The Adaptive Optics Revolution: A History (review)

For thirty-five years, Dr. Robert Duffner has been historian for the Air Force Research Laboratory and its predecessors at Albuquerque. He knows defense R&D and understands the high-technology systems it produces. Moreover, he has known many of the key figures who spawned The Adaptive Optics Revolution, gaining their confidence over the years as a colleague eager to chronicle their accomplishments. He has applied this knowledge and understanding to produce a masterful history of one of the most complex technologies to emerge from the cold war.

The story of adaptive optics (AO) begins with the proliferation of orbiting objects that followed the launch of Sputnik in 1957. This proliferation spawned a military requirement for detailed images of space objects in order to determine their purpose and capabilities. Attempts to acquire such images by using telescopes were thwarted by atmospheric distortion. AO systems resolve this problem by first sampling light along an appropriate atmospheric path and using this sample to generate commands that change the face of a deformable mirror. The deformed mirror introduces an artificial distortion that corrects for atmospheric distortion, allowing a telescope to produce sharper images. The deformable mirror illustrates the complexity of AO technology. These devices came to include hundreds of actuators, spaced millimeters apart. Moving a distance measured in microns, these actuators reshape the paper-thin surface of the mirror as many as 8,000 times a second.

In describing the origins of AO, Duffner emphasizes the importance of a steady funding source for risky research projects. Here, the key player was the Defense Department’s Advanced Research Projects Agency (DARPA), which aimed to “rattle scientists out of their traditional comfort zone,” encouraging revolutionary developments rather than incremental changes (p. 12). DARPA began by supporting both the development of an initial AO concept by a civilian research firm (Itek) and studies of atmospheric effects on light at the air force’s Rome Air Development Center (RADC). DARPA then funded a joint RADC-Itek project that produced an AO prototype that was successfully demonstrated in 1981.

The effectiveness of early AO systems was limited by their sampling process. To generate their control signals, these systems sampled light gathered by the telescopes they served. This was acceptable when observing bright objects. However, draining light from a dim object seriously degrades its image. This difficulty was resolved by using a laser to create a bright spot in the atmosphere near the path followed by light being collected by a telescope. Backscatter light from this “guide star” provided the sample used to generate control signals. While an improvement, the small amount of light produced by early guide stars continued to limit AO effectiveness.

The resolution of this limitation emerged from the summer 1982 meeting of the Jasons, a convocation of civilian scientists that advises the Defense Department on technical issues. Princeton physicist Dr. Will Happer suggested creating a guide star by using a properly tuned laser to excite sodium atoms in the mesosphere. Once developed, sodium guide stars proved to be significantly brighter than those based on backscatter. In 1991, the air force declassified its work on guide stars and made its AO technology available to civilian astronomers. A measure of the importance of AO to modern astronomy is the fact that virtually every large telescope produced after 1991 has been equipped with an AO system.

Since the development of AO involved an extensive amount of work with lasers, it is not surprising that AO would also be applied to the enhancement of laser weapons. Here, Duffner discusses two principal developments: the Airborne Laser Laboratory, which used its laser to down several Sidewinder missiles in the 1980s, and the air force’s airborne laser, which destroyed a tactical ballistic missile in February of 2010. The airborne lasers and several large telescopes are among the AO applications Duffner describes in his final chapters. Overall, Duffner’s volume amounts to a superb case study of how the Defense Department develops the arcane technologies that have...