How to build a telescope

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The type of telescope you will build in this activity is also called a monocular or a spyglass.



  1. Start at a short edge and roll one of the pieces of cardstock into a tube so the outer edge of the tube lines up with the edge of the concave lens. (For an 8.5″×11″ piece of cardstock, the resulting tube should be 8.5″ long).
    A hand holding a piece of cardstock rolled into a tube so it is the same diameter as the lens
  2. Tape the lens to the end of the tube and use additional tape to hold the tube together. Optionally, mark this end of the tube somehow (for example, draw a dot or a small eye) so you can remember that this is the end you look into.
    Cardstock tube with a lens taped to one end and additional pieces of tape holding the tube shape in place.
  3. Roll up the other piece of cardstock, starting along a short edge, so it fits inside the first piece. Use a piece of tape to secure the end of the second tube so it does not unroll. Slide the second tube inside the open end of the first tube.
    A second piece of cardstock rolled up and inserted into the first piece of cardstock.
  4. Tape the convex lens to the open end of the second tube.
    Concave lens taped to the other end of the cardstock tube.
  5. It is time to test your telescope! Hold your telescope with both hands and aim it at a distant object (something across the room or outside). Close one eye and look through the telescope with your other eye. Make sure you are looking through the first tube (the one with the concave lens). If everything looks smaller instead of bigger, flip the telescope around.
  6. Slide the second tube in and out to focus your telescope. Keep adjusting the focus until your view through the telescope is not blurry.

    Think about:
    What do you see? How much bigger do objects look when you look at them through your telescope compared to looking at them with just your eyes?
    Scenery with trees, clouds, and a power line. Part of the picture shows the view through the telescope, where the leaves and power line are magnified.

What Happened?

Your telescope should make far-off objects look several times bigger! In the picture above, look at how much bigger the leaves and power line look in the telescope than they do in the rest of the picture. Depending on how far away the object is, you will need to adjust your telescope to bring it into focus. It will not work on objects that are very close to you.

Digging Deeper

Your telescope is made using lenses. Lenses are curved pieces of glass or plastic that bend rays of light that travel through them. Lenses can make objects look bigger or smaller. Sometimes we only use a single lens to make something look bigger, like a magnifying glass.

Your telescope has two different types of lenses. This helps you see far-off objects. The eyepiece (the lens you look into) is a concave-concave lens, also called a diverging lens. Concave means curved inward. A concave-concave lens is curved in on both sides. The objective lens (the lens at the far end) is a convex-convex lens, also called a converging lens. Convex means curved outward. A convex-convex lens is curved outward on both sides. When you look through the telescope, light passes through the objective lens, then through the eyepiece, and finally into your eye. The lenses bend the light to make objects that are far away look bigger.

The type of telescope you built in this activity is called a monocular. Mono means one. Binoculars have two sets of lenses so you can look into them with both eyes.

See the link about telescopes in the Additional Resources section to learn more about different types of telescopes and the lenses they use.

For Further Exploration

  • Can you use a second pair of lenses and build binoculars?
  • Try making a telescope with two convex-convex lenses instead of a convex-convex lens and a concave-concave lens. What is different about the resulting image?
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