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Details of Adaptive optics

Adaptive optics is a technology to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion. It is commonly used on astronomical telescopes to remove the effects of atmospheric distortion, or astronomical seeing. Adaptive optics works by measuring the distortion and rapidly compensating for it either using deformable mirrors or material with variable refractive properties. While the technique was theoretically understood for some time, it was only advances in computer technology during the 1990s that finally made the technique practical. Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the primary mirror geometry itself. The simplest form of adaptive optics is tip-tilt correction, which corresponds to correction of the tilts of the wavefront in two dimensions (equivalent to correction of the position offsets for the image). This is performed using a rapidly moving tip-tilt mirror which makes small rotations around two of its axes. A significant fraction of the aberration introduced by the atmosphere can be removed in this way. Tip-tilt mirrors are widely used in night time and solar telescopes, to correct the aberration introduced by the atmosphere on the light path and improve image quality over what would be possible according to the atmospheric seeing. Tip-tilt mirrors are effectively segmented adaptive optics mirrors having only one segment which can tip and tilt, rather than having an array of multiple segments which can tip and tilt independently.

When light from a star or another astronomical object enters the Earth's atmosphere, turbulence (introduced, for example, by different temperature layers and different wind speeds interacting) distort and move the image in various ways (see astronomical seeing for a full discussion). Images produced by any telescope larger than a few centimeters are blurred by these distortions. For example, an 8-10 m telescope (like the VLT or Keck) can produce AO-corrected images with a resolution of 30-60 milli-arcsecond resolution at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.

An adaptive optics system tries to correct these distortions, using a wavefront sensor which takes some of the astronomical light, a deformable mirror that lies in the optical path, and a computer that receives input from the detector. The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a few milliseconds; the computer calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror is reshaped accordingly.

In order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array of pixels in a wavefront sensor, either using an array of small lenslets (a Shack-Hartmann sensor), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixellated map of the wavefronts is fed into the deformable mirror and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of the astronomical object to be known - even Solar System objects which are not point-like can be used in a Shack-Hartmann wavefront sensor, and time-varying structure on the surface of the Sun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp. Because a science target is often too faint to be used as a reference star for measuring the shape of the optical wavefronts, a nearby brighter guide star can be used instead. The light from the science target has passed through approximately the same atmospheric turbulence as the reference star's light and so its image is also corrected, although generally to a lower accuracy.

The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where a guide star of sufficient luminosity (for current systems, about magnitude 12-15) can be found very near to the object of the observation. This severely limits the application of the technique for astronomical observations. Another major limitation is the small field of view over which the adaptive optics correction is good. As the distance from the guide star increases, the image quality degrades. A technique known as "multiconjugate adaptive optics" uses several deformable mirrors to achieve a greater field of view.

An alternative is the use of a laser beam to generate a reference light source (a Laser guide star, LGS) in the atmosphere. LGSs come in two flavors: Rayleigh guide stars and sodium guide stars. Rayleigh guide stars work by propagating a laser, usually at near ultraviolet wavelengths, and detecting the backscatter from air at altitudes between 15-25 km. Sodium guide stars use laser light at 589 nm to excite sodium atoms in the mesosphere and thermosphere, which then appear to "glow". The LGS can then be used as a wavefront reference in the same way as a natural guide star - except that (much fainter) natural reference stars are still required for image position (tip/tilt) information. The lasers are often pulsed, with measurement of the atmosphere being limited to a window occurring a few microseconds after the pulse has been launched. This allows the system to ignore most scattered light at ground level; only light which has travelled for several microseconds high up into the atmosphere and back is actually detected.

Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include speckle imaging, aperture synthesis, lucky imaging and space telescopes such as NASA's Hubble Space Telescope.

Uses of adaptive optics