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How Telescopes Work


Hans Lippershey of Middleburg, Holland, gets credit for inventing the refractor in 1608, and the military used the instrument first. Galileo was the first to use it in astronomy. Both Lippershey's and Galileo's designs used a combination of convex and concave lenses. About 1611, Kepler improved the design to have two convex lenses, which made the image upside-down. Kepler's design is still the major design of refractors today, with a few later improvements in the lenses and the glass to make them.

Refractors are the type of telescope that most of us are familiar with. They have the following parts:

  • a long tube, made of metal, plastic, or wood
  • a glass combination lens at the front end (objective lens)
  • a second glass combination lens (eyepiece)

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Diagram of a refractor showing the light path inside.

The tube holds the lenses in place at the correct distance from one another. The tube also helps to keeps out dust, moisture and light that would interfere with forming a good image. The objective lens gathers the light, and bends or refracts it to a focus near the back of the tube. The eyepiece brings the image to your eye, and magnifies the image. Eyepieces have much shorter focal lengths than objective lenses.

Achromatic refractors use lenses that are not extensively corrected to prevent chromatic aberration, which is a rainbow halo that sometimes appears around images seen through a refractor. Instead, they usually have "coated" lenses to reduce this problem. Apochromatic refractors use either multiple-lens designs or lenses made of other types of glass (such as fluorite) to prevent chromatic aberration. Apochromatic refractors are much more expensive than achromatic refractors.

Refractors have good resolution, high enough to see details in planets and binary stars. However, it is difficult to make large objective lenses (greater than 4 inches or 10 centimeters) for refractors. Refractors are relatively expensive, if you consider the cost per unit of aperture. Because the aperture is limited, a refractor is less useful for observing faint, deep-sky objects, like galaxies and nebulae, than other types of telescopes.

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