Telescopes are delicate instruments and need protection against daylight, atmospheric precipitations and dust. Therefore a closed enclosure is needed when the telescope is not observing the sky. Traditionally this building was topped by a rotating dome with an opening slit in order to give a free field of view to the telescope during the observations.
On the other hand it has been also known for some time that the very presence of the dome around the telescope is the cause of seeing effects that degrade the quality of the images observed. This was often expressed by astronomers as "the best dome is no dome", a paradox which is not always true since vibrations induced by the wind on an exposed telescope may degrade image quality as much as seeing, but which illustrates the dilemma faced by the designers of telescope enclosures.