Design of the Science-Fold Mirrors for the Gemini Telescopes

As a part of the Acquisition and Guidance Unit for the Gemini project a light-weight, 50cm flat mirror has been designed at the Fraunhofer Institute for Applied Optics and Precision Mechanics in Jena as a subcontractor of the Carl Zeiss Jena company. A light-weight design of the mirror and its mount was essential since the total mass of the whole assembly including the positioning system was limited to 50 kg while interferometric quality of the mirror surface was required for arbitrary orientation. The overall surface error was below 54 nm r.m.s. while 27 nm was achieved in the central part. The mirror was fabricated from low-expansion glass ceramics to avoid thermally induced deformations. By milling pockets into its rear surface the mass of the mirror was reduced by 70%. The mirror is mounted kinematically via six solid-state hinges to three steel levers. The levers are connected to the mount frame at their centers via ball-and-sphere joints. This arrangement determines the position of the mirror uniquely while it allows for the thermal expansion of the mount frame. The position of the mirror as well as its tilt around an axis perpendicular to the optical one may be controlled a precision of 20µm and 3 arcsec, respectively. The tilt axis is driven directly by two high-torque motors. To avoid an excessive power consumption of the motors the torque of the mirror had to be compensated for by a counterweight mechanism. The mirror may be deployed into the optical path using spindle driven linear rails.