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Disturbance Compensation for Extremely Large Telescopes

The Institute for Systemdynamics develops methods for disturbance estimation and compensation in several projects to achieve diffraction limited imaging in large telescopes. Most of the projects are in cooperation with the Max Planck Institute for Astronomy (MPIA).
[Photo: ESO]

Current Research Projects

Disturbance Compensation for the Extremely Large Telescope

The largest ground-based telescope, the Extremely Large Telescope (ELT), with a primary mirror diameter of 39 m is currently built in the Atacama desert in Chile. The diffraction limit will be five times and the light collection area 25 times larger than the current 8m-class large telescopes. First light of the telescope is expected in 2024, where the telescope is equipped with three instruments

  • MICADO (Multi Adaptive Optics Imaging Camera for Deep Observations),
  • METIS (Mid-infrared ELT Imager and Spectrograph),
  • HARMONI (High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph).

The Max Planck Institute for Astronomy (MPIA) is involved in the development and construction of MICADO and METIS within international consortia, where the Institute for Systemdynamics (ISYS) supports the MPIA in the field of advanced Adaptive Optics (AO) control.

Atmospheric turbulences and wind induced structural vibrations at the telescope disturb the images of the science cameras. To achieve diffraction-limited observations with the first light instruments, the ELT respectively the first light instruments are equipped with AO. The standard AO-control loop uses a wavefront sensor and a deformable mirror to measure and correct these disturbances.

 

Image of the ELT

 

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Development of new concepts for the compensation of high frequency structural vibrations at the ELT

In this project we investigate the impact of structural vibrations at the ELT on the performance of the MICADO instrument and the compensation of those within the AO control loop. Current AO control concepts are extended with an additional accelerometer-based disturbance feedforward control to compensate the vibrations.

 

Picture of MICADO

 

Due to the telescope mirror mounts high frequency vibrations (5 Hz – 50 Hz) are induced into the optical path and cannot fully mitigated for the observation with faint natural guide stars. The exposure time of the wavefront sensor is increased for a better signal-to-noise ratio, where the AO loop bandwidth is reduced. Therefore, the high frequency vibrations cannot be measured and are not available for compensation.

The ISYS works on a new control concept, where the structural vibrations are measured with additional accelerometers at the telescope mirrors. The vibrations can be measured with a faster sample rate and used for a disturbance feedforward control. This control concept has already been tested at the Large Binocular Telescope (LBT) for the compensation of the optical path difference due to vibrations in the LBT Interferometer (LBTI). In this project we extend the method for the compensation of the tip-tilt modes in the AO.

Goals of the project:

  • Design and construction of a laboratory test setup to reconstruct the imaging distortions and the compensation
  • Development of an accelerometer-based disturbance feedforward control for the tip-tilt modes
  • Optimal distribution of the disturbances to M4 and M5 in connection with the disturbance feedforward control
  • Transferring the results to MICADO and ELT

This project is funded by the Federal Ministry of Education and Research.

Partner

Max Planck Institute for Astronomy (Heidelberg, Germany)

Responsible

Dipl.-Ing. Martin Glück

Development of advanced AO Control Algorithms for METIS

One goal of METIS is to find new exoplanets and to analyze their atmosphere in the mid-infrared regime (3 - 19 µm). In order to achieve the goals of METIS, images with maximum contrast are needed, which can only be achieved with a very fast (> 500 Hz) and extremely precise AO. In this project, we develop advanced AO controllers, which compensate atmospheric turbulences and structural vibrations for diffraction-limited observations.

 

CAD-Picture of METIS

 

The AO control loop of METIS consists of

  • a pyramidal wavefront sensor to detect the image disturbances and
  • the deformable mirror M4 and the tip-tilt mirror M5, to compensate these disturbances.

The deformable mirror M4, equipped with 5316 actuators, can be deformed with a high spatial resolution and has a fast settling time. Because the M4-actuator strokes are limited, M4 cannot fully compensate the image disturbances dominated by tip-tilt. Therefore, the AO-control loop uses the tip-tilt mirror M5 additionally to M4. Since the mirror M5 has a diameter of approximately 2.5m, it can only provide limited compensation of high frequency disturbances.

Goals of the projects:

  • Creating an AO end-to-end simulation
    • modelling of atmospheric turbulences and structural vibrations
    • modelling the dynamic behavior of the active mirrors M4 and M5 at the ELT
  • Development of advanced model-based AO control concepts
    • handling of M4-actuator errors
    • data-driven models for the compensation of atmospheric turbulences
    • concepts for splitting the distrubance compensation between M4 and M5
Further information on METIS:
Partner

Max Planck Institute for Astronomy (Heidelberg, Germany)

Responsible

M.Sc. Philip Neureuther

Completed Research Projects

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.

 

Picture of the LBT

 

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 was 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
Partner

Max-Planck-Institut für Astronomie (Heidelberg, Germany), Large Binocular Telescope Observatory (Tucson, Arizona, USA)

Responsible

Dr.-Ing. Michael Böhm

Publications

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  • 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 Technology2016doi: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 Society442pp. 2446-24552014doi:10.1093/mnras/stu1012
  • 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 InstrumentationEdinburgh, UK2016pp. 99062Rdoi: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 InstrumentationEdinburgh, UK2016doi: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 InstrumentationEdinburgh, UK2016doi: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, Australia2015doi: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 InstrumentationMontreal, Canada2014doi: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 InstrumentationMontreal, Canada2014doi:10.1117/12.2053769
  • A. Keck, J.-U. Pott & O. Sawodny“Accelerometer-based online reconstruction of vibrations in extremely large telescopes”IFAC World CongressCape Town, South Africa2014pp. 7467-7473doi: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 SystemsHangzhou, China2013pp. 249-255doi: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, Italy2013
  • 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 InstrumentationAmsterdam, Netherlands2012doi: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 InstrumentationAmsterdam, Netherlands2012doi:10.1117/12.927030
  • M. Böhm, J.-U. Pott & O. Sawodny“Störgrößenkompensation im optischen Pfad von Riesenteleskopen mittels aktiver Optik”2013

Contact

Dieses Bild zeigt Glück
Dipl.-Ing.

Martin Glück

Research Assistant

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M.Sc.

Philip Neureuther

Research Assistant