A telescope's continuous viewing zone (CVZ) is that
region of the celestial sphere that the telescope can view any
time astronomers choose to do so, i.e., there aren't times when the
telescope cannot view that region.
For space missions, areas that are sometimes blocked by the Earth
or Sun would be excluded.
The concept is of less use for Earth-bound telescopes, but
seems relevant to radio telescopes not close to the equator,
i.e., a region around the pole can be viewed day and night all year.
A telescope proposal may specify the CVZ needed to meet the proposed
observation's goals.