Refracting Telescope: Definition, Parts & Facts

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  • 0:00 What is a Refracting…
  • 0:45 History of Telescopes
  • 2:55 Parts of a Refracting…
  • 4:40 Using Refracting Telescopes
  • 5:25 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what a refracting telescope is and how it works, including each of the parts that make it up, and its relevance in historical and modern astronomy.

What is a Refracting Telescope?

Have you ever seen a stunning image of the universe released by NASA? They seem to get more and more incredible every year. But it's taken 400 years of telescope development to get where we are today. And it all started with the basic refracting telescope.

The Horsehead Nebula
The Horsehead Nebula

A telescope takes electromagnetic waves (which includes light) and focuses them at a point. This creates an image. An optical telescope does this job specifically with visible light, rather than ultraviolet, radio waves, or some other part of the electromagnetic spectrum.

There are two main types of optical telescope: refracting telescopes and reflecting telescopes. Reflecting telescopes use mirrors and historically came later. But the first kinds of telescopes to be made were refracting telescopes -- telescopes that magnify images of the sky using lenses.

History of Telescopes

As far as we know, the first refracting telescope was invented in the Netherlands. Like a lot of scientific history, we tend to focus on the Western world, and so it is possible that one was invented earlier. Certainly, as far back as Ancient Greece people understood that lenses could magnify images. But in the West, it took the development of eyeglasses to reach the sophistication necessary to build a refracting telescope. Although it's unclear who first invented the design, the opticians Hans Lippershey, Zacharias Janssen, and Jacob Metius have been credited as inventing the refracting telescope, in 1608. Even using the best glass of the time, the images from the first telescopes were still blurry.

In Venice, Galileo made significant improvements to their design in 1609. As an astronomer, he would use the telescope not to magnify things on the Earth, but to look at the planets and stars. He counted the moons of Jupiter, created maps of our own moon, and investigated sunspots.

Just two years later, in 1611, Kepler replaced the concave eyepiece in Galileo's telescope with a convex lens. This caused the image to be upside down, but it allowed you to see more in a single image and reduced eye-strain.

Unfortunately Keplerian telescopes still had distortion and aberration - these issues wouldn't be resolved for over a hundred years. All they could do was minimize the aberration by making telescopes super long.

Optical Telescope at Cincinnati Observatory
Optical Telescope at Cincinnati Observatory

But in 1758, John Dollond was finally able to solve these problems by attaching two very different pieces of glass together.

Little changed for a long time, but in recent years we've found ways to make the images even better. We've put telescopes in space to avoid the distortion created by the Earth's atmosphere. And using a form of complex mathematics called adaptive optics, computers can adjust images to compensate for the atmosphere here on Earth. These developments, along with high quality modern glass, have made refracting telescopes popular again with amateur astronomers, and they can create beautiful images.

Parts of a Refracting Telescope

Refracting telescopes focus light onto an eyepiece, sensor, or photographic plate using one or more curved glass lenses. The simplest kinds of refracting telescopes can have problems with a type of distortion called chromatic aberration. When light of different wavelengths (colors) enters the glass of such a telescope, each color refracts (bends) at different angles. This creates a blurry image, because the different colors of light are focused at different places.

Refracting Telescopes Suffer From Chromatic Aberration
Refracting Telescopes Suffer From Chromatic Aberration

Chromatic aberration can be fixed in modern refracting telescopes, but this requires multiple lenses.

The basic setup of the lenses in a refracting telescope looks something like the following:

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