Exam 4: Astronomical Telescopes and Instruments

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The magnification of a telescope is indeed determined by the combination of the telescope's objective lens or mirror (which collects light and creates an image) and the eyepiece (which magnifies that image). While it is true that you can change the magnification by swapping out eyepieces with different focal lengths, there are practical limits to how much you can magnify an image. Here are several reasons why you can't magnify objects to very large sizes simply by choosing an appropriate eyiece:

1. **Optical Limits**: Each telescope has an optimal range of magnification, which is determined by the quality and aperture of the telescope's optics. Beyond a certain point, increasing magnification will not improve the detail of the image and can actually make it worse.

2. **Diminishing Light**: As magnification increases, the brightness of the image decreases because the same amount of light is spread over a larger area. This can result in a very dim image that is difficult to see, especially for faint objects like distant galaxies or nebulae.

3. **Atmospheric Disturbance**: The Earth's atmosphere can distort the light from celestial objects, a phenomenon known as "seeing." High magnification makes these distortions more noticeable, resulting in a blurry or wavy image. There's a practical limit to how much you can magnify an image before atmospheric conditions degrade the view.

4. **Field of View**: High magnification reduces the field of view, which is the area of the sky you can see at one time. This can make it difficult to locate and track objects, especially those that move, like planets or satellites.

5. **Eye Relief**: Eye relief is the distance from the eyepiece lens to where the eye must be positioned to see the full field of view. High magnification eyepieces often have short eye relief, making them uncomfortable to use, especially for people who wear glasses.

6. **Resolution Limit**: Every optical system has a resolution limit, defined by the Dawes' limit for telescopes, which is a measure of the finest detail it can resolve. This limit is determined by the diameter of the telescope's aperture. Beyond this limit, no additional detail can be resolved, regardless of magnification.

7. **Practicality and Usability**: Extremely high magnifications can make the telescope very sensitive to vibrations and movements, making the viewing experience frustrating and the image unstable.

In summary, while changing eyepieces can alter the magnification of a telescope, the quality of the resulting image is constrained by the physical and optical limits of the telescope, the viewing conditions, and the practicality of using such high magnifications. It's often better to use a moderate magnification that provides a clear, bright, and stable image.

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