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.
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.
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
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.
|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.