Adaptive and Active Optics

The great telescopes of an earlier age, such as the 200-inch reflector on Mount Palomar in California that was for many years the world's largest, were constructed with a single large reflecting mirror. As astronomers sought ever-larger light-gathering power, it became difficult to make a single mirror of sufficient size that was of high enough optical quality. (Light-gathering power varies basically with the area of the mirror and thus it increases as the square of the diameter; the effective diameter of the mirror is often called the aperture of the telescope.)

Adaptive Optics

The cases discussed on this page are all examples of adaptive optics, where one uses the fast response of a computer to control the elements of a telescope and compensate for changes in the telescope or in the atmosphere that distort the light coming to the telescope. Adaptive optics represent an active area of research in astronomy and other areas where high resolution is required (for example, high-resolution satellite imaging of surface features on the Earth).

The Shape of Things to Come

Modern large telescopes use newer technologies that either combine multiple mirrors or use very thin large mirrors. In both cases, the exact shape of the telescope mirror can be adjusted in real time to partially compensate for light distortion in the atmosphere and gravitational distortion of the telescope as it moves to track objects in the sky.

Example: The Twin Kecks

For example, the twin 10-meter Keck telescopes at Mauna Kea in Hawaii are each composed of thirty-six hexagonal mirrors that are 1.8 meters in diameter (see image above; note the person in the center). These 36 mirrors combine to form an effective 10-meter mirror, but with each of the thirty-six elements constantly being adjusted by computer to optimize the telescope. In the Keck telescopes, the position of each segment relative to its neighbors is adjusted twice a second, with a precision of 1/1000 the width of a human hair (four nanometers, where a nanometer is 10^-9 meters).

Example: The Subaru Telescope

As another example, the 8.3-meter Japanese Subaru telescope, also on Mauna Kea, uses a single large mirror (the largest in the world). However, the mirror is so thin that 261 computer-controlled mechanical actuators can distort it continuously to maintain an optimal shape under all conditions. Incidentally, the telescope is named after the Pleiades star cluster (which is called "Subaru" in Japanese), not a car!

Resolution and Resolving Power

Resolution refers to the ability to distinguish two adjacent objects. The following figure illustrates the idea of angular resolution on the celestial sphere.

The ability of an optical instrument to resolve two separate objects is called its resolving power. Under ideal conditions, the smallest angle α in seconds of arc that can be resolved by a lens or mirror of diameter d for light of wavelength λ is limited by diffraction effects to

α = 2.1 x 10⁵ λ / d arc seconds

where d and λ are measured in the same units. This illustrates two important properties of the minimum angle that a telescope can resolve. It is inversely proportional to the diameter of the lens or mirror, and is proportional to the wavelength of light. Therefore, resolution improves for a larger telescope, but the longer the wavelength of light the poorer the resolution for a telescope of given size. As noted in the right panel, the theoretical resolution implied by the formula above is ultimately limited by seeing conditions, so that simply increasing the diameter of the telescope no longer improves the effective resolution beyond a certain point.