9.1.1

# Astronomical Telescopes

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## Telescopes

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.

### Images

• 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.

### 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.

### Definition of magnification

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

### 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 = $\frac{angle \;subtended\; by\; the\; magnified\; image}{angle\; subtended\; by\; the\; object\; without\; a\; lens\; in\; place}$

### 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 = $\frac{focal\; length\; of\; the\; objective\; lens}{focal\; length\; of\; the\; eyepiece}$
• $M=\frac{{f_o}}{{f_e}}$