太空光學遙測酬載之主鏡光機系統組裝

Primary Mirror Alignment and Assembly for a Spaceborne Cassegrain Telescope by Using CMM

福衛5號光學遙測酬載為國人第一套自主研發之光學系統，該光學系統為一種常應用在天文、太空領域之中、大型望遠鏡型態──卡賽格林式望遠鏡（Cassegrain telescope）。由反射式之主、次鏡與修正像差之折射式修正鏡組所組成，主鏡（primary mirror，M1）的型式是拋物面鏡，次鏡（secondary mirror，M2）則是雙曲面鏡。為符合火箭承載能力需求，亦可兼顧光學品質與結構強度，本文介紹之太空光學遙測酬載之主鏡透過雙足支撐結構（bipod flexure）與望遠鏡之主結構（main plate）結合，光學系統結構由六組碳纖維複合材料（carbon-fiber-reinforced polymer，CFRP）桿件所組成的桁架結構連結主鏡與次鏡的支撐結構。光機系統組裝除達到機械組裝之要求亦須兼顧光學成像品質，因此如何設計多自由度之調整機構與如何達到最佳之組裝型態是光機系統發展中重要的一環。本文介紹利用三次元量床CMM（coordinate-measuring machine）輔助太空光學遙測酬載主鏡姿態調整方法、調整治具（mechanical ground support equipment）的設計概念、主鏡次系統組裝程序，以及主鏡組裝後之光學品質檢測。 The currently developing spaceborne optical remote sensing payload is a typical Cassegrain telescope with 450 mm clear aperture. The primary mirror is made of ZERODUR® glass ceramic blank and lightweighted with hexagonal cell structure at a ratio about 50% for considering both effect of thermal deformation and the mass budget of the rocket. For this mirror, it is critical to reduce the astigmatism caused from the gravity effect, bonding process and the deformation from the mounting to the main structure of the telescope(main plate) . In this article, the alignment of primary mirror, MGSE for the alignment and assembly process and the optical performance test for the primary mirror assembly(PMA) are presented. To get the best optical performance of PMA, the bonding position of the bipod flexure relative to primary mirror is a critical issue and shall be analyzed by finite element method(FEM) before alignment and assembly process of PMA. The mechanical shim is the interface between the bipod flexure and main plate. It is used to adjust the bonding position for the bipod flexure relative to primary mirror and also compensate the manufacture tolerances of components and differences of local co-planarity errors to prevent the local stress while bipod flexure is screwed to main plate. After primary mirror assembly process, an optical performance test refer to “bench test” is used to analyze the astigmatism caused from the gravity effect and the deformation from the mounting or supporter. In an effort to achieve the requirement for the tolerance in PMA, the astigmatism caused from the gravity and deformation by the mounting force can be controlled within P-V 0.02 l at 632.8 nm. The consequence of these demonstrations indicates that the assembly technique for the lightweighted primary mirror meet the critical requirements for PMA of the telescope.