Active optical systems compensate for quasi-static distortions/aberrations in optical systems, e.g., those associated with imperfectly manufactured, misaligned or mis-positioned elements within the system. Adaptive optical systems compensate for more dynamic aberrations or disturbances, e.g., those associated with atmospheric turbulence in the optical path of a ground-based telescope.
An example of an active optical system is provided by a recent telescope project. The telescope's primary mirror is formed by bonding a paper-thin nanolaminate optical surface to an actuated silicon carbide substrate. The result is a very low areal density, lightweight mirror of potentially excellent figure and surface quality. And this potential is realized by using a set of actuators to carefully reshape the substrate and thereby compensate for any deformations introduced by the manufacturing or bonding processes or changes in the structural or thermal environment. In a demonstration of the system, a 300 nm rms primary surface was transformed to one of 40 nm rms.
PALAO, the JPL-built Palomar Adaptive Optics system, provides an example of an adaptive optics application. In December 2001, the combination of the Palomar High Angular Resolution Observer (PHARO, built by Cornell University) and PALAO recorded the occultation of the binary star NV0435215 + 200905 by Saturn's moon Titan. In the sequence, Titan's atmosphere is seen to have a lensing effect: as Titan passes in front of one of the stars, a partial image of the star appears to run around Titan's limb. Considerable information about Titan's atmosphere can be extracted from careful observations of this "flash" phenomenon. Without the compensation of atmospheric distortions afforded by PALAO, the stars, separated by 1.5 arcsec and previously thought to be a single star, would have been unresolved and the phenomenon unobservable.