Atmospheric Tomography in Adaptive Optics

Turbulent air motion in the Earth’ atmosphere causes index of refraction fluctuations. In atmospheric tomography one aims at reconstructing this atmospheric turbulence from light wavefronts of bright astronomical objects or laser beacons from several directions that are measured on the ground.

Atmospheric tomography is needed in tomographic Adaptive Optics (AO) systems in large ground-based telescopes. AO is a technology that improves image quality by means of deformable mirrors which correct for atmospheric turbulence in real-time, i.e., at around 500 Hertz. Therefore, fast algorithms which yield a good reconstruction quality on few layers are needed.

Figure 1: Atmospheric tomography

The atmosphere is approximated by several infinitely thin layers which are weighted with the strength of turbulence at the corresponding height. The choice of the number of layers, their heights and turbulence weights, is a crucial question for reconstruction quality and speed, in particular in the context of the new generation of Extremely Large Telescopes with mirror diameters of around 40m.

Figure 2: The 3-step approach for tomographic AO systems

Within the project described in yesterday’s post, I was working on a successful approach for tomographic AO systems with multiple guide stars, the 3-step approach presented in Figure 2. It decouples the problem into the reconstruction of the incoming wavefronts from wavefront sensor data, the reconstruction of the turbulent layers (atmospheric tomography) from the reconstructed incoming wavefronts and the computation of the optimal mirror shape (by projection) from the reconstructed atmosphere.

Such a flexible and fast solver allows to control tomographic ELT-sized AO systems in real time with existing hardware. This significantly contributes to bringing such complex AO systems into the realm of feasible implementations.