Disturbance Compensation for Extremely Large Telescopes

Responsible Dipl.-Ing. Michael Böhm (since 10/2011)
Dipl.-Ing. Alexander Keck (since 01/2012)
Dipl.-Ing. Martin Glück (since 10/2014)


Partner Max-Planck-Institute for Astronomy, LBTO

 

 

 

Description

The Large Binocular Telescope (LBT) is the worlds most advanced optical and infrared telescope. It utilizes two symmetrical mirror system each of which has a primary mirror with 8.4m in diameter and an adaptive secondary mirror with approximately 1m in diameter. Research in this field is also done at this Institute, see above.

During operation of the telescope many unwanted vibrations occur, due to wind forces or even heavy machinery needed to operate the telescope inducing disturbances of high frequency. For incoming light, this results in a difference in the optical path to the detector between the two symmetric mirror systems leading to fringe pattern.

Therefore, this project aims at finding methods for reliably estimating the telescope's vibrations using available acceleration sensors. From this, the optical pathway difference has to be estimated. Subsequently, a controller will be designed for correcting the disturbed optical path by the means of an opto-mechanical cerrection device keeping the maximum phaseshift between the two optical paths below 100nm. The underlying control algorithm for the movable piezo-actuated optical correction device has also been developed at the ISYS.

The picture above was taken at the telescope's construction site and one can already see the large primary mirrors (attached to the red supporting structure) and the two adaptive secondary mirrors (at the extension arms painted black). The two adaptive secondary mirrors can deform their surface using 672 voice coil actuators each. This is used for real time phase shift correction of the incoming light to account for atmospheric turbulences.

The project can be divided into five major parts:

  • Mathematic modeling and simulation of the telescope
  • Comparison of the simulation results to real measurement data
  • Identification, validation and adjustment of the telescope model
  • Observer design to estimate vibrations and the optical pathway difference
  • Controller design for the opto-mechanical correction device

 

The 39,3 meter (main mirror diameter) European Extremely Large Telescope (E-ELT) will beequipped with a new camera design, the MICADO (Multi Adaptive Optics Imaging Camera for Deep Observations), which permits to improve the E-ELT's spatial resolution by a factor of five and the light gathering power by a factor of twenty compared to existing giant telescopes.
MICADO, the First Light camera of the E-ELT, is developed by the Max-Planck-Institute for Astronomy with support from ISYS.

The aim of this project is to compensate for image distortions caused by atmospherical turbulences and image distortions caused by wind-induced mechanical vibrations of the telescope's structure using two separated control systems. The goal is to reduce the disturbance load on the classic adaptive optics control loop by adding a model-based disturbance feedforward loop to the system.

The classic adaptive optics control loop consists of a wavefront sensor to obtain an optical measurement of the image distortions caused by atmospheric turbulences and mechanical vibrations of the telescope's structure and a deformable mirror to compensate for these disturbances. Typically, this feedback loop is used to compensate for both disturbance sources.

Since the two disturbance sources typically have spectral components which are far apart, there are contradictory requirements for the optimal tuning of the adaptive optics control loop. So far, the control loops use very large sampling rates on the wavefront sensor to compensate for high-frequency disturbances, largely deterorating the resolution during observations of faint stars.

The new control approach which is already used by the ISYS for the Large Binocular Telescope (LBT), utilizes accelerometers to measure the movements of the optical system. Because this main source for high-frequency distortions is taken care for by the accelerometers, the wavefront sensor can be run at much smaller sampling rates, permitting the observation of faint stars.

The project is divided into the following parts:

  • Design and construction of a laboratory test setup to reconstruct the imaging distortions and the compensation
  • Development of a dynamical model of the mechanical system and the optical components
  • Design of a model-based disturbance feedforward loop which permits to decrease the bandwidth of the adaptive optics feedback loop
  • Transfering the results to the E-ELT's design

 

Partner

The LBT- and the E-ELT-Project is realized in closed cooperation between the Max-Planck-Institute for Astronomy in Heidelberg, which also funds this research.

Publications
Journal Papers
  • M. Böhm, J.-U. Pott, M. Kürster, O. Sawodny, D. Defrère & P. Hinz, “Delay Compensation for Real Time Disturbance Estimation at Extremely Large Telescopes”, IEEE Transactions on Control Systems Technology, 2016, doi:10.1109/TCST.2016.2601627
  • M. Böhm, J.-U. Pott & O. Sawodny, “Real Time Vibration Compensation for Large Telescopes”, Monthly Notices of the Royal Astronomical Society, 442, pp. 2446-2455, 2014, doi:10.1093/mnras/stu1012
Conference Papers
  • M. Böhm, J.-U. Pott, J. Borelli, P. Hinz, D. Defrère, E. Downey, J. Hill, K. Summers, A. Conrad, M. Kürster, T. Herbst & O. Sawodny, “OVMS-plus at the LBT. Disturbance compensation simplified.”, SPIE Conference on Astronomical Telescopes and Instrumentation, Edinburgh, UK, 2016, pp. 99062R, doi:10.1117/12.2231268
  • M. Glück, J.-U. Pott & O. Sawodny, “Simulation of an Accelerometer-based Feedforward Vibration Suppression in an Adaptive Optics System for MICADO”, SPIE Conference on Astronomical Telescopes and Instrumentation, Edinburgh, UK, 2016, doi:10.1117/12.2231852
  • A. Keck, J.-U. Pott & O. Sawodny, “Accelerometer-based online reconstruction of fast telescope vibrations from delayed measurements”, SPIE Conference on Astronomical Telescopes and Instrumentation, Edinburgh, UK, 2016, doi:10.1117/12.2231667
  • A. Keck, J.-U. Pott & O. Sawodny, “Accelerometer-based online reconstruction of vibrations from delayed measurements”, IEEE Conference on Control Applications (CCA), Sydney, NSW, Australia, 2015, doi:10.1109/CCA.2015.7320666
  • M. Böhm, J.-U. Pott, O. Sawodny, T. Herbst & M. Kürster, “Real Time Estimation of Differential Piston at the LBT”, SPIE Conference on Astronomical Telescopes and Instrumentation, Montreal, Canada, 2014, doi:10.1117/12.2055833
  • A. Keck, J.-U. Pott & O. Sawodny, “Accelerometer-based position reconstruction for the feedforward compensation of fast telescope vibrations in the E-ELT/MICADO”, SPIE Conference on Astronomical Telescopes and Instrumentation, Montreal, Canada, 2014, doi:10.1117/12.2053769
  • A. Keck, J.-U. Pott & O. Sawodny, “Accelerometer-based online reconstruction of vibrations in extremely large telescopes”, IFAC World Congress, Cape Town, South Africa, 2014, pp. 7467-7473, doi:10.3182/20140824-6-ZA-1003.00265
  • M. Böhm, J. Pott, M. Kürster & O. Sawodny, “Modeling and Identification of the Optical Path at ELTs – a Case Study at the LBT”, IFAC Symposium on Mechatronic Systems, Hangzhou, China, 2013, pp. 249-255, doi:10.3182/20130410-3-CN-2034.00014
  • A. Keck, J.-U. Pott & O. Sawodny, “An accelerometer based dual loop approach to minimize the impact of fast telescope vibrations seen by the E-ELT/MICADO wavefront sensors”, Adaptive Optics for Extremely Large Telescopes Conference (AO4ELT), Florence, Italy, 2013
  • M. Böhm, T. Ruppel, J.-U. Pott, O. Sawodny, T. Herbst & M. Kürster, “Modelling the optical pathway of the Large Binocular Telescope”, SPIE Conference on Astronomical Telescopes and Instrumentation, Amsterdam, Netherlands, 2012, doi:10.1117/12.926293
  • A. Keck, J.-U. Pott, T. Ruppel & O. Sawodny, “Development of new concepts to minimize the impact of fast telescope vibrations seen by the E-ELT/MICADO wavefront sensors”, SPIE Conference on Astronomical Telescopes and Instrumentation, Amsterdam, Netherlands, 2012, doi:10.1117/12.927030
Other Publications
  • M. Böhm, J.-U. Pott & O. Sawodny, “Störgrößenkompensation im optischen Pfad von Riesenteleskopen mittels aktiver Optik”, 2013