In a reflecting astronomical telescope, if the objetcive (a spherical mirror) is replaced by a parabolic mirror of the same focal length and aperture, then

Options

A. the final image will be erect
B. a large image will be obtained
C. the telescope will gather more light
D. spherical aberration will be absent

Answer

To solve this question, we need to analyze the effects of replacing the objective spherical mirror of a reflecting astronomical telescope with a parabolic mirror of the same focal length and aperture. Let's break down the solution step by step.

### Step 1: Understanding the Telescope Configuration
In a reflecting astronomical telescope, the objective mirror is responsible for gathering light from distant objects and forming an image. The original configuration uses a spherical mirror, which can introduce spherical aberration.

**Hint:** Recall the basic structure of a reflecting telescope and the role of the objective mirror.

### Step 2: Identifying Spherical Aberration
Spherical aberration occurs when light rays that strike the mirror near its edge do not converge at the same focal point as those that strike closer to the center. This can lead to a blurred image.

**Hint:** Consider how the shape of the mirror affects the convergence of light rays.

### Step 3: Benefits of a Parabolic Mirror
A parabolic mirror, unlike a spherical mirror, focuses all incoming parallel rays (from distant objects) to a single focal point. This eliminates spherical aberration, leading to a clearer and sharper image.

**Hint:** Think about the geometric properties of parabolic mirrors and how they differ from spherical mirrors.

### Step 4: Implications of Using a Parabolic Mirror
By replacing the spherical mirror with a parabolic mirror of the same focal length and aperture, we can conclude the following:
- Spherical aberration will be absent.
- The telescope will gather light more effectively, potentially leading to a larger and clearer image.

**Hint:** Reflect on how the absence of spherical aberration improves image quality.

### Step 5: Conclusion
Based on the analysis, the most significant change when replacing the spherical mirror with a parabolic mirror is that spherical aberration will be absent. This leads to improved image quality and light gathering capability.

**Final Answer:** Spherical aberration will be absent.