GLAS: A Laser Beacon for the WHT
n 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 (AO) system on the 4.2-m William Herschel Telescope (WHT).
Such a laser guide star system will amplify the fraction of sky available
to AO 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.
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 it also allows observations to go 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 it is at the heart of the future generation of extremely
At the 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. Examples of science areas that may profit from
the laser facility 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.
Since January this year work started on designing the various components
of the laser beacon system. Although maybe not a project of a very large
scale, the complexity is quite significant and offers various challenges
for engineers and astronomers alike. The project will be a joint endeavour
with, besides ING, participation from the University of Durham, the ASTRON
institute in the Netherlands, the University of Leiden, and the Instituto
de Astrofísica de Canarias. Below we will set out the main components
and challenges of the laser system and summarise the performance prospects.
Laser Guide Stars Basics
||Figure 1. The Durham laser experiment in action
at the WHT in April 2004. [ JPEG | TIFF ]
The idea is simple: a laser beam is used to generate a point source as high
as possible in the sky, projected towards the same area as where the telescope
is pointing. That laser beacon illuminates the atmospheric turbulence above
the telescope and is used for sensing the corrugation of the wavefront caused
by that turbulence. The higher the laser beacon is projected the better it
is, as in that way it best approximates a source from infinity.
There are basically two ways to produce a laser beacon: either exploiting
a layer of relatively high sodium density in the atmosphere at some 90 km,
or ‘just’ using back scatter in the atmosphere. The sodium laser option is
technologically very demanding for reason of laser technology and for the
implications it has on the design of the AO system. The Rayleigh laser, however,
is somewhat easier as it can use existing off-the-shelf laser technology
which is also much less expensive and easier to maintain. The Rayleigh has
however the disadvantage that the beacon will at best be at an altitude of
some 20 km. The lower elevation implies that atmospheric turbulence very high
in the atmosphere will not (properly) be sensed. Turbulence close to observatory
will be well measured, and therefore it is often referred to as ground-layer
AO. This feature has given the name to the laser project for the WHT: GLAS,
for Ground layer Laser Adaptive optics System (or better in Dutch, Grondlaag
Laser Adaptieve optiek Systeem).
Evidence built up over the years indicate that ground-level turbulence often
dominates, a nice example of which is shown in the paper in this Newsletter
by García et al. Over the next several months more solid experimental
data will be gathered about the turbulence characteristics.
First of all, the Rayleigh laser system is designed to work in conjunction
with existing AO equipment (NAOMI) and its ancillary instrumentation and
infrastructure like the INGRID IR camera and the OASIS integral-field spectrograph.
A powerful 25 to 30W pulsed laser will be focussed to some 20 km altitude
from a launch telescope mounted behind the secondary mirror. The pulse will
produce a short (tens of meters) column of light that travels through the
atmosphere. The Rayleigh back scattered light from this pulse will find its
way back to the telescope. About 10% of all the light is scattered into the
atmosphere, but of course in all directions and along the full depth of the
atmosphere. Only a very small fraction of the laser light returns to the
telescope and can be used to sense the turbulence, hence the need for a powerful
laser in order to produce a beacon that is bright enough to serve for AO.
|Figure 2. System diagram for the GLAS and NAOMI
system. [ JPEG | TIFF
Only photons returning from a certain set altitude range are useful to us.
Hence unwanted photons have to be blocked from entering the detector. This
will be done using a combination of a geometric filter that will obstruct
most of the unwanted light, and a very fast electro-optical Pockels cell
shutter. The timing of this Pockels cell shutter opening will be slaved to
the laser pulse signal, and open exactly when the Rayleigh scattered light
from an altitude of 20 km returns to the telescope. The very short period
during which the shutter remains open sets the length in the atmosphere over
which the laser beacon will extend.
Having passed the shutter, the Rayleigh back-scattered light will be detected
by a wavefront sensor system that measures the instantaneous wavefront shape
from the laser guide star. The results from this measurement, some 300 times
per second, will provide the demanded shape that the deformable mirror of
the AO system will have to take in order to correct for the wavefront corrugation.
So far the situation is very similar to a ‘standard’ natural guide star AO
system, except that laser light is used rather than light from a star. However,
as the laser light travels through the atmosphere twice along more or less
the same path, the measured wavefront does not contain information on the
overall image shift (tip-tilt) caused by the atmosphere. Hence to measure
the tip-tilt component still a natural guide star is required, but such star
can be quite faint and may be relatively far away from the science object.
The existing wavefront sensor system will be dedicated to tip-tilt
measurements on a star. The requirement for having such star near the science
object still poses a limitation on the effective sky coverage. To maximise
our chances of finding a suitable star even more, the existing wavefront
sensor will be upgraded with a Low-Light-Level CCD that has virtually zero
read noise and would give us an extra magnitude in faintest detectable star.
As can be seen in the adjacent figure
Stuik, Leiden) the conservatively estimated sky coverage will be extremely
good, even at the galactic pole.
|Figure 3. Representation of the sky coverage for
finding a star brighter than R=18 within a search diameter of 1.5 arcminutes
(courtesy Dr Remko Stuik, Leiden University). [ JPEG
| TIFF ]
The laser light scattered into the atmosphere of course has to be blocked
from entering the science instruments, both at the WHT as at other telescopes
at the observatory. Within NAOMI a dichroic mirror will block the laser light
from going into the science beam. But the situation with other telescopes
is more complicated and requires a coordination of laser operation and the
pointing of all telescopes that might be affected in order to avoid that
some telescope will inadvertedly cross the laser beam. Much experience with
this problem has been obtained at Mauna Kea observatory where such a laser
traffic control system has been put into operation. A similar system will
be put into operation at La Palma. The system will collect pointing information
and inform all telescopes whether or not there is a risk of crossing the
laser beam. If necessary the laser beam will automatically be intercepted.
In preparation for this project, various performance predictions were carried
out by Richard Wilson at Durham University. As the main scientific niche
for AO at the WHT rests with the visible light OASIS integral-field spectrograph
the focus is on achieving moderate but significant improvements of image
quality down to 0.6nm. It is unrealistic with current technology to aim for
high Strehl ratios at these wavelengths. But as the calculations below show,
image FWHM will improve very significantly at short wavelengths and performance
in the near IR is even better.
| FWHM (")
|| FWHM (")
|Faint tip-tilt star on-axis
|Typical seeing (0.74")
|Good seeing (0.54")
|Faint tip-tilt star at 60 arcsec
|Typical seeing (0.74")
|Good seeing (0.54")
The model calculations were designed to deliver realistic figures for the
expected improvement of image quality as a function of seeing, wavelength,
natural guide star brightness, and distance of the natural guide star to
the science object. A typical profile of atmospheric turbulence strength
with height was assumed. The following table shows a few of the model results.
The model calculations indicate very attractive improvements in image quality
when NAOMI will be used with a laser beacon. But of course above all, the
laser enhancement will provide such performance for nearly any point in the
sky, thus opening up the exploitation of AO to surveys of large number of
The GLAS project 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. Moreover, an added
attraction of the laser system is that it can serve as a testbed for concepts
of future laser systems at much larger telescopes.
Progress on this project will be reported in future articles in this Newsletter.
If you are excited about the prospects as we are, and interested in working
with us to define detailed scientific plans, don’t hesitate to contact us.
*Email contact: René Rutten (firstname.lastname@example.org