The Stratospheric Observatory For Infrared Astronomy (SOFIA) reached its full operational capability in 2014 and completed hundreds of observation flights. Since its installation in 2002, the Secondary Mirror Mechanism was subject to thousands of operating hours equivalent to millions of load cycles. During the aircraft heavy maintenance in fall 2014, a four month time window enabled the removal of the mechanism from the telescope structure for service and improvements. Next to visual corrosion- and crack-inspection of the flexures, critical electronic components (in particular the set of three eddy current position sensors that determine the mirror tilt) were replaced. Moreover, a detailed temperature dependent position calibration of the system was performed in a cold chamber to improve the pointing accuracy. Until then, a simple temperature independent linear gain was used to translate the sensor output voltage into a position. For accurate positioning across the whole temperature range, a temperature dependent correction function had to be developed. This calibration would have cost hours of observing time when performed in flight which made it an essential goal for completion during the maintenance period. An autocollimator was used as optical reference camera to measure the tip-tilt position of the secondary mirror in the cold chamber. Using this calibration setup, a pattern of many mirror positions in the tip-tilt domain was approached at several temperature points to provide a high resolution data set for the new multidimensional calibration function. Follow-up in-flight verification measurements confirmed a large improvement in pointing accuracy as soon as the temperature measurements were included into the position correction. Improvements of up to a factor of 10 were especially noticed in the lower temperature range. This contribution provides an insight into the work performed during the SOFIA - Secondary Mirror Mechanism maintenance with the focus on the temperature dependent position calibration.
The M5 Field stabilization Unit (M5FU) for European Extremely Large Telescope (E-ELT) is a fast correcting optical
system that shall provide tip-tilt corrections for the telescope dynamic pointing errors and the effect of atmospheric tiptilt
and wind disturbances.
A M5FU scale 1 demonstrator (M5FU1D) is being built to assess the feasibility of the key elements (actuators, sensors,
mirror, mirror interfaces) and the real-time control algorithm. The strict constraints (e.g. tip-tilt control frequency range
100Hz, 3m ellipse mirror size, mirror first Eigen frequency 300Hz, maximum tip/tilt range ± 30 arcsec, maximum tiptilt
error < 40 marcsec) have been a big challenge for developing the M5FU Conceptual Design and its scale 1
The paper summarises the proposed design for the final unit and demonstrator and the measured performances
compared to the applicable specifications.
A 42 meters telescope does require adaptive optics to provide few milli arcseconds resolution images. In the current
design of the E-ELT, M4 provides adaptive correction while M5 is the field stabilization mirror. Both mirrors have an
essential role in the E-ELT telescope strategy since they do not only correct for atmospheric turbulence but have also to
cancel part of telescope wind shaking and static aberrations. Both mirrors specifications have been defined to avoid
requesting over constrained requirements in term of stroke, speed and guide stars magnitude. Technical specifications
and technological issues are discussed in this article. Critical aspects and roadmap to assess the feasibility of such
mirrors are outlined.
The paper presents an innovative technical solution which provides a combined damping and isolation interface with the appropriate transmissibility characteristics between a vibrating base and a sensitive payload, typically an optical terminal/telescope. The novelty of the solution is primarily found in the implementation of uncoupling and magnification of the incurred vibrations by means of flexures combined with the implementation of energy dissipation by means of a linear electro-magnetic actuator to constitute a passive integrated resistor-damped electromechanic lever block. By means of frictionless flexible lever systems, the amplitude of the payload vibrations is adapted to the optimal range of the actuator with a magnification by a factor ranging typically between 10 and 30. Passive viscous damping is obtained by simply short-circuiting the electro-magnetic motor and can be adapted by setting the impedance of the shorting connection. The desired stiffness is provided by the passive springs of the elastic motor suspension and by the stiffness of the lever flexure blades. The mobile mass of the motors also provide a reaction mass which, like damping and stiffness, is amplified by the square of the lever factor. A theoretical model of resistor-damped electromechanical lever blocks has been established. A particular property is it the good attenuation of excited vibrations only over a set frequency range. Above this range the interface properties rejoin the ones of a rigid connection. This performance makes this type of isolators particularly suitable for integration into multi-layer vibration control systems where sensitive equipment is protected by a mix of passive and active damping/isolation devices acting optimally at different frequency ranges. Experiments performed with a dummy load (80 Kg) representative of a satellite based optical terminal demonstrated the efficiency of the system in protecting the payload by passive damping for vibration excitations of amplitude up to 0.15 mm and frequency up to about 120 Hz achieving an attenuation of the eigenmodes of the load structure by more than 20 dB.
Active and adaptive structures, also commonly called 'smart' structures, combine in one integrated system various functions such as load carrying and structural function, mechanical (cinematic) functions, sensing, control and actuating. Originally developed for high accuracy opto-mechanical applications, CSEM's technology of flexure structures and flexible mechanisms is particularly suited to solve many structural and mechanical issues found in such active/adaptive mechanisms. The paper illustrates some recent flexure structures developments at CSEM and outlines the comprehensive know-how involved in this technology. This comprises in particular the elaboration of optimal design guidelines, related to the geometry, kinematics and dynamics issues (for instance, the minimization of spurious high frequency effects), the evaluation and predictability of all performance quantities relevant to the utilization of flexure structures in space (reliability, fatigue, static and dynamic modeling, etc.). material issues and manufacturing procedures.