Educator Features

Build a Telescope

The student will construct a simple refracting telescope and calculate the magnification.

diagram of a refracting telescope
Illustration of a refracting telescope

Science and Mathematics Standards

Science Standards
  • Science as Inquiry

  • Physical Science

  • Mathematics Standards
  • Communication

  • Computation/Estimation

  • Theory

    In a telescope, the lens held next to your eye is called the eyepiece and is usually a short focal length lens or a combination of lenses. The lens at the other end of the telescope is called the objective lens. Light from a distant object is focused by the objective lens to form an image in front of the eyepiece. The eyepiece acts as a magnifier and enlarges that image. The magnification of the telescope can be found by dividing the focal length of the objective by the focal length of the eyepiece.

    • 2 converging lenses (convex lenses)

    • telescoping tubes (mailing tubes)

    • manila file folder

    • scissors

    • knife or saw

    • glue

    • 1 white poster board

    • red and black tape

    This telescope will be constructed using the same lenses that were used in the experiment named, "Focusing Light With a Lens," page 49 in the Educator Guide: Optics - Light, Color and their Uses.

      1. The mailing tubes will be the body of the telescope with the smaller one sliding inside the larger one. The length of the assembled telescope will be a little longer than the sum of the focal lengths of the two lenses. Add the value of the focal lengths of the short and long lens together. Divide that length by two and then add another inch. Cut both of the tubes to that length with a knife or saw.

      2. Use the scissors to cut out two circles from the manila paper that are the same size as the diameter of the mailing tube. These circle frames will mount and center the lenses on the tube. With a knife, cut out circles that are slightly smaller than the diameter of the lenses in the center of the paper frame circle. Glue the lenses to the center of the frame. The shorter focal length lens will be the eyepiece. Glue that framed lens to the end of the smaller tube. Glue the other framed lens to the end of the larger tube.

      3. Slide the two cardboard tubes together. You have now assembled a simple refracting telescope. Look through the eyepiece of your telescope and focus it on a distant object. Slide the two cardboard tubes in and out until you have a clear image. What do you observe?

      4. Use the red and black tape to make stripes on the white posterboard (see illustration on page 55 in the Educator Guide: Optics - Light, Color and their Uses) to use as a chart.
    diagram showing parts of a telescope
    Assembly illustration of a telescope

    Observations, Data, and Conclusions

      1. To compute the power or magnification (M) of your telescope, you will use the focal lengths computed in the experiment named, "Focusing Light With a Lens," page 49 in the Educator Guide: Optics - Light, Color and their Uses. Insert the number for each previously computed focal length into the following equation:

      the formula for magnification

      The magnification of my telescope is?

      2. Evaluate your calculated magnification. Stand at one end of the room and look at the chart with red and white stripes, and black and white stripes. Look directly at the chart with one eye and look through the telescope with the other eye. This may be a little difficult at first, but with a little practice you will find that you can do it.

      3. How much is the chart magnified?

      a chart that helps you test your telescope
      Telescope magnification chart

      4. Do you think the amount of magnification observed through your telescope matched the magnification you computed for your telescope?

      5. In observing objects through your telescope, did the image appear clear?

      6. How was the observed image oriented?
    Comment: The useful magnification of a telescope is limited by diffraction. This diffraction limit is about 10 times magnification per inch of diameter of the objective lens.

    Example: an objective lens 2 inches in diameter will provide a realistic telescope power of 20 times.

    Optics: Light, Color and Their Uses Guide

    Excerpt from the Optics: Light, Color and Their Uses Guide