An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

An → optical system in which the → optical axis of the → aperture is not coincident with the mechanical center of the aperture.

→ off; → axis; → optical; → system.

1) Of, pertaining to, or applying optics or the principles of optics.
2) Of or pertaining to sight or vision; visual; of or pertaining to the eye.

From → optic + → -al.

→ optic.

An imperfection in the imaging properties of a lens or mirror. The main aberrations are → chromatic aberration, → spherical aberration, → coma, → astigmatism, → field curvature, → distortion.

→ optical; → aberration.

The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).

→ optical; → activity.

The mathematical evaluation of an optical system to determine its basic optical properties and image quality characteristics.

→ optical; → analysis.

An instrument used to test lenses by utilizing the → optical transfer function. It consists of a HeNe laser, a beamsplitter and two mirrors.

→ optical; → autocorrelator.

Line passing through the optical center and the center of curvature of a → spherical mirror or → lens. Same as → principal axis.

→ optical; → axis.

A track or table on which sources, lenses, mirrors, and other optical components can be mounted and moved. It is used in optics experiments.

→ optical; bench, M.E., from O.E. benc "long seat;" cf. Da. bænk, M.Du. banc, O.H.G. banch.

Miz "table," originally "preparations for entertaining a guest; guest;" Mid.Pers. mêzd "offering, meal;" nurik, nuršenâxti, → optical.

Of a thin lens, a point situated at the geometrical center of the lens, through which an incident ray passes without being deviated.

→ optical; → center.

Any device such as a → lens, → prism, → mirror, and/or other similar objects used in an → optical system.

→ optical; → component.

An astronomical object with usually weak emission in the → visible found to be the optical representation of the object radiating chiefly in other → wavelengths of the → electromagnetic spectrum.

→ optical; → counterpart.

The transmittance of a point on a photographic negative equal to the log to the base 10 of the reciprocal of the transmittance through the negative at that point.

→ optical; → density.

1) A measure of how much radiation is absorbed when traveling through a medium (such as the atmosphere of a star or the interstellar medium) from the source to a given point. It depends on the type of medium and the frequency of radiation. It is defined as a dimensionless quantity dτ_λ = κ_λ ρ dx, where κ_λ is the extinction coefficient, ρ the density, and dx the path length.
2) The ratio of the intensity of radiation (light or radio waves) incident on a ring to that emerging from the opposite face of the ring, expressed as a natural logarithm. If the reduction in intensity is by a factors of e (= 2.718), the ring is said to have an optical depth of 1. Normal (or normalized) optical depth is optical depth corrected for oblique (non-vertical) viewing. In ring studies, the terms optical depth and optical thickness are generally used interchangeably. These terms generally refer to a particular wavelength (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ optical; → depth.

A pair of stars that lie close to each other in the sky by chance, but are not physically associated, in contrast to a true → binary star.

→ optical; → double star.

A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote internal reflection. It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.

→ optical; → fiber.

A perception of visual stimuli in which what is perceived is in a way different from the way it is in reality. Same as visual illusion.

→ optical; → illusion.

An instrument that either processes light waves to enhance an image or analyzes light waves to determine one of a number of characteristic properties.

→ optical; → instrument.

An → astrophysical jet which is visible in the optical range of the electromagnetic radiation.

→ optical; → jet.

(polar flattening) The ratio of the difference between equatorial and polar diameters to the equatorial diameter. A sphere has an oblateness of 0; an infinitely thin disk has an oblateness of 1 (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ optical; → oblateness.

In → geometric optics, the distance a light ray would travel in a vacuum in the same time it travels from one point to another, a specified distance, through one or more optical media.

→ optical; → path.