Why Do Astronomical Telescopes Use Mirrors Over Lenses?

In summary, large astronomical telescopes use a mirror instead of a lens for the objective optic due to the loss and distortion of light passing through a lens, difficulty in making and supporting large lenses, and the issue of chromatic aberration. These factors make a mirror a more practical and efficient choice for large telescopes.
  • #1
leehom
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Why do very large astronomical telescopes use a mirror rather than a lens for the objective optic?
 
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  • #2
Light must pass through the lens, this causes loss and distortion. A mirror does not have this problem. Large lens are very hard to make and the above problems are intensified with size. You will only find small refractors for this reason.
 
  • #3
In addition, there is the "strength" issue. A large mirror can be supported all along its back while a lens can only be supported arround its perimeter.

Also, a refracting telescope has its lens at the "top", pointed toward the heavens, while a reflector has its mirror at the bottom. Since those are the heaviest parts of large telescopes, large refractors are "top heavy".

Finally, the reason Newton built the Newtonian reflector to begin with: chromatic aberation. One type of glass refracts different colors of light differently. Using a lens made of a single type of glass instead of a point of light, you see a series of circles of different colors. You can fix that by making the lens of different kinds of glass so that the chromatic aberation of one "corrects" that of the other but for large lenses that is difficult and expensive.
 

1. Why do astronomical telescopes use mirrors instead of lenses?

Astronomical telescopes use mirrors instead of lenses because mirrors can gather and focus light over a larger area, resulting in a brighter and clearer image. Lenses, on the other hand, can only gather and focus light at their edges, resulting in a dimmer and less sharp image.

2. How do mirrors in telescopes work?

The curved shape of the mirror in a telescope allows it to reflect light and direct it towards a focal point, where an eyepiece or camera can then gather the light and create an image. The larger the mirror, the more light it can gather, resulting in a brighter image.

3. What are the advantages of using mirrors in telescopes?

The main advantage of using mirrors in telescopes is their ability to gather and focus more light, resulting in a clearer and brighter image. Additionally, mirrors are easier to manufacture and maintain compared to lenses, and they are not affected by chromatic aberration, which can distort images in lenses.

4. Are there any disadvantages to using mirrors in telescopes?

The use of mirrors in telescopes can result in a phenomenon called spherical aberration, where the edges of the mirror do not focus light as accurately as the center. This can be corrected by using a parabolic shape for the mirror or adding additional mirrors to the telescope design.

5. Can lenses be used in astronomical telescopes?

Yes, lenses can be used in astronomical telescopes, but they are typically only used in smaller, more portable telescopes. Lenses have a limited ability to gather light and can distort images due to chromatic aberration, making them less ideal for larger and more precise telescopes used in astronomy.

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