Absolute tilt from a laser guide star: A first experiment

Absolute tip-tilt recovery using a tilt signal measured on a Laser Guide St ar is a central problem in the framework of the development of Adaptive Opt ics Systems reaching full sky coverage down to visible wavelengths. In the past few years, various techniques aimed at solving this problem have been proposed. However only a couple of these has been recently tested in practi ce. We report about an experiment aimed at evaluating the performance of on e of these techniques called the `Elongation Perspective' technique. Our ex periment has been performed using the ALFA system in Calar-Alto (Spain) and involves the simultaneous operation of the 3.6 m and the 2.2 m telescopes at the Observatory. This article describes the telescope configuration used , as well as the data reduction process carried out in order to estimate th e scientific object tilt. The technique performances are discussed in terms of the residual tilt error variance and related correlation coefficient. T he analysis shows that, despite the low SNR of our measurements, the atmosp heric tilt variance is reduced to 80% of its initial value corresponding to a correlation coefficient of about 0.6. To get a better estimate of the pe rformance achievable using this technique, the tilt error variance due to p hoton noise in the laser measurement is estimated and removed from the obta ined tilt error variance. When this correction is done, this variance is re duced to about 50% of its initial value, showing that the use of this techn ique can give rise to a significant reduction of the scientific object imag e motion.