The Adaptive Optics Revolution: A History

5: Fugate’s Rayleigh Guide Star Experiments

89 F I V E Fugate’s Rayleigh guide Star Experiments The Jasons’ 1982 summer meeting represented a major turning point in the development of adaptive optics. Rett Benedict became convinced that DARPA should fund parallel programs to investigate two different techniques for creating laser guide stars to measure wavefrontdistortion —theRayleighbackscatteringmethodandthesodium-layer guide star. Each would cost an initial $100,000, with several million dollars to follow over the next few years.1 The Rayleigh effort was led by Bob Fugate at Sandia Optical Range at Kirtland Air Force Base. At the same time, Charles A. Primmermann and Ronald A. Humphreys from Lincoln Laboratory headed the sodium guide star investigation at White Sands Missile Range in southern New Mexico. Lincoln used a dye laser built by Avco for its experiments. Although both programs began by looking at stationary space objects, their efforts to measure atmospheric turbulence would in the long run be applied to the Air Force’s antisatellite mission.2 Once it had received the Jasons’ recommendations, DARPA, through Rett Benedict, approached Petras Avizonis in July 1982 at the Air Force Weapons Laboratory to get the first Rayleigh backscatter experiment under way. Avizonis, a no-nonsense sort of guy, was the technical advisor to the lab’s advanced radiation technology office and turned out to be very | Five 90 supportive of the experiment. He believed the ideal place to carry out this experiment was at the Lab’s Sandia Optical Range and selected Bob Fugate to be in charge.3 TheobjectiveofFugate’sfirstproof-of-conceptexperimentatSORwasto test whether the Rayleigh artificial guide star would work in the real world. As discussed earlier, only a tiny percentage of stars are bright enough that a telescope can use their light to measure atmospheric distortion and still have enough light left to record an image—thus the need for artificial guide stars. Researchers knew that at lower altitudes—where the air is denser and most atmospheric turbulence occurs—the backscatter return is stronger than at higher altitudes. They also knew that shorter-wavelength lasers produced more backscattered photons than longer-wavelength lasers.4 The question became, could the Rayleigh backscatter from a laser beam focused at an altitude of 5 kilometers (about 3 miles) be used to accurately measure the extent of distortion (phase errors) induced on the laser wavefront? To confirm that the measurements were of the highest quality , Fugate’s team needed to compare the data from the guide star with an independent reference—light from a real star. (The difference between the wavefronts of the star’s light and the laser’s backscatter, as discussed in chapter 4, is the focal anisoplanatism error—and the theory, which needed to be verified, was that this error would be small enough that the measurements would still be valid.) The narrow laser beam was directed within a few microradians of Polaris, the North Star. The starlight and the laser backscatter light would travel through nearly identical paths from the atmosphere to a telescope. Polaris was chosen because it does not move in the sky more than one degree in a 24-hour cycle, which allowed the experiment to be conducted without a gimbaled telescope. For most tests, Fugate explained, “the beam was focused at a range of 5 kilometers and the sensor was range-gated to accept backscattered light from 4.5 to 5.5 kilometers.” The gating mechanism allowed scientists to pick the exact altitude of the Rayleigh backscatter they wanted to view.5 Fugate and his small team of military, civilian, and contractor personnel began designing and building equipment in fall 1982 and started testing in late spring 1983. Members of the team included lieutenants Bruce R. Boeke and Richard A. Cleis, Raymond E. Ruane and Lawrence M. Wopat from the Weapons Laboratory, and contractors David L. Fried, George A. Ameer, Fugate’s Rayleigh guide Star Experiments 91 Steven L. Browne, Phillip H. Roberts, and Glenn A. Tyler from the Optical Sciences Company. David Fried worked closely with Fugate to design the experiment. It did not involve a highly sophisticated telescope and optical system sitting high on a mountaintop. Instead, Fugate configured the hardware components of the experiment in a garage-like facility on the east side of the hill at SOR. This facility was originally known as the FTT building, the old home of the field test telescope used to support the Airborne Laser Laboratory program in the early...