= Solution
Atmospheric refractive-index fluctuations distort the incoming wavefront. In a Kolmogorov model, the long-exposure seeing width is approximately $0.98\lambda/r_0$, where the <Fried parameter> scales as $r_0\propto\lambda^{6/5}$. High-quality sites and longer <wavelengths> can improve the observing conditions, but do not eliminate the distorted phase. Short coherence times also require rapid correction or exposure.
<Adaptive optics> uses a guide source, <wavefront sensor>, <deformable mirror> and fast feedback loop. Tip-tilt correction removes image wander; higher-order actuators correct finer spatial distortions. Finite actuator spacing, sensor <noise>, delay and changing turbulence leave residual errors. A nearby natural <guide star> limits sky coverage, while a laser beacon supplies higher-order reference information over more directions. Conventional laser-guide-star systems still need independent image-motion information, commonly from a natural star.<Laser Guide Star for Adaptive Optics by European Southern Observatory>[ESO, tip-tilt information and sky-coverage limitation]{ref} <Anisoplanatism> limits the corrected field; laser finite-altitude sampling and variable <PSFs> need their own <calibration>.
<Lucky imaging> selects favorable short exposures and aligns them, trading exposure efficiency and <photon> statistics for reduced blur. <Speckle interferometry> uses short-exposure spatial-frequency information and suitable phase/statistical reconstruction instead of simply averaging away the speckles. Both need adequate time resolution, detector performance and <calibration>; selection cannot restore information never recorded. Interferometric methods can also recover fine structure with phase control or closure information. These approaches correct or infer wavefront effects; they do not make clouds, absorption or every atmospheric amplitude fluctuation disappear.<Adaptive Optics by European Southern Observatory>[wavefront sensing, deformable-mirror correction and atmospheric limitations]{ref}
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