An Etymological Dictionary of Astronomy and Astrophysics
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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.

A member of a rare class of pulsars, such as the → Crab pulsar and → Vela pulsar, which can be detected in the visible spectrum.

→ optical; → pulsar.

A process in which light energy is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion. The technique was developed by 1966 Nobel Prize winner Alfred Kastler in the early 1950's.

→ optical; → pumping.

A collection of lens, prisms, mirrors, and/or other devices, placed in some specified configuration, to act on light (reflect, refract, disperse, polarize, etc.) and perform some definite optical function.

→ optical; → system.

Same as → optical depth.

→ optical; → thickness.

The function that provides a full description of the imaging quality of an optical system. A combination of the → modulation transfer function (MTF) and the → phase transfer function (PTF) , the OTF describes the spatial (angular) variation as a function of spatial (angular) frequency.

→ optical; → transfer; → function.

The optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground.

→ optical; → window.

Adverb of → optical.

→ optical + -ly.

Nurâné, from nur, → light, + -âné "-ly;" nurikâné, from nurik, → optical, + -âné "-ly."

Relating to → optical activity.

→ optically; → active.

The qualifier of a medium in which the → optical depth is large, significantly larger than 1.

→ optically; → thick.

A wind with the → sonic point located at large optical depth for continuum. In particular, → Wolf-Rayet star winds are → optically thick. However, the outer parts of W-R winds are → optically thin for continuum, and in those regions the matter flow is driven by the same mechanism as in the winds of OB stars.

→ optically; → thick; → wind.

The qualifier of a medium in which the → optical depth is large, significantly smaller than 1.

→ optically; → thin.