In a reflecting telescope, a secondary mirror is used to :

Options

A. make chromatic aberration zero
B. move the eyepiece outside the telescopic tube
C. reduce the problem of mechanical support
D. remove spherical aberration

Answer

To solve the question regarding the function of a secondary mirror in a reflecting telescope, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Components of a Reflecting Telescope**:
- A reflecting telescope primarily consists of a primary mirror that gathers light and focuses it to a point. A secondary mirror is also present, which plays a crucial role in the optical system.

2. **Identifying the Role of the Secondary Mirror**:
- The secondary mirror is positioned in such a way that it reflects the light gathered by the primary mirror. This reflection allows the light to be directed towards the eyepiece.

3. **Analyzing the Options Provided**:
- The options given are:
1. Reduce the problem of mechanical support.
2. Remove spherical aberration.
3. Make chromatic aberration zero.
4. Move the eyepiece outside the telescopic tube.

4. **Evaluating Each Option**:
- **Option 1**: While mechanical support is important, the secondary mirror does not primarily address this issue.
- **Option 2**: Spherical aberration is minimized by the shape of the primary mirror, not specifically by the secondary mirror.
- **Option 3**: Chromatic aberration is more related to lenses than mirrors, and the secondary mirror does not eliminate it.
- **Option 4**: The secondary mirror allows the eyepiece to be positioned outside the main tube of the telescope, making it more convenient for viewing.

5. **Conclusion**:
- Based on the analysis, the correct function of the secondary mirror in a reflecting telescope is to move the eyepiece outside the telescopic tube.

### Final Answer:
- Hence, the correct option is **Option 4**: Move the eyepiece outside the telescopic tube.

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