This concept covers the construction and optical function of the Kepler-type internal-focusing telescope used in theodolites, including the formal definitions of diaphragm/cross wire, line of sight, and line of collimation, and the distinction between external and internal focusing systems. It further covers the temporary adjustments of a theodolite — setting, levelling, centring, and focusing — as a dependent, iterative sequence required at each survey station to bring the instrument's vertical axis over a ground point and its horizontal plate into a true horizontal plane before angle observation. It belongs to the domain of surveying (specifically theodolite/total-station instrumentation), and relates to the parent discipline through the method of repetition, a technique for improving angular measurement precision and eliminating instrumental graduation errors by combining face-left and face-right observations.
This concept covers the construction and optical function of the Kepler-type internal-focusing telescope used in theodolites, including the formal definitions of diaphragm/cross wire, line of sight, …