The SOAR Telescope developed by NOAO and sited on Cerro Pachon, Chile is a 4.1-meter Ritchey-Chretien design incorporating active optics (AO). The AO system is composed of PC-hosted control software, a solid primary mirror supported by 120 electro-mechanical actuators, a lightweighted 600 mm secondary mirror supported by a six degree-of-freedom hexapod mechanism, and a lightweighted 600 mm tertiary mirror controllable over a range of ±100 μrad in two axes with a bandwidth of 50 Hz. The tertiary mirror assembly is in turn mounted on an azimuthal bearing that allows the output bundle of the telescope to be directed to one of five science instruments located at nasymth and bent-cassegrain foci. This paper discusses the active tertiary mirror assembly from the perspective of a control system designer. After a brief overview to establish the tertiary mirror's place in the overall AO system architecture, the paper presents the requirements that drove the design and some of the design’s salient electrical and mechanical features. A model representing electrical and mechanical aspects of the mirror and controller is presented and observed performance metrics such as frequency response, NEA, and measures of servo robustness are compared with values predicted by this model. The paper discusses a number of the design challenges which arose from the requirement to control a massive load with great precision and over a relatively large bandwidth and concludes with the "lessons learned."
The details of the design and the achieved performance of the recently completed primary mirror assembly for the SOAR telescope are described in this paper. The mirror is unique in that it uses a solid 4.3 meter diameter ULE lightweight meniscus primary mirror only 100 mm thick. The ULE substrate weighs approximately 3200 kg. The figure of the primary mirror surface is controlled with 120 electro-mechanical actuators that are force feedback controlled. Details of the design of the 4.1 meter clear aperture F=1.75 active primary mirror are presented. The figure control actuators are force controlled to correct aberrations induced by thermal and elevation changes affecting the telescope. A Calibration Wavefront Sensor provides input data to generate lookup tables used to develop the force commands at all zenith angles and temperatures.
During the design development phase of the program the mirror was analyzed using NASTRAN finite element modeling. During optical metrology testing the ability to accurately induce shapes into the mirror surface including Zernike polynomials was compared to the NASTRAN predictions. Details of the results of this testing will be presented including the magnitude of force to impose specific Zernike polynomial shapes and the fidelity of the resulting shapes compared to the analysis.
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. The 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.
The SOAR Telescope project has completed development of the Active Optical System (AOS) software system. This paper describes the two Computer Software Components (CSCs) that are part of the SOAR/AOS software. The first CSC is referred to as the Operations Control (OpCon) Software. The OpCon Software contains all of the software necessary for running and monitoring the Adaptive Optics Control System (AOCS). This includes the software to run the Primary Mirror Assembly (PMA), to command the Secondary Mirror Assembly (SMA) and the Turret Controller, to set the modes of the Tip/Tilt mirror, and to monitor and report status from the status data acquisition board. It includes the command and data interface to the Telescope Control System (TCS). It includes the AOCS state logic and the input routines for reading the database of command vectors. The second CSC is called the Database Generation (DBGen) Software. The DBGen Software contains the software that generates the database of PM force vectors and SM command vectors. This software uses either theoretical data or measured wavefront data to build the databases.
This paper focuses particularly on the PMA actuator control software. We describe the use of Nastran modeling data for initial deployment of the telescope and the concept for using actual measured data for calibration optimization. We also describe the software implementation designed to allow the actuator control system to meet its timing requirements during telescope slew and to meet the primary figure requirements during telescope observations.
The SOAR Telescope project has embarked on the development of a very high quality 4.2-meter diameter optical telescope to be sited on Cerro Pachon in Chile. The telescope will feature an image quality of 0.18 arc seconds, a moderate field of 11 arc minutes, a very large instrument payload capacity for as many as 9 hot instruments, and an Active Optical System optimized for the optical to near IR wavelengths. The active optical system features a 10 cm thick ULETM primary mirror supported by 120 electro- mechanical actuators for a highly correctable surface. the 0.6 meter diameter secondary is articulated by a hexapod for real time optical alignment. The 0.6-meter class tertiary will provide fast beam steering to compensate for atmospheric turbulence at 50 hertz and a turret for directing the light to either of two nasmyth or three-bent cassegrain ports. Both the secondary and tertiary are light- weighted by machining to achieve cost-effective low weight mirrors. This paper discusses the unique features of this development effort including many commercial products and software programs that enable its technical feasibility and high cost efficiency.