Atmospheric extinction attenuates starlight through absorption and scattering. It depends on wavelength and path length, and is described by an optical depth or an extinction coefficient in astronomical magnitudes per airmass.
An atmospheric window is a wavelength interval of relatively high atmospheric transmission. Infrared photometric bands are largely chosen to lie in such windows.
Atmospheric refraction bends a ray through the vertical refractive index gradient, shifting its apparent direction. Because of optical dispersion, the shift varies with wavelength.
Atmospheric dispersion is the wavelength dependence of atmospheric refraction. It spreads a broadband stellar image along the vertical direction toward the zenith.
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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Atmospheric optics is the study of how light interacts with the Earth's atmosphere, leading to various optical phenomena. This field encompasses the understanding of how atmospheric conditions—such as the presence of particles, water vapor, and gases—affect the propagation and perception of light. Some key phenomena studied in atmospheric optics include: 1. **Refraction**: The bending of light rays as they pass through layers of air with different temperatures and densities, leading to phenomena like superior and inferior mirages.