The line passing through the second nodal point of the objective of a 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 levelling instrument, it is perpendicular to the standing axis (vertical axis) of the instrument. When the telescope of a transit or theodolite is rotated about its horizontal axis, the collimation axis describes a plane called the collimation plane.