the most common seeing measurement is the full width at half maximum (fwhm) of the optical intensity across the "seeing disc" (the point spread function for imaging through the atmosphere). the fwhm of the point spread function (loosely called seeing disc diameter or ""seeing"") is the best possible angular resolution that can be achieved by an optical telescope in a long-exposure image, and corresponds to the fwhm of the fuzzy blob seen when observing a point-like source (such as a star) through the atmosphere. the size of the seeing disc is determined by the seeing conditions at the time of the observation. the best conditions give a seeing disk diameter of ~0.4 arcseconds and are found at high-altitude observatories on small islands such as mauna kea or la palma.