anisoplanatism

Anisoplanatism is the variation of atmospheric optical distortions across a field of view caused by the finite vertical distribution of atmospheric turbulence. In practice, imaging through the Earth's atmosphere, light from objects at different positions in the sky traverses different turbulent layers, so the wavefront distortions measured along a reference line of sight do not fully apply to nearby lines of sight. This leads to a point spread function that changes with angular separation from the guide star used for adaptive optics or wavefront sensing, reducing image sharpness away from that reference direction.

In astronomy and optical communications, anisoplanatism is most pronounced for ground-based observations. Adaptive optics systems correct the wavefront using a guide star, often in a single direction; since turbulence varies with angle, the correction degrades with distance from the guide star. The effect depends on the vertical distribution of refractive index fluctuations, described by Cn^2(h), and on wavelength. A commonly cited concept is the isoplanatic angle, the angular scale over which the PSF remains similar; the angle generally increases with longer wavelengths and decreases with higher altitude turbulence.

Mitigation strategies include multi-conjugate adaptive optics, which uses multiple guide stars and deformable mirrors conjugate to different altitudes, as well as ground-layer adaptive optics and laser guide stars to extend corrected fields. Anisoplanatism remains a fundamental limit for wide-field high-resolution imaging from the ground.