Calar Alto ALFA and the sodium laser guide star in Astronomy

ABSTRACT Adaptive optics (AO) coupled to laser guide star systems is crucial to future ground based astronomical observations. It allows correction of image distortion caused by the Earth's turbulent atmosphere, over a hugely larger fraction of the sky than achieved by using only natural stars. Yet there are still very few such systems producing any sort of scientific results. ALFA , now offered on a shared risk basis as a user-instrument at Calar Alto Observatory in Spain, is continuing to improve its performance during closed loop operation on both natural and laser guide stars (LGS). The ability to close the loop on the LGS through thin cirrus cloud (which would remain unseen during normal observing) has the potential to increase the number of nights previously considered suitable for the laser by a factor of about two. In particular, science observations carried out on such a night are described. As part of the TMR † network for Laser Guide Stars at Large Telescopes we are studying the distribution of atoms in the mesospheric sodium layer and its' evolution over time. Additionally, a new experiment to provide an on-line monitor of the mesospheric sodium layer has been proposed and the results of a simulation are presented. This study will be of importance to large telescopes with laser stars at good astronomical sites where accurate statistics of the sodium layer are required, both for optimal scheduling of observations and for keeping the wavefront sensor focussed on the LGS