We have so far assumed that the seeing phenomenon is associated to turbulent temperature fluctuations. An apparent seeing effect can, however, also happen in absence of turbulence fluctuations if steady variations of temperature, hence index of refraction, are present along and across the light beam of the telescope. This situation may occur when the air inside the enclosure is globally at a different temperature than the exterior and it experiences a temperature gradient across the aperture. In such conditions the light rays across the aperture will be steadily refracted along slightly different paths and the focused image will experience a blurring effect similar to seeing (but no particular image motion).
Because the phenomenon is stationary or has anyway a very slow evolution in time, it can be eliminated by a ventilation of the telescope volume. The image blurring will also be readily corrected by an active optics telescope. One such a case is illustrated by Wilson[7] .
While such effects may have been confused with "turbulent" seeing in the past, it should be clear that this "quasi-static" image blurring is a different phenomenon, much simpler to understand and to correct, either by providing ventilation at some time intervals or by an active optics system. We will therefore consider hereafter only the seeing caused by turbulent fluctuations of the index of refraction.