collimate-¹ In physics and astronomy, to render parallel to a certain line or direction; to render parallel, as rays of light; to adjust the line of sight or lens axis of an optical instrument so that it is in its proper position relative to other parts of the instrument. ² In photogrammetry, to adjust the fiducial marks of a camera so that they define the principal point; adjustment for collimation. See collimation; collimation adjustment.

collimation-¹ The process of bringing the optical elements of an optical system into proper relationship with each other. The process of bringing the collimated system into proper relationship with the pointing mechanism is called alinement. ² Adjusting the fiducial marks in a camera so that the lines through them intersect at the principal point.

collimation, error of—The angle between the line of collimation (line of sight) of a telescope and its collimation axis. When the collimation adjustment of an instrument is perfect (which is never the case), the line of collimation and the collimation axis coincide, and the error of collimation is zero. Usually, the adjustment is carried to where the error is so small that it can be considered to be negligible in many classes of work; or in precise work, after the adjustment is made, the residual error is either determined by observation and applied as a correction or is eliminated from the result by a suitable program of observations. Error of collimation is a systematic error and in a series of observations is usually treated as being of the constant-error type.

collimation, line of—The line through the second nodal point of the objective (object glass) of a telescope and the center of the reticle; line of sight, sight line, pointing line, aiming line of the instrument. The center of the reticle is defined by the intersection of cross-hairs or by the middle point of a fixed vertical wire or of a micrometer wire in its mean position. In a leveling instrument, the center of the reticle may be the middle point of a fixed horizontal wire.

collimation adjustment—The process of bringing the line of collimation of a telescope into close agreement with the collimation axis. Also called “adjustment for collimation.”

collimation axis—The line through the second nodal point of the objective (object glass) of the telescope and perpendicular to the axis of rotation of the telescope. In a surveyor\’s transit or theodolite, the collimation axis is perpendicular to the horizontal axis of the telescope. In a leveling instrument it is perpendicular to the vertical axis of the instrument. When the telescope of a transit is rotated about its horizontal axis, the collimation axis describes a plane called the “collimation plane.”

collimation correction—See correction, collimation [LEVELING].

collimation error-¹ Vertical—when the zero on the zenith circle is not aligned with the vertical axis of a theodolite. ² Horizontal—when flopping the scope does not result in a 180° horizontal angle.

collimation plane—The plane described by the collimation axis of the telescope of a transit or theodolite when rotated around its horizontal axis.

collimator—A device consisting of a convergent achromatic lens with a mark placed in the plane of its principal focus so that rays from the mark through the lens emerge along parallel lines.

collimator, auto—A collimator provided with a method of illuminating its cross-hairs in such a manner that, when a reflecting image is placed normal to the emergent-light beam, the reflected image of the cross-hairs will appear to be coincident with the cross-hairs themselves. This device is used in the calibration of optical and mechanical instruments. The mark in a collimator can be viewed from very short distances as if it were at an infinite distance, and can therefore be used in place of a distant mark when making any adjustment of the line of sight (line of collimation) of an instrument. In adjusting a surveying instrument, the telescope of another surveying instrument can be used as a collimator, the reticle furnishing the mark; or the telescope of a discarded instrument can be placed on a special mounting to form a permanent installation. In some astronomical instruments, a vessel of mercury, placed directly under the instrument, is used as a collimator. A prismatic eyepiece used with such an instrument is sometimes called “collimating eyepiece.” A collimator of special design may also be constructed for a particular purpose. See also collimator vertical.

collimator, vertical—A telescope so mounted that its collimation axis can be made to coincide with the vertical (or direction of the plumb line). The vertical collimator serves as an optical plumb line; it may be designed for use in placing a mark on the ground directly under an instrument on a high tower or in centering an instrument on a high tower directly over a mark on the ground. See also collimator, auto.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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