Dome seeing, in its general meaning the seeing generated inside a telescope enclosure, can be distinguished into different phenomena:
From the engineering standpoint, there are two different avenues to prevent the occurrence of dome seeing in a telescope enclosure. One way consists in achieving a very tight thermal control of the telescope air volume. Chilling the floor of the telescope volume below the exterior air temperature, with the establishment of a stable stratification, does prevent the occurrence of dome seeing. In the CFHT observatory, where such a cooling system is installed, no seeing related to convection flow inside the dome can be detected, except for mirror seeing due to free convection on the surface of the primary. And even that phenomenon penalizes the telescope only for a fraction of the observing time, because the dome thermal conditioning keeps also the temperature of the primary mirror always at a close range with the dome's ambient air.
The second possibility consists in planning the natural ventilation of the telescope volume by the wind. The NTT, typical example of this configuration, can achieves virtually zero local seeing in this manner. However, when the ventilation is reduced by closing the back louvers a significant seeing reappears. Also, when the mirror is warmer than ambient air, mirror seeing appears present in the NTT.
Thus in conclusion, both general evidence and engineering experience points to mirror seeing as the main issue toward the removal of local seeing effects in large telescopes. This aspect will therefore be analyzed in more detail in the next section.