Atmospheric turbulence produces changing refractive index fluctuations. These alter optical path lengths, distorting wavefronts and causing astronomical seeing and atmospheric scintillation.
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.
Atmospheric scintillation is the fluctuation of received stellar intensity produced by propagation through atmospheric turbulence. A phase-only deformable mirror does not generally remove these amplitude fluctuations.
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