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

The point on an elliptic orbit at the greatest distance from the principal focus. Also knwon as → apocenter.

From → apo- "away from, off," + → focus.

From apâ-, → apo-, + kânun→ focus.

The main focus in → Cassegrain telescope.

→ Cassegrain telescope; → focus.

An → optical system in which the → beam of light from the → primary mirror is reflected down through the instrument's → polar axis by a path bent like an → elbow. Since the focus remains fixed with respect to the Earth, light can be analyzed with permanently installed instruments. In addition long → focal lengths allow higher → spectral dispersions.

From Fr. coudé "elbowed," from coude "elbow," L. cubitus; → focus.

Kânun, → focus; ârenjvâr "elbow-like," → elbow.

To cause to deviate from accurate focus.

→ vâ- + → focus.

1) (n.) A point where parallel light rays from an object are gathered together by a lens or a concave mirror. It is the place where the clearest image of a distant object forms. Also called focal point. See also → focal distance.
2) (v.) To adjust the eyepiece or objective of a telescope so that the image is clearly seen by the observer.

From L. focus "hearth, fireplace," of unknown origin,

Kânun "hearth, fireplace."

Of an optical system, being in focus or brought into focus; adjusted to produce a clear image.

Past participle of → focus.

The act of bringing into focus.

Noun of → focus.

In an altazimuth-mounted telescope, a focal point to one side of the tube, created by placing a third deflecting mirror in the optical path. This extra mirror directs the beam along the altitude axis, and through a hole in the supporting trunnions. Nasmyth focus has the advantage of remaining at a fixed position relative to the telescope wherever the instrument is pointed. Moreover, bulky or heavy instruments can be mounted there on a permanent platform, which rotates only in azimuth.

Named after the inventor James Nasmyth (1808-1890), who first used it in the 19-th century.

The focus obtained by diverting the converging light beam of a reflecting telescope to the side of the tube.

→ Newtonian; → focus.

The focal point of a large primary reflecting mirror. This focus actually falls at a point just within the upper structure of the telescope and provides a large field of view.

→ primary; → focus.