offaxis

Off-axis refers to a location or direction that lies outside the central axis of a system, object, or reference frame. It is a relative term, indicating that something is displaced from a defined reference line or centerline rather than being aligned with it.

In optics and astronomy, off-axis designs place optical components or fields away from the central axis. This approach can eliminate obstructions, such as a secondary mirror, and reduce certain stray-light effects or diffraction spikes. Off-axis telescopes and coronagraphs use portions of larger parent optics offset from the axis, trading some symmetry for a wider usable field and unobstructed light paths. However, off-axis configurations often introduce asymmetries in the point-spread function and other aberrations that must be corrected in design and data processing.

In imaging and interferometry, off-axis arrangements describe the tilt of reference or illumination beams relative to the main axis. For example, off-axis digital holography uses a tilted reference beam to separate the desired signal from the zero-order and twin images, enabling clearer reconstruction of the sample.

In particle physics, off-axis beams refer to particle streams directed at detectors placed at an angle to the primary beamline. Off-axis detector positioning can yield different energy spectra or event characteristics, a principle employed in neutrino experiments to tailor the observed neutrino population and study oscillations and other phenomena.

Across disciplines, the term emphasizes a departure from a centerline or main axis, prompting trade-offs between field of view, symmetry, aberrations, or spectral properties.