Adaptive optics systems on large telescopes have limited
sky coverage if natural stars are the only sources used to monitor atmospheric
irregularities. To extend sky coverage for adaptive optics, artificial laser guide
stars must be incorporated into the system. The art of creating and detecting
laser guide stars is a challenge, to say the least.
Two separate specialties have developed during the 20-year
history of this research endeavor. One centers on the elegant process of exciting
sodium atoms that float in a tenuous layer some 95 km above the surface of the
Earth. The other specialty the topic of this article is more "brute
force" in character and relies on laser illumination of both molecules and
dust particles in the Earth's atmosphere at somewhat lower altitudes than
the sodium layer. In the jargon of adaptive optics, the first type of artificial
star is called a "sodium guide star" and the second a "Rayleigh
The first Rayleigh guide star system was developed at Starfire
Optics Range (Kirtland Air Force Base) under the direction of Dr. Robert Fugate
and was in full operation during an 8-year period in the early to mid 1990's.
The Starfire system used a powerful copper vapor laser emitting light at green
wavelengths. It was focused at the relatively low altitude of 10 km.
The second Rayleigh guide star system is now in place at the Mt. Wilson 2.5-m
telescope. It was developed at the University of Illinois with NSF funds from
the Advanced Technology and Instrumentation Program. The full system adaptive
optics, Rayleigh laser guide star, and science cameras is called UnISIS:
University of Illinois Seeing Improvement System. The author's primary collaborator
in the UnISIS development is Prof. Scott Teare from New Mexico Tech.
The UnISIS Rayleigh guide star light is created with an excimer
laser. Excimer lasers are the industry standard for LASIK eye surgery and UV silicon
foundry work. Depending on the gas mixture loaded into the laser chamber, excimers
will emit pulsed radiation at 6 different wavelengths ranging from 157 nm to 351
nm. The UnISIS laser is loaded with xenon and fluorine and thereby works at the
longest of these wavelengths, 351 nm. This particular wavelength is very attractive
for laser guide star work because the Earth's atmosphere is relatively transparent
(but not too transparent!) at 351 nm and Rayleigh scattering is strongest at the
short wavelengths. (For LASIK eye surgery, excimer lasers are loaded with krypton
and fluorine and thereby emit at 248 nm, a wavelength that is strongly absorbed
by the cornea.)
30 Watt UnSIS excimer laser with its top removed. Vent
hose in foreground removes spent laser gas - xenon and flourine in a neon buffer,
all of which are specific to 351 nm laser emision
At Mt. Wilson, the laser beam is projected off the full 2.5-m
primary mirror and is focused 18 km above the telescope (20 km above sea level).
This altitude is high enough to position the artificial laser star above nearly
all of the strong layers of atmospheric turbulence, but the fact that the light
returning from a laser guide star at 18 km fills a conical volume (rather than
a cylindrical volume like the star light) does degrade the adaptive optics performance.
In short, the higher the guide star the better the performance. There is no doubt
that the newer generation of Rayleigh laser guide stars will move even higher
into the atmosphere. With newer and more powerful lasers becoming available, an
altitude range of 30 km to 35 km seems most attractive.
The primary challenges in building and operating laser guide
star systems are: (1) dealing with very high power laser systems at astronomical
facilities that have as their primary mission the detection of extremely faint
astronomical signals, (2) dealing with the logistics of reliably operating complex
laser equipment developed as experimental rather than industrial systems, and
(3) satisfying hazard-avoidance both within the observatory building and in the
airspace above the telescope. Just a few interesting aspects of these challenges
are mentioned here.
Specialized optical systems must be installed to capture
the relatively faint Rayleigh laser guide star return signal in the presence of
the much brighter low altitude laser light that is scattered by the atmosphere
on its up-link path. This problem can be handled gracefully because both the copper
vapor laser used at Starfire Optical Range and the excimer laser used with UnISIS
are pulsed laser systems. For example, the length of the UnISIS laser pulse is
less than 20 nanoseconds. Since the roundtrip light travel time to 18 km is 120
microseconds, there is sufficient time for fast electronic shutters (based on
polarized light and a so-called Pockel's cell) first to hide the wave front
camera from the burst of low altitude Rayleigh scattered light and then open it
a time precisely in sync with the fainter return signal from 18 km.
Laser reliability has been a key factor in nearly all laser
systems deployed at telescopes. In a few cases, systems have failed to work because
the laser power has been too low, but more commonly the systems will work but
only some fraction of the time. Solving this problem becomes a matter of good
management. Everyone runs under very tight budget constraints, but having a reliable
system is a necessity.
The Federal Aviation Agency (FAA) is well aware that the
astronomy community is actively developing and installing laser guide star systems
at major observatories. One special beauty of UnISIS is the inability of humans
to see 351 nm light. This fact, plus the benign manner in which the excimer light
is projected to altitude, made it possible to classify the UnISIS laser guide
star system as a Class I laser from the perspective of pilots and the FAA. The
time-consuming airplane countermeasures that other laser guide star systems must
face are not an issue at Mt. Wilson Observatory. This represents a major simplification
in the operation of UnISIS compared to other laser guide star systems.
The future of Rayleigh laser guide star work looks very bright
for several reasons. First, the development of industrial-quality laser systems
means that robust and powerful laser systems are entering the market every year.
One of many possible examples is the new release by Lambda Physik of a system
they call "Lambda Steel", an excimer laser designed for LCD manufacturers.
Second, Rayleigh guide stars are flexible tools for astronomers because they can
be focused and detected at all altitudes up to 35 km.
This last characteristic makes it possible to use Rayleigh
laser guide stars to
(1) monitor atmospheric turbulence as a function of altitude
above an observatory,
(2) easily correct for ground-layer turbulence, i.e. atmospheric distortions that
originate in telescope domes and the atmospheric boundary layer, and
(3) develop the 3D tomography of turbulence for forefront multi-conjugate adaptive
There is no shortage of new opportunities!