KEYWORDS: Telescopes, Fluctuations and noise, Data modeling, Sensors, Adaptive optics, Adaptive optics, Control systems, Control systems, Signal processing, Turbulence, Charge-coupled devices, Device simulation
The Magellan Telescope Adaptive Optics System (MagAO) is subject to resonance effects induced by elements within the system instrumentation, such as fans and cooling pumps. Normalized PSDs are obtained through frequency-based analysis of closed-loop on-sky data, detecting and measuring vibration effects. Subsequently, a space-state model for the AO loop is obtained, using a standard AO loop scheme with an integrator-based controller and including the vibration effects as disturbances. Finally, a new control alternative is proposed, focusing on residual phase variance minimization through the design and simulation of an optimal LQG control approach.
Mechanical vibrations affect the performance in modern adaptive optics systems. These structural vibrations induce aberration mainly in tip-tilt modes that reduce the accuracy of the astronomical instrument. Therefore, control actions need to be taken. With this purpose we present a laboratory demonstration of vibration rejection of tip-tilt modes using closed-loop control, inducing vibration on the test bench via an eccentric motor with controllable frequency, in order to simulate the structural vibrations mentioned above. We measure the laser vibration and its tip-tilt aberration using a camera and a Shack Hartmann Wave Front Sensor. The control action is carried out by a Fast Steering Mirror (FSM).
Frequency-based analysis and comparisons of tip-tilt on-sky data registered with 6.5 Magellan Telescope Adaptive Optics (MagAO) system on April and Oct 2014 was performed. Twelve tests are conducted under different operation conditions in order to observe the influence of system instrumentation (such as fans, pumps and louvers). Vibration peaks can be detected, power spectral densities (PSDs) are presented to reveal their presence. Instrumentation-induced resonances, close-loop gain and future challenges in vibrations mitigation techniques are discussed.
The adaptive optics system performance depends on multiple factors, including the quality of the laser beam before being projected to the mesosphere. Cumbersome procedures are required in the laser system to optimize the laser beam in terms of amplitude and phase. However, aberrations of the laser beam are still detected during the operations. The performance of laser projection systems can be improved compensating the effects of aberrations in the laser source or misalignment in the transfer optics before the laser beam propagating through the aperture. Despite the algorithm previously reported predict effective amplitude and phase correction is strongly dependent of an accurate DM characterization and transfer optics alignments. The use of feedback makes the system response better in presence of modeling error and external disturbances. A 2-DM closed loop approach for amplitude and a phase correction is designed. Finally the results of simulations and comparisons are discussed.