Laser Guide

The yellow light from one of the four 8-meter VLT telescopes is a laser guide star for Adaptive Optics (AO), a technology to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion, most commonly used to remove the effects of atmospheric distortion. AO systems require a wavefront reference source in order to correct atmospheric distortion and because bright stars are not available in all parts of the sky, the system shine a laser into the atmosphere to create a yellow artificial “star” in the 95 km altitude sodium cloud around the Earth.