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Adaptive Optics (AO) Glossary of Terms

Altitude Conjugated.
When an AO system uses optics to form an image of a given turbulent layer (corresponding to a specific altitude) on an optical element.

Anisoplanatism and Isoplanatic Patch. Atmospheric turbulence is distributed in a volume (through approximately the first 20 kilometers above the telescope), so the phase perturbation measured along a line of sight is only valid for that line of sight. Going off axis, the degradation (anisoplanatism) is gradual, and there is a certain area in the sky where the error will be small. This angular area is called the "isoplanatic patch" (typically ~ 30 arcseconds in radius at 2.2 microns, scaling at the wavelength to the 1.2 power).

Cone Effect. Since a laser guide star is created at a finite altitude its light returning to the telescope forms a cone. In comparison, the light from an astronomical object (at essentially an infinite range) forms a cylinder. This difference has several detrimental effects on the ability to correct the wavefront distortions.

Classical AO. Uses a natural guide star as a reference source. It also uses a single deformable mirror and a single wavefront sensor. This is in contrast to laser guide star AO, multi-conjugate AO, or ground layer AO systems.

Deformable Mirror (DM). A device used in AO systems that can be deformed to match the (essentially) arbitrary shapes of the incoming wavefront. A deformable mirror operates within a limited range of spatial frequencies. Typically, these mirrors have dozens to hundreds of control elements (usually actuators, or miniature pistons attached to the back of the mirror that push or pull it).

Extreme AO (ExAO). A type of AO system in which the order of correction (number of elements in the deformable mirror/wavefront sensor) is very high. The goal is to achieve a very high correction quality, or a very high Strehl ratio.

Ground Layer Adaptive Optics (GLAO). An AO variant in which the deformable mirror is conjugated to the ground, and only the ground layer turbulence is measured and compensated for. This leads to only a partial correction, but on a field of view that can reach several arcminutes across.

Laser Guide Star (LGS). An artificial star created by exciting the sodium atoms located in the Earth's atmospheric sodium layer, at an altitude of 90 to 100 kilometers. A narrow, powerful laser, emitting yellow/orange light at 589nm is used to excite the sodium atoms.

Multi-Conjugate Adaptive Optics, (MCAO). Multiple wavefront sensors are used to measure the turbulence in different directions in order to reconstruct it in the entire 3-D volume over the telescope's field. Measuring not only how much turbulence there is, but also the altitude at which it lies, allows a set of deformable mirrors, conjugated to various altitudes, to enlarge the size of the AO compensated field.

Natural Guide Star (NGS). A star or any natural object that is bright and compact enough to be used by a wavefront sensor.

Sodium Laser. A laser used to deliver a yellow/orange beam at 589nm (sodium D2 line) light that excites sodium atoms in an atmospheric layer at an altitude between 90 and 100 kilometers.

Strehl Ratio. A measure of the quality of an image when close to the diffraction limit of an optical system. The Strehl ratio is the ratio of the maximum intensity of the actual image to the maximum of a perfectly diffraction limited image (provided both images are normalized in total intensity).

Tip-tilt Correction. Compensation of image motion only.

Tomography. The act of reconstructing 3-D information from a set of 2-D maps (or measurements). In the case of AO this is from turbulence measurements in multiple directions.

Wavefront. A light wave, characterized in a given X, Y plane by an intensity and a phase.

Wavefront Sensor. Device to measure the phase of a wavefront (that is, the deformations, i.e. bumps and holes). There are two primary types, a Shack-Hartmann and curvature which are described below:

Shack-Hartmann Sensor. Measures a wavefront phase by many contiguous square lenses in the beam. The deviation of the images at the focus of each lens gives a direct measurement of the "tilt" of the wavefront in front of the lens. From there, one can reconstruct the entire wavefront using various estimation algorithms.

Curvature Sensor. The ripples in swimming pools, once propagated through few feet of water, create bright caustics at the bottom. Curvature sensors use the same property of light to retrieve the phase from a curvature image, i.e. an image formed at a plan intermediate between the pupil plane and the image plane.

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