Line of collimation or principal line of sight: It is the imaginary straight line joining the optical center of the objective glass with the point of intersection of the crosshairs of the diaphragm.
Bubble tube axis: It is an imaginary line on the longitudinal curve at the midpoint of the tube. It is also called bubble line. When the bubble is in the center, it is horizontal.
Binocular axis: It is a line connecting the optical center of the object glass with the center of the eyepiece.
Vertical axis: It is an imaginary line passing through the center line of the axis of rotation.
Instrument Height (HI): Instrument height is the elevation of the plane of collimation or plane of sight when the instrument is properly aligned. When the adjustment surface is leveled correctly, the line of collimation will rotate in a horizontal plane called the plane of collimation or plane of sight.
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