Adaptive Optics

What is AO?

Adaptive Optics refers to optical systems which adapt to compensate for optical effects introduced by the medium between the object and its image.

Under ideal circumstances, the resolution of an optical system is limited by the diffraction of light waves. This so-called "diffraction limit" is generally described by the following angle (in radians) calculated using the light's wavelength and optical system's pupil diameter:



where the angle is given in radians. Thus, the fully-dilated human eye should be able to separate objects as close as 0.3 arcmin in visible light, and the Keck Telescope (10-m) should be able to resolve objects as close as 0.013 arcsec.

In practice, these limits are never achieved. Due to imperfections in the cornea and lens of the eye, the practical limit to resolution only about 1 arcmin. To turn the problem around, scientists wishing to study the retina of the eye can only see details about 5 (?) microns in size. In astronomy, the turbulent atmosphere blurs images to a size of 0.5 to 1 arcsec even at the best sites.

Adaptive optics (AO) provides a means of compensating for these effects, leading to appreciably sharper images sometimes approaching the theoretical diffraction limit. With sharper images comes an additional gain in contrast -- for astronomy, where light levels are often very low, this means fainter objects can be detected and studied.

Why AO?

Why Adaptive Optics?

In astronomy, the effects of atmospheric blurring can be avoided by going into space. However, facilities like the Hubble Space Telescope are extremely costly to build and operate, and despite their expense, space-based telescopes remain relatively small. To compare HST and the Keck Telescopes, HST cost roughly 20 (?) times more to build and launch, yet Keck has 20 times the light gathering area and -- potentially -- 4-5 times better resolution.



One technique that has been developed for overcoming atmospheric blurring is speckle interferometry, in which hundreds of very short exposures ("specklegrams") are later analyzed to reconstruct the unblurred image. However, because the specklegrams must be short exposures and at the same time have good signal-to-noise, speckle interferometry is limited to imaging very bright objects. Furthermore, results can only be seen following a lengthy reconstruction process.



Adaptive optics compensates for atmospheric turbulence while the observations are in progress. In principle, very faint objects can be imaged in long exposures, provided there is a bright "reference beacon" nearby to allow the AO system to analyze the atmospheric effects. Furthermore, AO's real-time nature means that spectroscopy becomes possible on very small angular scales, and it follows that fainter objects can be studied because less of the night-sky background needs to be included in the light being analyzed.



The nuclear region of the nearby galaxy NGC 7469, with and without AO (from CFHT).

Vision science does not have any means of avoiding the imperfections in the cornea and lens in living subjects, so AO is the only option for studying living retinal tissue. Furthermore, a full adaptive optics system can compensate for micro-fluctuations in eye muscles, which means that the eye does not have to be temporarily paralyzed while under examination.


How Does an Adaptive Optics System Work?

There are several AO systems working in both astronomy and vision science, but all work in closely similar fashions.

Consider a beam of parallel light passing through a vacuum; a slice across this beam will contain some pattern of phases which will move (uniformly) at the speed of light along the beam. If the beam passes through a uniform medium, its speed is slowed but the pattern of phases still moves together. In a non-uniform medium, however, some parts of the beam are slowed more than others, leading to distortions in the uniform wavefront.

All AO systems work by determining the shape of the distorted wavefront, and using an "adaptive" optical element -- usually a deformable mirror -- to restore the uniform wavefront by applying an opposite cancelling distortion.




The most basic systems use a point source of light as a reference beacon, whose light is used to probe the shape of the wavefronts. This may be a bright star, or in the case of vision research a laser spot focused on the retina. Light from this reference source is analysed by a wavefront sensor, and then commands are sent to actuators (pistons) which change the surface of a deformable mirror to provide the necessary compensations. For the system to work well, it must respond to wavefront changes while they are still small; for the earth's atmosphere, this means updating the mirror's shape several hundred times a second!