Grond-laag Laser Adaptieve optiek Systeem
Ground-layer Laser Adaptive optics System
A Rayleigh laser beacon for NAOMI

In January 2004 the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) announced its full support for the proposed development of a laser beacon for the NAOMI Adaptive Optics system on the 4.2-m William Herschel Telescope. Such a laser guide star system will amplify the fraction of sky available to adaptive optics observations at visible and infrared wavelengths from a few percent to nearly 100%. In terms of astronomical research, this translates into radical progress as it opens up high spatial resolution observations from the ground to nearly all types of science targets. In combination with the existing and planned instrumentation, the WHT will offer a highly competitive facility to the astronomical community, exploiting a window of opportunity before similar capability will exist on 8-m class telescopes.

Picture taken of a laser experiment at the WHT conducted by the Astronomical Instrumentation Group of Durham University.

Astronomical and Strategic Motivation

Adaptive optics (AO) techniques allow ground-based observers to obtain spatial resolutions better than a tenth of an arcsecond by correcting the image blurring introduced by the Earth's atmosphere. Hence the resulting image sharpness not only carries the advantage of distinguishing finer structure and avoiding source confusion in dense fields, but also allows observations to reach significantly fainter, as the sky background component reduces with the square of the angular resolution. For these reasons, AO instrumentation is being planned for nearly all large telescopes, and is at the heart of the future generation of extremely large telescopes. At the 4.2-m William Herschel Telescope (WHT), AO recently came to fruition with the commissioning of the common-user AO system, NAOMI, and an aggressive instrument development programme. A main practical limitation for AO is the availability of bright guide stars to measure the wavefront distortions, which has caused AO in general to produce fewer science results than one might have expected from its potential. By using an artificial laser guide star this limitation is largely taken away, thus opening up AO to virtually all areas of observational astronomy and to virtually all positions in the sky. In particular, it opens up the possibility of observing faint and extended sources, and will enable observations of large samples, unbiased by the fortuitous presence of nearby bright stars. With a laser guide star facility, a 4-m class telescope situated on a good observing site like La Palma is highly competitive for AO exploitation next to the larger telescopes. Scientific exploitation of AO encompasses all fields where achieving high spatial resolution and/or sensitivity is needed to progress. The laser guide star system for the WHT will take many areas of research from the current situation where single objects can be studied, which happen to be near a bright natural guide star, to a situation where all objects of interest can be studied under the same conditions, regardless of their position in the sky. This implies progress from case studies, to studies of well-defined samples of objects. Examples are the search for brown dwarfs and disks around solar type stars in obscured star formation regions, super massive black holes, dynamics of nearby galaxy cores, circumnuclear starbursts & AGN, gravitational lenses, and physical properties of moderately high redshift galaxies.

System description

The Rayleigh laser system is designed to work in conjunction with existing AO equipment and ancillary instrumentation and infrastructure. A 25W pulsed laser will be projected to 15km altitude from a launch telescope mounted behind the secondary mirror. The somewhat low altitude implies that turbulence nearer the ground is best illuminated, and hence this is usually referred to as ground-layer adaptive optics. The Rayleigh back-scattered light will be detected by a new wavefront sensor system to measure the wavefront shape from the laser guide star, and provide corrections to the existing deformable mirror of the AO system. A Pockels cell range-gate system that is synchronized with the laser pulses will set height and duration of the laser return beam. The existing wavefront sensor system will be dedicated to tip-tilt correction measurements using a nearby natural guide star. A natural guide star will still be required in conjunction with a laser beacon, because image displacement due to atmospheric turbulence is cancelled as the laser light traverses the same turbulence twice. The natural guide star can be much fainter than those required for normal adaptive optics operation, and hence a much larger sky coverage is achieved.

Scientific invitation

The laser development will open up a new exciting area of astronomical exploitation for the William Herschel Telescope. There is much work ahead, and much to learn on how to optimally use the future new facility. If you are excited about the prospects as we are, and interested in working with us to define detailed scientific plans, contact René Rutten at