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A system of wires, hairs, threads, etched lines, or the like, placed normal to the axis of a telescope at its principal focus, by means of which the telescope is sighted on a star, signal, or target, or by means of which appropriate readings are made on some scale, such as a levelling or stadia rod. See also cross wires, graticule, diaphragm.

In photogrammetry, a mark such as a cross or system of lines lying in the image plane of a viewing apparatus and used singly as a reference mark in certain types of monocular instruments or as one of a pair to form a floating mark as in certain types of stereoscopes.

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