Principal Focus


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principal focus

[′prin·sə·pəl ′fō·kəs]
(optics)

Principal Focus

 

in optics, the point where a beam of light rays converges after passage through an optical system when the incidence of the beam on the system is parallel to the optical axis of the system. When parallel rays in a beam diverge after passage through the optical system, the designation “principal focus” applies to the point of intersection of the straight lines that extend from the rays leaving the system. Conversely, a beam of rays originating at the focus becomes parallel to the axis of the optical system after passage through the system. The term “principal front focus” applies to a beam of parallel rays leaving the system and “principal back focus” to a beam of parallel rays entering the system. Both principal foci are located on the optical axis of the system.

In astronomy, the term “principal focus” is often used to describe the surface on which an image of the section of the celestial sphere under observation is constructed (either by the main mirror of the reflector or by the objective of the refractor). To achieve a correction of coma or enlargement of the clear image field, a corrector lens (for example, a Ross lens) is placed in front of the principal focus in the reflector. In the largest reflectors an observation booth is mounted at the principal focus.

References in periodicals archive ?
As was true for SARS, the principal focus of WHO-recommended nonpharmaceutical interventions is not at international borders but at national and community levels (4).
The essays are grouped into three sections: "In the Beginning was Watts," "Hymns and the Ordering of Protestant Life," and "Hymns as Good (or Bad?) Theology." The principal focus is on Protestants in the United States, but individual chapters also touch upon English-Canadian Protestantism and the use of Protestant hymns in Roman Catholic hymnals.
O Papel: How is this US$ 50 million investment allocated and what is the principal focus?
The principal focus of the Engaging Members Working Group was enticing more members to become involved in the Institute's far-flung activities.
In part, it may be that pre 9/11's principal focus was on stamping out illegal drugs, rather than bothering about the smuggling activities in West African countries or the petty crimes in the USA.
"It's our principal focus at the moment," Grayme Anthony, president of Houston Lake, says.
The principal focus, though, is on the years following the Civil War, a period in which miscegenation laws in the United States became more universal, when transgressions were punished more severely, and when many states redefined the boundary between white and nonwhite racial identities in an effort to make "whiteness" more exclusive.
The principal focus in the chapters on the eighteenth through twentieth centuries is on the acting styles and notable performances of such major actors and actresses as David Garrick, Sarah Siddons, Edmund Kean, the Booths, Henry Irving and his leading lady Ellen Terry, and Laurence Olivier.
With this amount of clear, concise, complete information so readily accessible by the responsible parties, improving communication among them became a principal focus. Enter, the alpha paging system, which uses text messaging via a Motorola Alphamate II pager, the Internet, or a software program; the SNF and hospital chose the Alphamate.
This acquisition is a departure from Globex's principal focus in Canada and to a lesser extent, the U.S.
We must avoid the all-too-common tendency to promote a platform or a medium versus what General Jumper and I consider the principal focus of our modernization efforts--capabilities.
"Our principal focus is still paper, but you need to deal with other waste material as well."

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