There are several levels of air turbulence: atmospheric turbulence, local turbulence and internal turbulence.

• Atmospheric turbulence is caused by movements of air masses of inhomogeneous temperature, generally at high altitude. One of the major tasks of the amateur who wants to obtain high resolution results is to find a place (at least for its holidays !) where the atmosphere turbulence is small as often as possible. If he is unfortunate and lives in an area where seeing is almost always bad, he has to be very persevering because he will spend most of his nights waiting good conditions, as seeing is unpredictable and can change very rapidly.
• Local turbulence is caused by close objects like heated buidings, cars...or the observer himself ! It can be more easily avoided than atmospheric turbulence.
• Internal turbulence is located inside the instrument, if this instrument is not in thermal equilibrium. This kind of turbulence can be greatly diminished with some precautions: the telescope must be left outside as long as possible before use (of course out of direct lighting by the sun !). The air is more stable in a closed tube, but usually it is more difficult to reach thermal equilibrium with such an instrument.

In addition to air movements inside the tube (they can be seen on a defocused star), bad thermal equilibrium can lead to distortions of the mirrors. The upper face of a mirror cools down more quickly than its other face, then its shape changes and spherical aberration appears. It is very common to see, in a hot telescope, a strong spherical aberration that disappears slowly during the night. It is evident that a very good mirror in bad thermal equilibrium becomes a mediocre mirror, and to benefit from all the lambdas measured in the laboratory (where temperature is controlled !), thermal equilibrium must be irreproachable. Moreover, this deformation of mirrors changes their focal lenght and leads to defocusing (see The focusing).

Because of the air turbulence, the image of a star given by a telescope is usually not the perfect pattern that books show us: it is agitated or blurred. In most of the cases, both effects come together in variable amounts, depending of the seeing and the diameter of the telescope. Thanks to short exposure times, CCD can fight against agitation of the image by 'freezing' it. However, contrary to adaptive optics, CCD can do nothing when the image is blurred.

For more information about turbulence, read High Resolution Astrophotography (see bibliography).