Astronomical Telescopes | Free Notes & Practice

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

Test yourself on Astronomical Telescopes

Why can we see distant objects more clearly with a telescope?
How is the size of an object measured?
What is the correct equation for the magnification of a telescope?

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