MICADO is the Multi-AO Imaging Camera for Deep Observations, a first light instrument for the Extremely Large Telescope (ELT). The instrument provides imaging, astrometric, spectroscopic and coronographic observing modes. MICADO will be assisted by a Single-Conjugate Adaptive Optics (SCAO) system and the Multi-conjugate Adaptive Optics RelaY (MAORY). The instrument will provide a narrow (19′′) and a wide (51”) Field of View. MICADO can operate in the so-called stand-alone mode in the absence of MAORY with the SCAO correction alone. Here, we present the opto-mechanical design of the Relay Optics (RO), the optical system relaying the ELT focal plane to an accessible position for MICADO using the SCAO-only stand-alone observing mode. The RO consists of an optical bench made of carbon fiber reinforced plastic (CFRP), an optical assembly made of three flat mirrors with motorized piston-tip-tilt mounts and three additional powered mirrors of up to ~500 mm in diameter, the MICADO calibration assembly, and a cover to protect all opto-mechanical components on top of the bench. A 9-point whiffletree support, combined with a thermal compensation system is implemented for the critical mirrors. The static and the dynamic performance of the MICADO RO are investigated through a detailed Finite Element Model (FEM), the results are combined with a Zernike basis representation of the mirror surface deformations performed in Zemax for assessing the optical performance.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) employs an airborne telescope with a 2.7m primary mirror. The telescope structure is composed of carbon fibre with major parts of steel for the suspension and balancing components. It is exposed to harsh environmental conditions and subject to vibration excitation due to aircraft motions and turbulence from the airflow coming into the telescope cavity. To meet pointing requirements and improve image stability there are ongoing efforts on various components of the telescope system, one of which is the implementation of an Active Mass Damping (AMD) control system: Based on accelerometer signals, reaction mass actuators impose forces onto the support structure to dampen the vibration of optical components. The system has been designed, implemented and preliminary tested in the early years of SOFIA’s scientific operation, but concerns about the structural integrity of the primary mirror and new requirements regarding software qualification have prevented the activation and further development for several years. These concerns being addressed, we are now in the process of reactivating the AMD system on the support structure of the primary mirror. Recent ground tests and in-flight jitter measurements indicate that the damping system is very efficient at eliminating the excitation of targeted structural modes of the telescope structure at 40 to 80 Hz and the first bending modes of the primary mirror at 175 Hz, resulting in a significantly improved image quality. This paper presents the analysis of those measurements and discusses options for future development.
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