An Etymological Dictionary of Astronomy and Astrophysics
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A short distance from the target, in right ascension, where the telescope is pointed for various purposes.

→ alpha; → offset.

In → CCD detectors, same as → bias and → offset.

→ bias; → offset.

For a four dimensional → space-time, the → Schwarzschild metric is the only solution of → Einstein's field equations which describes the gravitational field created by a spherically symmetrical distribution of mass. The theorem implies that the gravitational field outside a sphere is necessarily static, and that the metric inside a spherical shell of matter is necessarily flat.

The theorem was first demonstrated in 1923 by George David Birkhoff (1884-1944), an American mathematician; → theorem

A abbreviated notation for various functions associated with quadratic differential forms. Each Christoffel symbol is essentially a triplet of three indices, i, j and k, where each index can assume values from 1 to 2 for the case of two variables, or from 1 to n in the case of a quadratic form in n variables. Christoffel symbols appear in many calculations in geometry where non-Cartesian coordinates are used. These symbols are fundamental in the study of tensor analysis.

Named after Elwin Bruno Christoffel (1829-1900), a German mathematician; → symbol.

1) A designated limit beyond which the passage of something must be stopped.
2) A device that cuts off a transmission of photons.

→ cut; → off.

Boré, from bor- present stem of boridan "to → cut" + noun suffix -é.

Filter rejecting all light with wavelengths on one side of the cutoff wavelength.

→ cutoff; → filter.

The electrode voltage which reduces the value of a dependent variable, e.g. anode current, to a specified low value.

→ cutoff; → voltage.

Wavelength at which the transmittance of a filter, or the detectivity of a detector, has fallen to one-half its peak value.

→ cutoff; → wavelength.

A short distance from the target, in → declination, where the → telescope is pointed for various purposes.

→ delta; → offset.

In the → Fanaroff-Riley classification, sources with R_FR < 0.5. Fanaroff and Riley (1974) found that nearly all sources with luminosity L(178MHz) ≤ 2 × 10²⁵h₁₀₀^-2 W Hz^-1 sr^-1 were of class I. FR-I → radio jets are thought to be → subsonic, possibly due to mass entrainment, which makes them amenable to distortions in the interaction with the ambient medium.

→ Fanaroff-Riley classification; → class.

In the → Fanaroff-Riley classification, → radio sources with hotspots in their lobes at distances from the center which are such that R_FR > 0.5. The → radio jets in FR-II sources are expected to be highly → supersonic, allowing them to travel large distances.

→ Fanaroff-Riley classification; → class.

A classification scheme for distinguishing a → radio galaxy from an → active galaxy based on their → radio frequency  and → luminosity and their kpc-scale appearance. Analyzing a sample of 57 radio galaxies from the → 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz, Fanaroff & Riley (1974) discovered that the relative positions of regions of high and low → surface brightness in the → lobes of extragalactic → radio sources are correlated with their radio luminosity. They divided the sample into two classes using the ratio R_FR of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar, to the total extent of the source up to the lowest brightness contour in the map. → Fanaroff-Riley Class I (FR-I) , → Fanaroff-Riley Class II (FR-II). The boundary between the two classes is not very sharp, and there is some overlap in the luminosities of sources classified as FR-I or FR-II on the basis of their structures. The physical cause of the FR-I/II dichotomy probably lies in the type of flow in the → radio jets.

Bernard L. Fanaroff and Julia M. Riley, 1974, MNRAS 167, 31P; → classification.

Same as → Greisen-Zatsepin-Kuzmin cutoff.

→ Greisen-Zatsepin-Kuzmin cutoff.

The radiation law which states that at thermal equilibrium the ratio of the energy emitted by a body to the energy absorbed by it depends only on the temperature of the body.

Gustav Robert Kirchhoff (1824-1887), a German physicist who made major contributions to the understanding of electric circuits, spectroscopy, and the emission of black-body radiation from heated objects; → law.

The point on the → Hertzsprung-Russell diagram of a star cluster at which stars begin to leave the → main sequence and move toward the → red giant branch. The main-sequence turnoff is a measure of age. In general, the older a star cluster, the fainter the main-sequence turnoff. Same as → turnoff point.

→ main sequence; → turnoff.

(adverb & preposition) From a place or position; at a distance in space or time. So as to be separated from support.

M.E., from O.E. of "away, away from;" cf. Du. af "off, down," Ger. ab "off, from, down;" PIE *apo- "off, away," → apo-.

Away from a place.

→ off; cognate with Av. and O.Pers. apā "away from, from," as below.

Ap-, from apâ-, from Av. and O.Pers. apā "away from, from;" cf. Skt. apa "away, off;" L. ab- "from, away;" Hittite appa; Gothic af-; Ger. ab-; E. of, as above; PIE base *apo- "off, away"

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.

Of computers, operating independently of, or disconnected from, an associated computer.

→ off; → line.

An observation when the telescope is pointed away from the source in order to measure the sky background contribution.

→ off-; → source; → observation.