D-type ionization front
pišân-e yoneš-e gune-ye D
Fr.: front d'ionisation de type D
An → ionization front of → H II regions whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 104 K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C12 / 2C2, where C1 and C2 are the sound speed ahead and behind the front respectively), the front is called D critical.
Fr.: face, front
1) The part or side of anything that faces forward.
→ ionization front.
From O.Fr. front "forehead, brow," from L. frontem "forehead," perhaps lit. "that which projects," from PIE *bhront-, from base *bhren- "to project, stand out."
Pišân, from pišâni "front, forehead," from piš
"before; in front," from Mid.Pers. pêš "before, earlier;"
O.Pers. paišiya "before; in the presence of" + -ân suffix
of place and time.
A device containing a radio-frequency amplifier and associated cryogenic systems, routers, and converters (mixers), whose input is the voltage from a receptor and whose output is an intermediate-frequency signal. → back-end.
→ front + end, from O.E. ende, from P.Gmc. *andja, originally "the opposite side," from PIE *antjo "end, boundary," from base *anta-/*anti- "opposite, in front of, before."
Piš-tah, from piš, → front, + tah "end;" Mid.Pers. tah "bottom." The origin of this term is not clear. It may be related to Gk. tenagos "bottom, swamp," Latvian tigas "depth;" PIE *tenegos "water bottom."
A border between two countries.
From O.Fr. fronter, from front "forehead, brow," → front.
Marz, from Mid.Pers. marz "boundary;" Av. marəza- "border, district," marəz- "to rub, wipe;" Mod.Pers. parmâs "contact, touching" (→ contact), mâl-, mâlidan "to rub;" PIE base *merg- "boundary, border;" cf. L. margo "edge" (Fr. marge "margin"); P.Gmc. *marko; Ger. Mark; E. mark, margin.
Fr.: front d'ionisation
An abrupt discontinuity between an H II region and the molecular cloud in which it has formed. In this transition region interstellar gas changes from a mostly neutral state to a mostly ionized state.
R-type ionization front
pišân-e yoneš-e gune-ye R
Fr.: front d'ionisation de type R
A spherical → ionization front of → H II regions that moves radially outward from the → exciting star at a velocity much higher than → sound speed in the surrounding cold neutral gas of uniform density (ahead of the front). R-type ionization fronts corresponds to early evolution of H II regions, and will eventually transform into → D-type ionization fronts. If the motion of the front is supersonic relative to the gas behind as well as ahead of the front, the front is referred to as weak R. The strong R front correspond to a large density increase across the front.
Shack-Hartmann wavefront sensor
hessgar-e pišân-e mowj-e Shack-Hartmann
Fr.: analyseur de front d'onde
An optical device, a modern version of the → Hartmann test, used for analyzing the wavefront of light. Theses sensors can be used to characterize the performance of optical systems. Moreover, they are increasingly used in real-time applications, such as → adaptive optics to remove the wavefront distortion before creating an image. It consists of a microlens array placed in front of a CCD array. A planar wavefront that is transmitted through a microlens array and imaged on the CCD array will form a regular pattern of bright spots. If, however, the wavefront is distorted, the light imaged on the CCD will consist of some regularly spaced spots mixed with displaced spots and missing spots. This information is used to calculate the shape of the wavefront that was incident on the microlens array.
Named after the German astronomer Johannes Hartmann (1865-1936), who first developed the method, and R. V. Shack, who in the late 1960s replaced the screen by a microlens array; → wavefront; → sensor.
pišân-e toš, ~ šok
Fr.: front de choc
The boundary over which the physical conditions undergo an abrupt change because of a → shock wave.
Fr.: front chaud
Meteo.: A leading edge that advances in a mass of air and replaces cooler air by warm air.
pišân-e mowj, mowj-pišân
Fr.: front d'onde
The locus of adjacent points possessing the same phase in the path of a wave motion. Its surface is uniform (spherical or plane) and normal to propagation direction in an isotropic medium. → wavefront distortion.
aršâyeš-e pišân-e mowj
Fr.: correction de front d'onde
cowlegi-ye pišân-e mowj
Fr.: distortion de front d'onde
The disruption of the spherical shape of a wavefront due to atmospheric turbulence which makes the adjacent points in the wavefront out of phase.
hessgar-e pišân-e mowj
Fr.: analyseur de front d'onde
In adaptive optics, a device that analyzes the light sample coming from the wavefront and determines the error in each part of the beam. The wavefront sensor used in adaptive optics is a → Shack-Hartmann type, which works in conjunction with a deformable mirror.
gerâ-ye pišân-e mowj
Fr.: inclinaison du front d'onde
The average slope in both the X and Y directions of a → wavefront or phase profile across the pupil of an optical system.