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a lens or mirror telescope on a parallactic, or equatorial, mounting (seeTELESCOPE MOUNTING). A clock mechanism rotates the telescope around a polar axis at a rate of one rotation per 24 sidereal hours, so that the celestial object being observed, which moves because of the apparent diurnal rotation of the celestial sphere, remains in the field of view of the telescope. Graduated circles with measuring devices are secured to the axes to direct the telescope at the celestial object according to its hour angle and declination and to enable the approximate determination of its coordinates. Auxiliary astronomical tubes mounted on the primary one, called the finder and the guide, are used to locate the necesssary object and to keep the instrument directed at it during photography.
Equatorials may be visual (equipped with an ocular that has a micrometer for differential measurements) or photographic (with plateholders). (SeeASTRONOMICAL INSTRUMENTS AND DEVICES.)