For centuries, astronomers have observed the night sky through telescopes, peering through Earth’s shimmering atmosphere and wishing for a little more clarity. Seen from Earth, the stars twinkle, not by magic, but because light from distant stars or galaxies is distorted as it passes through Earth’s turbulent atmosphere.
This natural distortion, known as “seeing,” is caused by temperature variations and air currents that limit the sharpness of even the most powerful ground-based telescopes, blurring stars, planets, and whatever else might be out there. Although astronomers had been dealing with the issue of air turbulence for centuries, it wasn’t until 1953 that astronomer Horace W. Babcock developed a plan to un-twinkle the stars.
In a ground-breaking paper based on his work at the Mount Wilson Observatory and published in Publications of the Astronomical Society of the Pacific, Babcock theorized that the distortion of starlight caused by Earth’s atmosphere could be corrected in real time by using deformable mirrors and electronic control systems to measure wavefront distortions—which occur when light waves are affected by atmospheric conditions—and correct them quickly to follow the rapidly changing patterns of atmospheric turbulence. At the time of publication, however, there were no computer systems that could handle the workload to make this theory viable.
Perhaps realizing how far ahead his ideas were, Babcock wrote in the manuscript, “In this paper a method is proposed which, although subject to severe limitations, seems to offer in principle a means of compensating or correcting for the effects of atmospheric turbulence.” In time, this method would become known as adaptive optics (AO) and it laid the groundwork for a revolution in observational astronomy.
Today, AO is defined as measuring the atmospheric distortions in the incoming light from a star or other object and [sending] electronic signals to a deformable mirror that can change its shape rapidly to correct for the distortions.
One way to understand this concept is to imagine taking a picture through a wavy piece of glass, which would result in a distorted image. Now imagine if you could bend another piece of glass in just the right way to correct the distorted waves of light from the first piece of glass. In the AO system built for the Mount Wilson Observatory’s 100-inch telescope, the light reflected from the telescope mirror is divided into hundreds of smaller beams. Some of these beams are bent by atmospheric turbulence. The system computes the altered shape of a mirror surface that would realign the separate beams so that they are all going in the same direction, then sends a signal to a deformable mirror to change its shape, resulting in an undistorted beam.
Along with computational power, other early challenges to realizing AO included making and controlling deformable mirrors as well as wavefront sensor technology. The world had to wait for technology to catch up to Babcock’s genius.
At the time of Babcock’s proposal, the astronomy community considered his concept too technically complex or speculative for real-world implementation, but the US Department of Defense (DoD) had other ideas. As Robert W. Duffner explains in The Adaptive Optics Revolution: A History, during the Cold War, DoD and the Air Force recognized the potential of AO for surveillance and imaging from satellites and thus worked in secret on developing the first practical AO system.
The turning point for AO came in the late 1980s and early ’90s, when parts of the military’s research were declassified. Suddenly, the astronomical community had access to decades of hidden progress. Combining that knowledge with rapidly advancing computer processing power, real-time sensors, and precision mechanics, astronomers began building AO systems for telescopes, observing planets, stars, and distant galaxies with unprecedented clarity from the ground.
Horace W. Babcock. Photo credit: Mount Wilson and Los Campanas Observatories.
One way around the atmospheric distortion issue, of course, is to launch telescopes into space. Although more costly overall, space-based telescopes offer clear, detailed images as they are free from atmospheric turbulence, weather conditions, and light pollution. Now, AO allows ground-based observatories to perform many of the same functions at a fraction of the cost of space telescopes, and with larger mirrors—for example, the twin 10 m telescopes of the Keck Observatory versus the 2.4 m mirror of the Hubble Space Telescope.
The Giant Magellan Telescope (GMT) at Las Campanas Observatory is one of the first to incorporate AO into its core design. The GMT uses seven deformable mirrors, each only 2 mm-thick and flexible enough to be reshaped up to 2,000 times-per-second. In some instances, AO allows GMT to produce images 4- to 16-times sharper than the James Webb Space Telescope. As GMT chief scientist Rebecca Bernstein put it, “Adaptive optics will let us form the sharpest possible images for single objects, like planets, and will boost the resolution by 50% even over the widest field of view of the telescope, which will give GMT the best combination of sensitivity and field of view of any extremely large telescope”.
And AO isn’t just for astronomy anymore, having found a second life in other fields such as high-resolution retinal imaging, laser communications, and microscopy. In each of these fields, AO corrects for optical distortions—whether they come from Earth’s atmosphere or from biological tissue.
Horace Babcock was 90 years old when he died in Santa Barbara, California, on 29 August 2003. His obituary in the LA Times noted his “uncommonly reserved nature” and “abiding modesty” about his accomplishments.
Born 13 September 1912, in Pasadena, California, he was the only child of Harold D. Babcock and Mary G. Henderson. His father, an astronomer well-known for his work in solar spectroscopy and magnetic fields of the sun, worked at the Mount Wilson Observatory, so young Horace grew up immersed in the world of astronomy and instrumentation.
The elder Babcock was a great influence on his son, teaching him to appreciate nature by taking him on hikes around Mount Wilson and encouraging his interests in photography and how things work. They would later collaborate on academic papers, such as “Mapping the Magnetic Fields of the Sun” and “The Sun’s Magnetic Field, 1952–1954.” Together, they developed the solar magnetograph, an instrument that allowed measurement of magnetic fields on the surface of the sun to unprecedented precision.
Torn between his interests in astronomy and engineering, Babcock earned an undergraduate degree in structural engineering at the California Institute of Technology (Caltech) in 1934 and a PhD in astronomy from the University of California, Berkeley, in 1938. His dissertation focused on magnetic fields in sunspots—an early sign of his lifelong interest in astrophysical magnetism and precise observation.
Babcock’s doctoral thesis introduced observational evidence suggesting the presence of unseen mass in galaxies, an early hint of what would later be known as dark matter. Although this work was not widely recognized at the time, it laid the groundwork for later astronomers, in particular Vera Rubin and Kent Ford in the 1970s, who confirmed similar rotational curves in several galaxies and brought the dark matter concept into the mainstream.
In 1946, Babcock joined the Mount Wilson and Palomar observatories as a staff member. He served as director of the observatories from 1964 until his retirement in 1978. One of his many achievements during that time was pushing for the construction of an observatory in the Andes mountains. Today, the Carnegie Science telescopes at its Las Campanas Observatory in Chile’s Atacama Desert are among the most powerful tools available for astronomers. Babcock’s other work included studies of the glow of the night sky, the rotation of galaxies, and telescope design.
Babcock’s work bridged the gap between theoretical insight and technical capability and helped usher in a new era of astronomy. His legacy is a testament to how long-term scientific thinking—sometimes ignored or underappreciated—can eventually change our way of seeing.
Karen Thomas is Section Editor of Photonics Focus.
