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

Describing a lens or optical system with a convergent power of zero, whose object and image points stretch to infinity. Thus, → afocal system.

Afocal, from → a- + → focal.

Akânun, from a- negation prefix, → a- + kânun, → focus. Bikânun, from bi- negation prefix + kânun, → focus.

An optical system with object and image points at infinity.

→ afocal; → system.

The focal length of an imaging system, which consists of several lenses or mirrors.

→ effective; → focal length.

Of or relating to a focus; placed at or measured from a focus.

→ focus + → -al.

same as → focal length.

→ focal; → distance.

The distance between the optical center of a lens, or the surface of a mirror, and its focus.

→ focal; → distance.

A plane at right angles to the principal axis of a lens or mirror on which the best image is formed.

→ focal; → plane.

Same as → focus.

→ focal; → point.

The ratio of the → focal length of a reflecting surface or lens to its effective diameter, i.e. to its → aperture. The smaller the focal ratio, the smaller the image scale and the more luminous the image for a given aperture.

→ focal; → ratio.

An optical component or system for changing the image scale of a telescope to achieve a better match between the → seeing disk and the → pixel size.

→ focal; → reducer.

An approximate value of the → focal length, used for the classification of lenses, mirrors, or cameras, as opposed to the measured one.

→ nominal; → focal; → length.