Instrument Geometry – Definition of Line of Sight / Collimation in Survey Telescopes In a surveying telescope, the imaginary straight line passing through the intersection of the cross-hairs and the optical center of the objective is called what?

Introduction / Context:
Precise angle and level readings depend on a well-defined reference line within the telescope. Surveying terminology distinguishes between the line of sight (a general viewing direction) and the exact geometric line used for measurements, called the line of collimation. This question clarifies the correct definition used in instrument adjustment and observations.

Given Data / Assumptions:

• A telescope contains a cross-hair diaphragm and an objective lens.
• The cross-hair intersection defines the measurement reference point.
• Instrument is properly focused to place the image in the cross-hair plane.

Concept / Approach:

The line of collimation is rigorously defined as the straight line joining the optical center of the objective lens to the intersection of the cross-hairs. This is the line along which the instrument points for measurements. While some texts use 'line of sight' loosely as a synonym, in strict surveying adjustment procedures, 'line of collimation' is preferred for the exact reference used in collimation error tests and two-peg tests.

Step-by-Step Solution:

Verification / Alternative check:

Adjustment procedures (e.g., for theodolites) explicitly reference the line of collimation being perpendicular to the trunnion axis and coincident with the target direction when correctly adjusted.

Why Other Options Are Wrong:

'Axis of the telescope' often refers to the mechanical axis, not the optical measurement line. 'Principal focal ray' is not standard survey terminology. 'None of these' is invalid because a precise term exists.

Common Pitfalls:

Using 'line of sight' ambiguously; neglecting that small collimation errors cause systematic pointing bias unless eliminated by reversal or adjustment.

Final Answer:

Line of collimation