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Adaptive Optics
Newsletter
Volume 2
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Creating and Detecting Rayleigh Laser Guide Stars 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 guide star". 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.)
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
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