Surveying telescope optics – objective and diaphragm placement: Which statements correctly describe the internal arrangement of a typical surveying telescope (diaphragm location and objective lens components)?

Introduction / Context:
Surveying telescopes (in levels, theodolites, and total stations) use a compound objective and a reticle (cross-hair diaphragm) to provide a sharp, parallax-free image for precise readings on staffs and targets. Understanding where the diaphragm sits and how the objective is built improves field adjustments and troubleshooting optical issues.

Given Data / Assumptions:

• The instrument is a Keplerian telescope with an objective that forms a real image.
• A cross-hair diaphragm must lie at the image plane for parallax-free viewing.
• Achromatic correction is achieved using crown and flint glass elements.

Concept / Approach:
To eliminate parallax, the diaphragm is placed at the common focal plane of the objective–eyepiece system. Practically, this lies between objective and eyepiece, closer to the eyepiece so both the image and cross hairs are in focus together. Achromats pair a crown-glass double-convex element with a flint-glass concavo-convex (meniscus) element to reduce chromatic and spherical aberrations, yielding a sharp, flat field near the optical axis for accurate stadia and levelling work.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer exploded diagrams show the cross-hair ring just ahead of the eyepiece lens group, and achromatic objectives combining crown and flint elements with opposite dispersions to minimize secondary spectrum.

Why Other Options Are Wrong:

• Each individual statement is correct but incomplete alone; only the combined choice fully captures the internal arrangement.

Common Pitfalls:
Confusing “line of sight” with the precise “line of collimation”; placing the diaphragm away from the image plane causing parallax; assuming a single-element objective is adequate for precision work.

Final Answer:
All of the above