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in astronomy, an auxiliary operation that is performed when photographing heavenly bodies. In this operation, an observer, using micrometer screws or auxiliary drives of the telescope, holds some heavenly body in the crosshairs of an eyepiece micrometer mounted in the focal plane of an auxiliary telescope, known as a guiding telescope. (A displacement of the heavenly body from the crosshairs of a telescope that is rotating in conformance with the apparent diurnal motion of the sky may be caused by errors in the telescope’s manufacture, by atmospheric effects, or by a shift of the observed heavenly body relative to the stars.) Large astrographs often have a special device (a Ricci cassette) that makes it possible to use the optics of the photographic telescope itself for guiding.