Surveying telescope construction: where is the diaphragm (cross-hair reticle) mounted inside a telescope for correct imaging and focus?

Introduction / Context:
The diaphragm (cross-hair reticle) must coincide with the real image formed by the objective to eliminate parallax and allow precise pointing. Knowing its position helps understand eyepiece and objective focusing roles in levels and theodolites.

Given Data / Assumptions:

• Objective forms a real image at its focal plane inside the tube.
• Eyepiece magnifies this real image and the cross hairs together.
• Parallax-free viewing requires the reticle to be at the same plane as the objective’s image.

Concept / Approach:

The correct location is at the objective’s focal plane, which lies nearer the eyepiece end of the telescope tube for distant targets. The eyepiece is adjusted so the reticle is sharply visible; the objective focus brings the target’s image onto the same plane as the reticle.

Step-by-Step Solution:

Verification / Alternative check:

Parallax test: move the eye slightly; no relative motion between image and cross hairs indicates coincidence of planes—possible only with reticle at the objective’s focal plane near the eyepiece.

Why Other Options Are Wrong:

Mounting inside the eyepiece or near the objective end would not coincide with the objective image plane for general targets; mid-tube is arbitrary.

Common Pitfalls:

Assuming the reticle is part of the eyepiece; confusing the internal focusing lens with the reticle position.

Final Answer:

In the telescope at the end nearer the eye-piece (at the objective’s focal plane)