Adaptive Optics for Biological Imaging

Abstract

It is fascinating how optical technology transfers among disparate fields of science and engineering. The history of adaptive optics is a good case study. The story begins in 1953 when a visionary, Horace Babcock, who was an astronomer at the Mount Wilson and the Palomar observatories, proposed a method based on adaptive optics to correct in real time the atmospheric distortions that degraded ground-based telescope images. All ground-based telescopes suffer from atmospheric turbulence, which causes time-dependent inhomogeneities in the air refractive index. They are caused by nonstationary random processes. The wind shears mix various atmospheric layers and the temperature inhomogeneities result in time-dependent variations in the refractive index of the air. They distort the wavefronts and thus degrade the image. One alternative is space-based telescopes such as the Hubble Space Telescope. Another is to implement Babcock’s idea of a closed-loop system incorporating a wavefront sensor and a deformable mirror that can introduce real-time changes in the wavefront to compensate for the aberrations introduced by the atmospheric turbulence. Babcock’s prescient ideas were developed into instrumentation in the mid-1970s and in 1982 the Defense Advanced Research Projects Agency (DARPA) working with the United States Air Force completed a real-time adaptive optics system integrated with an optical telescope on Maui in Hawaii. The motivation was to obtain high-resolution images of Soviet satellites. One common wavefront sensor is the Shack-Hartmann wavefront sensor, which works with white light and also with extended sources (such as the Sun). The closed-loop adaptive optics system involves computer wavefront reconstruction, which is a classical inverse problem whose solution can be found, but the solution cannot be proved to be unique.