Optical design – Ramsden eyepiece (used in surveying telescopes): Which specifications correctly describe a Ramsden eyepiece construction and its focal relationships?

Introduction / Context:
The Ramsden eyepiece is common in surveying instruments because it balances field of view, eye relief, and distortion. Recognizing its construction ensures proper interpretation of cross-hair placement and parallax behavior during focusing.

Given Data / Assumptions:

• Each lens is plano-convex with equal focal length.
• Convex faces are oriented inward toward each other.
• Specified separations are approximate design ratios widely cited for Ramsden optics.

Concept / Approach:
A Ramsden eyepiece uses two equal plano-convex lenses with convex faces facing. Typical separation is about 2/3 f (f being the focal length of one lens). The diaphragm is placed near the eye lens side; with the chosen distances, rays emerging from a diaphragm point are approximately parallel after passing the eyepiece, aiding in clear viewing of the reticle and minimizing aberrations in the central field.

Step-by-Step Solution:

Verification / Alternative check:
Optical handbooks list two-lens eyepiece families (Huygens, Ramsden). Ramsden positions the diaphragm between lenses near the eye lens, providing a real image plane suitable for cross hairs and less chromatic aberration centrally.

Why Other Options Are Wrong:

• Choosing any single statement would omit other necessary design descriptors; the Ramsden is characterized by the combination.

Common Pitfalls:
Confusing Ramsden with Huygens (which places the diaphragm outside and uses different lens spacing and orientation); misplacing the diaphragm leading to parallax or poor reticle illumination.

Final Answer:
All of the above