The Powerful Green Laser of William Herschel Telescope

In the foreground we can see the open dome and powerful green laser from William Herschel Telescope (WHT) pointing high, up to the zenith, while in the right corner the Milky Way sets behind a layer of clouds in the horizon, below Roque de Los Muchachos mountain, in La Palma. The CANARY laser guide star is in use at the WHT during a few nights per semester, so this was a rare opportunity to capture it.

Development of new instrumentation is crucial for the continued scientific health of any telescope facility. At the WHT a vibrant development programme was in place, focusing on providing the widest possible science use of adaptive optics, a technique that greatly improves image quality by correcting for the degradation due to turbulent motions in the Earth’s atmosphere. Adaptive optics is now well established at several telescopes around the world. The potential of adaptive optics is huge because the improved spatial resolution allows the detection of sources and fine structures in complex systems that would otherwise not be resolved. Examples are the study of dense stellar clusters, cores of relatively nearby galaxies, and complex star-formation regions.

The technique of adaptive optics, although of huge potential, has its limitations. The requirement that a bright point source lies very close to the object of interest implies that less than 1% of the sky is accessible for the technique. There is, however, a solution to this problem, which is to create an artificial “star” by projecting a bright laser beam on the sky. Such a laser beacon assumes the role of the bright star, hence opening virtually all of the sky to observation with adaptive optics. The current developments at the WHT focus on the design and construction of such a laser beacon system, which will result in a dramatic enhancement of the science prospects of the telescope. Interestingly, adaptive optics and laser beacons are crucial for the next generation of extremely large telescopes that are currently being planned. These future telescopes, with unprecedented large primary mirror diameters of 30 or even 100m, require many solutions to be found for a range of technological problems. Thanks to the ongoing developments in this area at the WHT this telescope is well placed to play a key role as a testbed facility where techniques for these future telescopes can be explored under realistic conditions.