Optical Survey Instruments – Features of an Internal Focusing Telescope Which of the following statements correctly describe an internal focusing telescope used on theodolites and levels?

Introduction:
Modern surveying telescopes often employ internal focusing for robustness and sealed optics. Understanding which element moves and where it is placed helps in diagnosing parallax, collimation, and focusing issues in the field.

Given Data / Assumptions:

• An internal focusing design uses an additional focusing lens rather than moving the entire objective.
• The cross-hair diaphragm is fixed relative to the objective mount.
• Survey tasks require quick, smooth focusing without shifting external lengths.

Concept / Approach:

In internal focusing telescopes, the objective and diaphragm maintain a fixed separation to preserve image plane stability. Focus is adjusted by moving a negative (divergent) lens inside the tube, generally around the mid-section. This motion forms a virtual image at the cross-hair plane for targets at varying distances, while keeping the instrument compact and better sealed against dust and moisture.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer schematics and optics texts diagram internal focusing with a negative focusing lens and a fixed objective–diaphragm spacing.

Why Other Options Are Wrong:

'Only the objective lens moves' describes external focusing; it does not apply here.

Common Pitfalls:

Confusing removal of parallax (eye-piece and objective focusing) with the mechanical means by which focus is achieved inside the tube.

Final Answer:

All of the above