optical axis

The optical axis of a telescope (or other optical instrument such as a camera) is often described as the center line through an optical system, passing through the center of each lens and mirror. The term is used in the description of optics. This is a valid description regarding axisymmetric optics (lenses and mirrors), including many telescopes and cameras, but some telescopes (and analogous radio reflectors) are asymmetric, termed off-axis. What can be said of the optical axis regardless of whether the instrument is symmetric or not is that a ray of light along the optical axis is not refracted by a lens (i.e., its direction is not changed by the lens), and if reflected by any curved mirror, is reflected at the mirror's center-of-curvature. In the case of reflector telescopes, often the optical axis is not actually used optically, being blocked by the secondary mirror, or a fold mirror, or whatever is located at the prime focus, which blocks a portion of the light received through the instrument's aperture. A motivation for off-axis reflector telescopes (and radio reflectors) is to avoid this, preserving collecting area that would be blocked. However, being off-axis amplifies those aberrations that increase with distance from the optical axis.

Lens-based systems can also be asymmetric (off-axis), but this is relatively rare, likely most often for lenses used in illumination.