GLAS

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 about one 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. 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.

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.

At the time of writing the GLAS system is still under development and much testing remains to be done before accurate performance estimates can be presented. Adaptive optics performance strongly depends on the atmospheric turbulence conditions, even more so when using a laser guide star system. However, extensive model calculations have been carried out which aim to mimic the real AO and laser system as well as the atmosphere in order to achieve realistics predictions of what one might expect from the future operational AO-plus-GLAS Rayleigh laser system.

The key reason for building the GLAS laser system is to improve sky coverage for AO observations. Although the laser will guarantee the presence of a bright point source for high-order wavefront sensing, correcting the low-order tip-tilt mode still relies on the presence of a natural guide star. This then poses the limitation on sky coverage. Calculations of sky coverage based on anticipated GLAS performance measures are shown in the diagram below and indicate a dramatic improvement over natural guide star adaptive optics.

Sky coverage predictions based on achieving an R=17 limit for the tip-tilt natural guide star and a 2 arcmin patrol field. (courtesy Remko Stuik, Leiden).

Predictions have also been made with respect to the actual AO performance of NAOMI with GLAS. A summary of the results is shown in the following table, reflecting a realistic range of seeing conditions on La Palma where median seeing is 0.69". Calculations were done for r0 values of 0.11m, 0.14m and 0.19m, specified at 500nm, corresponding to natural seeing at 550nm of 0.90", 0.69" and 0.50" respectively. The table shows the FWHM values for the various combinations of seeing and wavelength, for the case of a natural guide star on-axis, and 1 arcmin off-axis. It should be noted that the results are based on statistical analysis and are therefore not exact. For reference, lambda/D as a measure of the diffraction limit is shown as well.

GLAS performance expectations; FWHM in arcseconds.

	r0	R	I	J	H
Natural tip-tilt star on-axis	0.19	0.12	0.09	0.09	0.10
	0.14	0.27	0.17	0.13	0.12
	0.11	0.41	0.29	0.18	0.16
Natural tip-tilt star 1' off-axis	0.19	0.18	0.14	0.13	0.12
	0.14	0.31	0.24	0.19	0.16
	0.11	0.45	0.35	0.24	0.20
lambda/D		0.03	0.04	0.06	0.08

The on-axis models predict near diffraction limited performance in the J and H bands for good seeing conditions. At shorter wavelengths the diffration limit will not be reached, and wavefront fitting errors are dominated by the performance limitations of the deformable mirror. Although Strehl ratios at short wavelengths will be low there will be a very attractive improvement in the delivered point spread function. It should be noted that any source as faint as R=17 may serve as a self-referencing tip-tilt source. Hence for any such point source good AO correction will be obtained. For fainter science targets or diffuse sources an off-axis natural guide star is required, causing some degradation of AO performance.

The GLAS project is led by the ING and carried out in collaboration with the University of Durham, Leiden Observatory, the ASTRON institute, and the IAC. 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, please contact us.