Development of an active optical system for the SOAR telescope

ABSTRACT This paper describes the design and summarizes the performance of the recently completed SOAR telescope Active Optical System (AOS). This system is unique in that it uses a thin, solid 4.3-meter diameter ULE™ lightweight meniscus primary mirror only 100 mm thick. The figure of the primary mirror surface is controlled with 120 electro-mechanical actuators that are force feedback controlled. Th e telescope is calibrated against the sky using a calibration wave-front sensor; as this calibration progresses, feedback forces, initially set from finite element analysis predictions, are replaced with sky database look-up tables. The system also includes a 0.6-meter diameter secondary mirror articulated by a hexapod for real-time optical alignment of the telescope, a 0.6-meter class tertiary mirror that also works as a 50 Hz tip tilt corrector to compensate for atmospheric turbulence and a rotary turret mechanism for directing the light to either of two nasmyth or three-bent cassegrain instrument ports. An operation control system interfaces with the telescope control system and each of the hardware assemblies. The paper provides an overview of the design of each assembly as well as summarizes results of performance testing the system. Key Words: Active Primary Mirror, Fast Steering Mirror, Articul ated Secondary Mirror, Lightweight Optics, Adaptive Optics 1. INTRODUCTION The SOAR telescope project is a collaboration between the University of North Carolina at Chapel Hill (UNC), Michigan State University (MSU), Brazil, and the United States National Optical Astronomy Observatory (NOAO), and sited on Cerro Pachon, Chile. The telescope is a 4.1-meter Ritchey-Chretien design incorporating active optics (AO). The SOAR Active Optics System consists of three opto-m echanical assemblies: the force actuator controlled Primary Mirror Assembly; the 5 degree of freedom articulated Second ary Mirror Assembly; and the Tertiary Mirror Assembly, which consists of a 50 Hz fast steering mirror on a rotary turret to direct the beam to one of five instrument ports in the telescope elevation ring. These three assemblies are controlled by and have command interface to the SOAR Telescope Control System (TCS) through the Control System Assembly (CSA). Telescope cables are provided to tie the AOS assemblies mounted on the telescope to the CSA and the TCS. The secondary mirror and tertiary mirror were coated at Goodrich with an enhanced aluminum coating providing thro ughput from the atmospheric transmission cutoff to well into the mid IR. The primary mirror was coated at the Cerro Pachon Gemini coating chamber. 2. OVERALL AOS REQUIREMENTS The far-field performance of the SOAR AOS system for the as-built and mounted primary, secondary, and tertiary mirrors was calculated using the Fourier Transform of the full AOS system phase map. The AOS system level wave-front error is generated based on the measured surfaces of the AOS optics and other error terms consistent with the AOS system level error budget shown in Figure 1. The system leve l values calculation is after th e actuator forces are adjusted to minimize the system rms wave front. Forces are constrained to a force range allowing 10 lbs margin. Actuator