Control for adaptive optics

Project members: M. Verhaegen, K.J.G. Hinnen, N. Doelman

Sponsored by: TNO-TPD

Adaptive optics (AO) is a technique to actively sense, estimate and correct the wavefront distortions that are introduced in a light beam as it propagates through turbulent media [21,28]. One important application is to counteract the effects of atmospheric turbulence in ground-based astronomical imaging, which results in a considerable improvement of the image resolution (Figure 29). Nowadays, most of the leading ground-based telescopes are equipped or being retrofitted with some kind of AO system. In this project we focus on the control aspects of adaptive optics. Our ultimate goal is to develop innovative control strategies for AO in general, with a main emphasis on systems dedicated to ground-based imaging.

The current generation of AO systems are often based on static control algorithms that are implemented as explicit matrix multiplications. These algorithms are usually derived from physical insights. The goal of this project is to apply modern control strategies to AO, which take into account the dynamics of the wavefront sensor, the deformable mirror and the turbulent atmosphere. The control of large AO systems poses a number of interesting research problems:

• Modeling the disturbances, i.e. the wavefront distortions introduced by the turbulent atmosphere In order to apply modern control strategies, a model of the disturbance is required. The main challenge is to model both the temporal as well as the spatial correlation of the wavefront distortions over the telescope aperture plane.

• Development of an algorithm to predict the wavefront distortion
  The wavefront sensor inherently introduces a delay. Therefore it is desirable to introduce a predictor to estimate the current wavefront distortion. To this end an accurate model of the distortions is required.

• Dimension of the control problem
  Current AO systems incorporate a few hundred to about one thousand sensors and actuators. The wavefront corrections have to be applied in real-time with a frame rate in the order of a few hundred Hertz, which imposes considerable demands on the computational power. This issue will become more and more important since the number of sensors and actuators of future AO systems is expected to increase. As the algorithms become more complex it may be necessary to consider distributed control.

• Non-linearities in the wavefront sensor and deformable mirror.

Figure 29: (left) Paranal Observatory, ``Credit European Southern Observatory (ESO)'' (right) Image of a galaxy with and without AO compensation ``Credit Canada, France, Hawaii Telescope (CFHT)''