Adaptive Optics

What is Adaptive Optics?
Light from a distant star or galaxy is distorted as it passes through the turbulent earth's atmosphere, preventing a telescope on the surface of the earth from forming sharp images. Instruments using a new method called adaptive optics can eliminate the blurring effect of the atmosphere. Thus images formed with the 100-inch telescope using adaptive optics are as sharp as those from NASA's Hubble Space Telescope. This is the most revolutionary technical development in astronomy since Galileo first used an astronomical telescope in 1609.

The History of Adaptive Optics at Mount Wilson Observatory
The atmospheric blurring of an astronomical image is obvious to viewers using even small telescopes. In 1953, Horace Babcock, an astronomer at the Mount Wilson Observatory, published a seminal paper on the new technology of adaptive optics -- a method of correcting for atmospheric disturbances of wavefronts in real time. The process described by Babcock involved measuring the distortions in the wavefront and correcting them very quickly in order to follow the rapidly changing patterns of atmospheric turbulence. The technology of the time was not up to the requirements and it was years before Babcock’s ideas could be implemented.

In 1957, physicist Robert Leighton partially corrected the atmospheric blurring at the 60-inch telescope on Mount Wilson to produce the sharpest photographs of Jupiter and Saturn up to that time. He did this by tilting the secondary mirror several times a second (a technique now called "tip-tilt" correction) to eliminate or reduce the rapid motion of the image. Tip-tilt yielded an improvement of about a factor of 2 or 3 in sharpness but an improvement by another factor of 10 was needed in order to eliminate atmospheric blurring altogether. Reaching this goal required a major investment in new technologies that could divide the light beam into many small elements and correct each element separately hundreds of times per second. These new techniques first became available in the 1970's, largely as a result of U.S. Department of Defense research on methods of keeping a laser beam sharply focused in the atmosphere.

How Adaptive Optics Works
Adaptive optics measures the atmospheric distortions in the incoming light from a star or other object and sends electronic signals to a deformable mirror that can change its shape rapidly to correct for the distortions. In the system built for the 100-inch telescope, the light reflected from the telescope mirror is divided into several hundred smaller beams or areas. Looking at the beam of light from a star, the system sees hundreds of separate beams, some of which have been deviated because of atmospheric turbulence. The electronic circuits in the system compute the altered shape of a mirror surface that would realign the separate beams so that they are all going in the same direction. Then a signal is sent to the deformable mirror to change its shape in accordance with these electronic signals, resulting in an undistorted beam.

Challenges for an Adaptive Optics System
There are two primary challenges for an adaptive optics system. The first involves having an optical system which is mechanically capable of providing a correction to the incoming light. For the adaptive optics systems on the telescopes at Mount Wilson Observatory, this is done by using two mirrors working in concert: a tip-tilt mirror to provide a coarse correction to the incoming beam of light, and a deformable mirror. This latter mirror can deform in such a way as to make the distorted light entering the telescope resemble light from outside the Earth's atmosphere.

The second challenge is presented by these distortions constantly changing. The deformable mirror must flex quickly to keep up with the changing effects of the incoming light. This requires that the optical components be controlled by a computer system fast enough to analyze the incoming light and send the appropriate commands to the mirror many times per second.

The complexity and speed of the system depend largely on the conditions of the atmospheric turbulence. If the turbulence is great the system will have to work harder and faster to achieve useful results than when the atmosphere is steady. Fortunately, the atmospheric steadiness at Mount Wilson is as good as any site in North America, rivaling the observatory sites in Hawaii and Chile. The adaptive optics systems at Mount Wilson have less work to do to get close to the diffraction limit of the telescope than at most other sites.

How does the system know what shape to make the deformable mirror? To determine the shape of the deformable mirror, the system needs to know the shape of both the distorted and undistorted image. For stars, the undistorted shape is typically just a point showing no detail. There are several techniques for determining the final, distorted shape of a point source at the Earth’s surface. The adaptive optics system currently in use at Mount Wilson uses a star near the telescope’s target as the source of the distorted wavefront. That is, it looks at a star as seen through the telescope close to the object under study and determines how it has been distorted from its expected appearance. This technique has several advantages:

However, it requires that the object being observed be close to a relatively bright star. Another technique uses a laser instead of a guide star. The laser beam is projected into the sky to a point very close to the object of interest in the sky. This allows any object to be observed even no bright stars are nearby. As long as the laser is bright enough, even very faint objects can be observed. A laser guide star adaptive optics system is also now under construction at the 100-inch telescope under the direction of Laird Thompson of the University of Illinois.