Astronomical Telescopes

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Telescopes are designed for viewing distant objects. They produce an image that is larger than the actual image, which can be seen up-close and clearly.

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  • Telescopes gather far more light than the eye.
    • This means that we can see dim objects as enlarged and much clearer.
  • An image is formed through an arrangement of lenses which can produce upright or inverted images.
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Telescope lenses

  • A telescope is made of two lenses:
    • The concave eyepiece - the lens that we look through, which focuses the light.
    • The convex objective lens - this is the front lens, which gathers the light from distant objects.

Telescope Images

An image seen through a telescope is greatly magnified so that we can see even dim and distant objects clearly.

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Definition of magnification

  • Magnification is the ratio between how large an object is without a lens and how large it is with a lens.
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Calculating magnification

  • We measure how large an object is using angles.
  • The angle from a line to the middle of the object and a line to the top of an object from the eye is called the subtended angle.
  • Lens magnification is given by the ratio between the subtended angles:
    • Magnification = angle  subtended  by  the  magnified  imageangle  subtended  by  the  object  without  a  lens  in  place\frac{angle \;subtended\; by\; the\; magnified\; image}{angle\; subtended\; by\; the\; object\; without\; a\; lens\; in\; place}
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Telescope magnification

  • A telescope has two lenses. This means that the magnification changes as each lens alters the image. The equation for the magnification of a telescope is:
    • Magnification = focal  length  of  the  objective  lensfocal  length  of  the  eyepiece\frac{focal\; length\; of\; the\; objective\; lens}{focal\; length\; of\; the\; eyepiece}
    • M=fofeM=\frac{{f_o}}{{f_e}}

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