Optical Principles – Basic Construction of a Refracting Telescope for Surveying Which of the following statements correctly describe the optical construction of a refracting telescope as used in theodolites and levels?

Introduction / Context:
Survey instruments use compact refracting telescopes to form clear, magnified images of distant targets (staff graduations, targets, prisms). Understanding the basic lens arrangement aids in diagnosing focus, parallax, and aberration issues during fieldwork and maintenance.

Given Data / Assumptions:

• Simple refracting telescope (no mirrors) with an objective and an eyepiece.
• Cross-hair diaphragm placed near the focal plane.
• Optical axis alignment is properly adjusted at the factory.

Concept / Approach:

The objective, a convex lens, collects light from the target and forms a real, inverted image near the focal plane where the cross-hairs lie. The eyepiece then magnifies this image for the observer. Both lenses are coaxial, ensuring that the line of sight is centered and that vignetting or off-axis aberrations are minimized. This configuration is the foundation for more advanced designs (e.g., achromatic objectives, Ramsden eyepieces) but the essential two-lens description remains valid.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer diagrams show a convex objective and composite eyepiece (often two lenses acting as a system), all centered on a single optical axis with the reticle at or near the focal plane.

Why Other Options Are Wrong:

Because (a)–(d) are all correct, any option denying one of them would misrepresent the telescope’s basic design.

Common Pitfalls:

Confusing refracting telescopes with reflecting designs (mirrors); assuming the eyepiece is a single simple lens—while conceptually true, in practice it is a compound pair arranged as a unit.

Final Answer:

All of the above