Which one is a disadvantage of the Internal Focusing Telescope?
Correct Answer: A. There are two separate tubes
📚 Detailed Explanation: Disadvantage of Internal Focusing Telescope = Two Separate Tubes
Why A (There are two separate tubes) is correct per the exam: An internal focusing telescope has an extra internal sliding concave lens element, which creates what the exam classifies as a “two separate tube” configuration. This is listed as a disadvantage because the double-tube design makes the interior harder to clean if dust or moisture enters. The remaining options (B, C, D) describe genuine advantages of internal focusing.
Internal vs. External Focusing Telescope
| Feature | External Focusing | Internal Focusing |
|---|---|---|
| Focusing mechanism | Outer slide tube moves the objective lens in/out | Internal concave lens slides forward/backward; outer tube is fixed |
| Tube design | Single sliding tube | Fixed outer tube + internal sliding lens carriage (exam classifies as two tubes) |
| Balance | Balance changes as tube extends | More balanced; outer tube fixed → option D is an advantage |
| Collimation | Line of collimation affected by tube sliding | Less affected → option C is an advantage |
| Tube movement | Outer tube slides → can collect dust/moisture | No outer tube movement (sealed) → option B (no sliding) is an advantage |
Note: The extra internal concave lens does reduce image brightness (less light transmission), which is another genuine disadvantage of internal focusing not listed in the options. The exam answer is A.
- Exam-identified disadvantage: two separate tube configuration (internal lens creates a compound internal assembly).
- Real-world disadvantages also include: reduced image brightness due to extra lens, more complex manufacturing.
- Options B, C, D are all genuine advantages of internal focusing telescopes.