Universe Colloquium: Making Stars: Sodium Laser Guide Stars in Terrestrial Astronomy

by Ronald Holzlöhner (ESO)

Wednesday 7 May 2014, 16:30 → 17:30 Europe/Berlin

seminar room (Excellence Cluster Universe, Boltzmannstrasse 2, Garching)

Terrestrial astronomy suffers from wavefront distortion due to atmospheric turbulence: The resolution limit in the visible, depending on the site, lies in the range of 0.5-2 arcseconds, corresponding to the diffraction limit of a telescope with only 6-25cm clear aperture. The detrimental effect of turbulence can be significantly mitigated with the help of adaptive optics, in which the shape of one or several mirrors in the optical train is actively controlled using a real-time closed loop system. The control signal is derived from the continuous observation of a bright star in the telescope field of view with wavefront sensors and updated at a rate of 500-1000 Hz. Since many areas of the night sky lack sufficiently bright stars, it has become common in large telescopes such as ESO's Very Large Telescope (VLT) in the Atacama desert to employ laser guide stars (LGS). To make these, a powerful laser beam is propagated onto the sky parallel to the optical axis. Sodium LGS excite a diluted natural sodium layer in the mesosphere at 80-110km altitude through resonant fluorescence at 589nm and thus provide artificial beacons to adaptive optics. In the presentation Ronald Holzlöhner will discuss the involved physics and highlight some of the technical challenges. The Universe Colloquium is followed by a social gathering with wine and cheese.