Control for adaptive optics
Project members: M. Verhaegen, K.J.G. Hinnen, N. Doelman
Sponsored by:
TNOTPD
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 groundbased
astronomical imaging, which results in a considerable improvement
of the image resolution (Figure 29). Nowadays, most
of the leading groundbased 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 groundbased 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 realtime 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.
 Nonlinearities 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)''

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