An internally focussing telescope in a surveying instrument uses:
Correct Answer: C. A concave lens between the objective and the eyepiece
📚 Detailed Explanation: Internal Focussing Telescope Uses a Concave Lens Between Objective and Eyepiece
Why C (A concave lens between the objective and eyepiece) is correct: In an internally focussing telescope, a movable concave (negative) lens is placed between the objective lens and the eyepiece diaphragm. Moving this lens along the optical axis changes the focal length and brings the image into focus without changing the external length of the telescope.
External vs. Internal Focussing Telescopes
| Feature | External Focussing | Internal Focussing |
|---|---|---|
| Focussing mechanism | Sliding the objective lens or diaphragm; telescope tube length changes | Moving a concave lens between objective and eyepiece; tube length stays constant |
| Tube length | Varies with focussing — dust enters | Constant — sealed tube; dust excluded |
| Additive constant (C) | C = f + d ≠ 0 (typically 30–50 cm) | C ≈ 0 (anallatic telescope) |
| Stadia formula | D = KS + C (must apply C) | D = KS (C = 0; simpler) |
| Common instrument type | Older instruments | Modern surveying instruments (anallatic) |
Anallatic Telescope: An internally focussing telescope designed so that the additive constant C = 0. This means D = 100S exactly, with no additive constant to calculate. The movable concave lens is the key to achieving C = 0.
- Internal focussing = uses a movable concave (negative) lens between objective and eyepiece.
- The concave lens slides along the optical axis to focus at different distances.
- External tube length remains constant → sealed, dust-free → modern preference.
- Internal focussing enables the anallatic property (C = 0 in stadia).