Internal focusing telescope – focusing travel: For an internal focusing surveying telescope that must focus at all distances beyond 4 m, the minimum axial sliding range required for the focusing lens is approximately how much?

Introduction / Context:
Internal focusing telescopes shift a small lens group within the telescope tube to achieve focus, keeping the tube length fixed and maintaining collimation. Design guidelines specify a minimum mechanical travel to cover a practical focus range used in surveying, typically from a few meters to infinity.

Given Data / Assumptions:

• Required focusing range: from beyond 4 m to infinity.
• Optical group is lightweight and moves axially by a small amount.
• Mechanical travel must be sufficient for the focus shift while preserving stability.

Concept / Approach:
The movement of the internal lens causes a change in the effective image plane position to bring objects at various distances into sharp focus at the reticle. Practical instrument designs indicate that a modest travel—on the order of a centimeter—covers from several meters out to infinity. Hence, a typical minimum travel approximates 10 mm for common magnifications and focal lengths used in levels and theodolites.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturers’ specifications and classical surveying texts often cite ~10 mm travel for internal focusing systems providing focus from a few meters to infinity without excessive mechanical complexity.

Why Other Options Are Wrong:

• 5 mm: often insufficient to cover from ~4 m to infinity.
• 15–20 mm: larger than typical minimum required; may be used in special optics but not necessary as a minimum guideline.

Common Pitfalls:
Confusing external focusing (tube length change) with internal focusing; overlooking the need to refocus the eyepiece on the reticle first to eliminate parallax.

Final Answer:
10 mm