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Seeing, in astronomy, is a measure of the image quality obtainable during a telescopic observation. It is measured by the width (in arcseconds) of an unresolved point source's image.

Under ideal conditions, a point source will look like an Airy pattern -- a central bright circle, surrounded by a series of increasingly fainter circles. This pattern is caused by Fraunhofer diffraction by the aperture of the telescope. The distance between the center of the bright circle and the first dark ring of the Airy pattern is the resolution of the telescope, known as the Rayleigh criterion. The value of the Rayleigh criterion is

θ (radians) = 1.22 &lambda / D

θ (arcseconds) = 206265 × 1.22 λ / D

where θ is the angular distance, &lambda is the wavelength of the light you're observing, and D is the diameter of the telescope. For visible light (average wavelength of 5000 angstroms) and a 1-meter telescope, the theoretical resolution is about a tenth of an arcsecond.

If you've ever used a telescope, even one much smaller than one meter in size, you probably know the resolution doesn't come close to that. This is because the Earth's atmosphere is a soggy, blustery, turbulent mess of eddies and currents. These disturbances in the atmosphere change its refractive index as functions of time *and* location, which wind up blurring any starlight passing through the air. Seeing is the observed resolution limit for a given observation, a quantity that changes from site to site, telescope to telescope, and night to night. When astronomers discuss the quality of a given observation, seeing is one of the quantities they state.

In practice, seeing is usually limited to no better than half an arcsecond, regardless of the size of your telescope. This means that the smallest details you can make out in a large telescope are about half an arcsecond across. This is quite a nuisance if, for example, you're trying to observe both stars in a close binary star system, looking for extrasolar planets, or trying to study the morphologies of galaxies halfway across the universe. If two points were closer together than about half an arcsecond, they would be blurred together into a single, amorphous blob. For major observatories, "sub-arcsecond" seeing is considered good, and seeing of 0.5 arcseconds or less is generally considered superb.

You can get partly around this by placing telescopes on top of high mountains like Mauna Kea, or Cerro Tololo, because you are putting the telescope above a large mass of air that light would otherwise have to pass through. But even then, seeing can still vary from hour to hour, night to night, and season to season, depending upon the state of the upper atmosphere. Another way around it is to try and measure the distortions of the light caused by turbulence in the atmosphere, and then adjust the optics of your telescope to cancel them out in real time. This technique is called adaptive optics, and it has been used in some of the larger observatories around the world for the past decade.

The ultimate way to avoid seeing problems altogether is of course to put the telescope outside of the Earth's atmosphere. The Hubble Space Telescope has nearly diffraction-limited resolution (about 0.02 arcseconds with a 2.5-meter mirror). As a result, it has produced some of the finest optical images in history. Of course, it cost 100 times more than a ground-based observatory, but who's counting?