off-axis telescope

An off-axis telescope is generally a reflector telescope in which the primary mirror purposely reflects the light somewhat sideways, to avoid blocking incoming light by further optics. It is off-axis because all the reflection is to one side of the primary mirror's optical axis. (Other telescopes generally reflect symmetrically around the optical axis.) 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 specify off-axis, 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.