以达到高质量高分辨率成像的要求。同时,反射镜加工过程中使用的重力卸载方案也难以沿用至组件阶段。针对重力误差测试过程中装配误差与三叶像差混叠以及检测光路对球差测试精度不足的问题,提出了翻转与卸载相结合的测试方案。基于不同像差的正交性,可以进行分别测试来逐项获取各像差。通过反射镜组建的翻转测试,分离装配误差与重力误差中的三叶像差。设计一定精度的卸载装置,通过卸载前后的对比测试,得到重力造成的球差等旋转对称像差。采取上述方案可以实现对全部重力误差的实测。利用1.3 m口径高轻量化反射镜组件进行了测试验证,其重力误差面形rms和在轨面形rms分别为0.192λ (λ=0.6328 μm)和0.023λ。Abstract: Bipod structure has the characteristics of static support and can isolate the additional mechanical load. Therefore, it has become one of the common support forms of large aperture space camera mirror assembly. When installing and adjusting on the ground, the surface figure of the mirror supported by the Bipod decreases due to the action of gravity. After the space camera enters the orbit, the surface figure of the mirror will change again with the release of gravity deformation. The gravity error of the mirror assembly is evaluated by finite element analysis method, and its accuracy is difficult to meet the requirements of high-quality and high-resolution imaging. At the same time, the gravity unloading scheme used in the mirror processing process is also difficult to be used to the component stage. In order to solve the problems of aliasing of assembly error and trefoil aberration and insufficient accuracy of spherical aberration test by detection light path in the process of gravity error test, a test scheme combining turnover and unloading was proposed. Based on the orthogonality of different aberrations, individual tests could be carried out to obtain each aberration item by item. Through the gravity turnover test of the mirror, the trefoil aberration in the assembly error and gravity error was separated. The unloading device with certain accuracy was designed. Through the comparative test before and after unloading, the spherical aberration caused by gravity, was obtained. By adopting the above scheme, the measurement of all gravity errors could be realized. The 1.3 m high-lightweight mirror assembly was tested. The gravity error surface figure (rms) and on-orbit surface figure (rms) are 0.192λ (λ=0.6328 μm) and 0.023λ, respectively.
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Key words:
- measurement /
- large aperture mirror /
- 0 g surface figure /
- gravity turnover /
- gravity unloading
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