Fabrication of the first GMT ASM reference body

In this study, we present the report about the first segment of the Giant Magellan Telescope (GMT) Adaptive Secondary Mirror (ASM) reference body. The GMT ASM comprises 7 segmented deformable mirrors, including a thin shell, actuators, and a reference body. In this formation, one critical role of the reference body is to provide the reference surface form for the thin shell mirror with supporting actuators. Thus, it is one of key components to succeed in development of GMT ASM. Through collaboration between the Korea Research Institute of Standards and Science (KRISS) and the University of Arizona (UArizona), the first segment of the off-axis reference body has been successfully developed. Produced from Zerodur, the reference body has a light-weighted pattern characterized by a set of non-parallel rib/wall structures. It has 675 cylindrical actuator holes along the surface’s normal vector according to the designed off-axis aspheric form. A 5-axis computer numerical control (CNC) machine fabricated this complex structure. The front surface was ground from a best-fit sphere to an off-axis asphere using a large polishing machine. A laser tracker was employed to meticulously measure the surface form errors of the front surface under the grinding step during the fabrication process. After polishing, the mechanical measurement on a large Coordinate Measuring Machine (CMM) and the optical measurement using Software Configurable Optical Test System (SCOTS) confirmed that the GMT ASM reference body adhered to the stringent surface form error requirement. This product has been shipped to AdOptica, Italy, for the final assembly of the ASM with actuators and the thin shell.