optical axis

The optical axis of a telescope or other optical instrument is the line along which a ray of light is not refracted by a lens, and if reflected by any curved mirror, is reflected at its center-of-curvature. In a telescope that is axisymmetric, the line is the axis of symmetry, through the middle. In the case of reflector telescopes, often the optical axis is not actually used optically, being blocked by the secondary mirror or whatever is located at the prime focus. The optical axis is of use in geometrical descriptions of the instrument's/telescope's optics.