Adaptive optics

Adaptive optics measures changing wavefront distortions and corrects them with deformable optical elements. In astronomy, it sharpens telescope images blurred by atmospheric turbulence.

Atmospheric turbulence: Irregular motion in the atmosphere that creates changing variations in air density and refractive index. It bends incoming starlight unevenly, producing the image blur adaptive optics corrects.
Optical wavefront: A surface connecting points in a light wave that share the same phase. Adaptive optics seeks to restore an approximately flat wavefront before it reaches the image.
Laser guide star: An artificial reference beacon created in the upper atmosphere by a laser for adaptive-optics wavefront sensing. It supplies a nearby reference when no suitably bright natural guide star is available.
Active optics: A telescope technique that slowly adjusts optical elements to correct structural or thermal deformation. Unlike adaptive optics, it corrects gradual telescope changes rather than rapid atmospheric turbulence.
Wavefront sensor: An optical instrument that measures the shape or phase variations of a light wavefront. It estimates the atmospheric distortion from incoming light, often using a guide star.
Atmospheric seeing: The blurring and image motion caused by atmospheric turbulence during astronomical observation. It describes the image degradation that adaptive optics aims to reduce.
Multi-conjugate adaptive optics: An adaptive-optics design using multiple deformable mirrors conjugated to different atmospheric altitudes. It corrects turbulence across a wider field than a single-conjugate system.
Lucky imaging: An imaging method that selects and combines the sharpest short-exposure frames affected by atmospheric turbulence. It avoids real-time mirror correction by keeping only moments when turbulence briefly produces sharp images.
Shack–Hartmann wavefront sensor: A wavefront sensor that uses a lenslet array to infer local wavefront slopes from spot displacements. Its lenslet pattern is a common way to measure distortion for astronomical correction.
Fried parameter: A measure of the transverse coherence length of light passing through atmospheric turbulence. It helps determine the spatial scale and actuator density needed for correction.

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