The Adaptive Optics Revolution: A History

Foreword

ix FoReWoRd Adaptive optics: What is it? Why should I care? For nearly 400 years, astronomers have lived with blurry images of the planets, stars, galaxies, and indeed the universe when looking through their ground-based telescopes. Looking through the atmosphere with a highmagnification telescope is like looking through shower glass with your eye. You can see shapes on the other side of the glass, but you can’t make out any detail—you couldn’t, for example, see people’s faces clearly enough to positively identify them. Earth’s atmosphere acts like a shower glass for astronomers. They can make out shapes, but only with limited clarity. Large telescopes provide two critical ingredients for making new discoveries about the universe: they gather a lot of light, and they have the potential to provide detail and clarity (resolution). Eight-, 10-, and soon-to-be 30-meter class telescopes certainly gather a lot of light, but because of the atmosphere, they only provide the same image detail as an 8-inch telescope in your backyard. The most important science (for example, investigating whether there is carbonbased life on planets outside the solar system) is critically dependent on the fine details. Astronomers are losing out on that science because of the atmosphere. The science lost by observations from the ground was the main justification for building the Hubble Space Telescope—astronomers wanted to get on the other side of the “shower glass,” where a large telescope can | x gather lots of light and provide the resolution they are entitled to based on the laws of optics, not the limits imposed by the atmosphere. In 1953 Horace Babcock, an astronomer at Mount Wilson and Palomar observatories in California, had an idea that would enable astronomers to remove in real time the distortions created by the atmosphere. His idea was revolutionary, since it removed the distortions from the optical waves before they were focused on film in a science camera. Today we call his concept adaptive optics. There are other imaging techniques for getting beyond the limits imposed by the atmosphere. One, still in use today, was to make many very-short-exposure images and process them afterward. This approach is, however, limited to very bright objects. Long-exposure techniques were not effective, because if the telescope’s resolving power was not sufficient, details would not be recorded in the first place. Babcock’s idea was somewhat complicated, and the technology really didn’t exist at the time to support a practical implementation. However, by the mid-1970s, sensors and opto-mechanical devices had matured sufficiently that the first demonstrations of the concept could take place. In 1982, the Defense Advanced Research Projects Agency (DARPA) and the U.S. Air Force put into service the first practical adaptive optical system, called the Compensated Imaging System (CIS), on the 1.6-meter telescope on top of Haleakala on the island of Maui in Hawaii. This system provided the U.S. Defense Department images of bright earth-orbiting satellites at nearly the full resolving power of a 1.6-meter telescope. The goal was to get detailed images of new Soviet satellites on the first or second revolution after their launch. It was a significant step forward to space situational awareness in the Cold War era. Adaptive optics works by analyzing the rapidly changing shape of light waves passing from the subject being imaged through the atmosphere into the telescope. The optical wave distortions are corrected by reflecting the wave off a mirror (called a deformable mirror) whose shape is adjusted to be the opposite of the measured distortion. After reflection off the deformable mirror, the wave shape is restored to what it was before it entered the atmosphere . Adaptive optics thus effectively removes the atmosphere’s distortion. The most obvious source of light waves for analyzing the distortions (most often called the guide star) is sunlight reflected from the subject being Foreword xi imaged. However, a serious limitation arises for dim objects: they don’t provide enough light for the adaptive optics sensor (called the wavefront sensor) to make a measurement. It’s all a matter of timing. The wavefront sensor must measure the distortions on the optical wave much faster than the rate at which the distortions are changing. The distorted waves change to a new shape about 100 times per second. However, objects of high scientific interest to astronomers arejust too...