An off-axis telescope is a telescope that lacks symmetry around a point regarding the optical paths. The term may sometimes be taken to mean that some of the optical elements (lenses or reflectors that focus) do not include an optical axis (a ray that passes straight through). An example is a telescope with a parabolic primary mirror, but the mirror does not include or encircle the paraboloid's vertex, rather it consists of a portion of the paraboloid completely to one side of its vertex. Some telescope types are built around the idea and others can be designed to use it or not. For example, a prime focus telescope may be designed to be off-axis so that the mirror is not blocked. Many radio telescopes use off-axis configurations: the Green Bank Telescope is an example.