Telescope Eyepieces – Construction of the Ramsden Eyepiece Used in Survey Instruments Which component arrangement correctly describes a Ramsden eyepiece commonly used on surveying telescopes?

Introduction:
The eyepiece enables comfortable viewing and carries the cross-hair diaphragm at or near the focal plane. The Ramsden eyepiece is favored in surveying for its relatively flat field and manageable aberrations at modest cost.

Given Data / Assumptions:

• Eyepiece must permit focusing of the observer’s eye on the cross-hairs.
• It should minimize distortion and chromatic blur for instrument use.
• Construction details matter for image quality and parallax control.

Concept / Approach:

A Ramsden eyepiece consists of two identical plano-convex lenses, with their convex surfaces facing each other and separated by a short distance. The cross-hair diaphragm is placed near the common focal plane, providing a clear image with reduced spherical aberration compared to a simple single lens. This configuration is standard in many theodolites and levels.

Step-by-Step Solution:

Verification / Alternative check:

Instrument optics references diagram Ramsden eyepiece as P-C + P-C with convex faces inward, distinguishing it from Huygens (two plano-convex with plane faces inward).

Why Other Options Are Wrong:

Generic 'two convex/concave' descriptions lack the specific orientation; mixed convex–concave is not Ramsden; a single doublet does not define this eyepiece type.

Common Pitfalls:

Confusing Ramsden with Huygens orientation; forgetting the diaphragm position relative to the focal plane.

Final Answer:

Two plano-convex lenses placed close together with convex faces facing each other