Seeing

When looking at the stars in the sky we're also looking trough earth's atmosphere. This layer of gas around our planet is always turbulent to some degree and distorts the light coming to us from space. This can be observed by the naked eye if the seeing is quite bad, as the stars in the night sky will appear to twinkle. Along with diffraction, this turbulence creates limitations on the maximum amount of useful magnification you can use. The severity of the turbulence is often described on a scale of 1 to 10, with 10 being close to perfect conditions.

Deep space objects
Because a star (or any other object) flickers the light will be spread out across a wider area on the imaging surface. Depending on the image scale this can create large and fuzzy stars. In this situation, the seeing is measured by the size of stars during a long exposure - the full width at half maximum. This being the width of a star measured at the point where the brightness drops to half of that at the center. The unit of measurement is arc seconds. This effect also applies to nebula and other extended objects, blurring the details within them.

Planetary imaging
From our point of view the planets in our solar system are very small objects requiring high magnification to resolve any detail. This means that the seeing conditions are the dominant factor in the quality of the resulting image. Because the planets have a high apparent magnitude (brightness), astrophotographers can use lucky imaging to get the most out of the seeing conditions available. This is a process where extremely short exposures are used to freeze the effects of seeing in that particular instant. The amount of exposures can be several thousand. These exposures are then averaged out by combining them and sharpened.