Astronomical seeing is the atmospheric blurring of a stellar image, conventionally expressed as the angular full width at half maximum of its long-exposure point spread function, usually in arcseconds. For a large aperture in the ideal Kolmogorov model, it is approximately radians, with Fried parameter .
The Fried parameter is the atmospheric coherence length for optical phase. In the standard inertial-range model, the phase structure function is . For unchanged atmospheric turbulence, . The NACO manual, section 3.1 describes this scaling and its role in adaptive correction.
Anisoplanatism is the loss of correlation between wavefront errors along different viewing directions. Correcting a guide star therefore leaves a larger residual for a sufficiently separated target.
The isoplanatic angle is the characteristic angular separation over which a single-direction atmospheric correction remains useful. It depends on the altitude distribution of atmospheric turbulence as well as on wavelength.
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