Optical Telescope Facts & Properties

Coralie Nettles, David Wood
  • Author
    Coralie Nettles

    Coralie has taught university physics and tutored high school and college students in STEM since 2012. She has a bachelor of science and doctorate in physics from the University of Newcastle. She has worked at universities and schools in three different countries.

  • 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.

Explore optical telescopes. Learn how optical telescopes are defined and understand their properties. Discover interesting facts about optical telescopes. Updated: 11/15/2021

Table of Contents


What is an Optical Telescope?

People use optical telescopes for many applications, such as astronomy, hunting, navigating and bird watching. But what is an optical telescope? The optical telescope definition is: a telescope for the visible part of the electromagnetic spectrum. So, what is a telescope? A telescope gathers and focuses light to create a magnified image of an object. Telescopes exist for all parts of the electromagnetic spectrum. There are radio telescopes and x-ray telescopes, along with optical telescopes.

Optical telescopes collect incoming light using an objective. The light is then focused to a point using lenses, mirrors or both. The focused light is then viewed through an eyepiece. There are three primary types of optical telescopes, based on how light is gathered and focused. These are

  • Refracting telescopes, which use lenses.
  • Reflecting telescopes, which use mirrors.
  • Catadioptric telescopes, which use a combination of lenses and mirrors.

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Optical Telescope History

The earliest telescopes relied on lenses for magnification. The use of lenses in this way dates back to the Ancient Greeks but it was not possible to manufacture good quality lenses. It wasn't until the invention of eyeglasses in the 13th century that widespread manufacturing of lenses for this use began.

The first optical telescope was built in 1608 by a Dutch eyeglass maker named Hans Lippershey. This telescope was a refracting telescope and was composed of a convex and a concave lens. This telescope only produced a three times magnification of what was being viewed. Rapidly following this, the optical telescope spread through the Netherlands and Europe.

In 1609, Italian astronomer Galileo Galilei began constructing his own telescope. His telescope consisted of a convex objective lens and a concave eyepiece. He spent several months refining and improving his design and was able to increase the magnification from three times to 23 times. Galileo then began making astronomical observations with his telescope. He discovered the moons of Jupiter, observed the topography of the moon and tracked sunspots. In this way, Galileo revolutionized astronomy.

Optical Telescope Facts

Some interesting telescope facts include:

  • Currently, telescopes are most commonly used in astronomy to view objects that are incredibly far away. However, the first telescopes were used to see incoming ships.
  • The first telescopes built were all refracting telescopes despite the theoretical advantages of reflecting telescopes. This is because it was simpler to construct lenses than spherical mirrors.
  • Optical telescopes can see about 13 billion light years away. This light has been traveling since the start of the universe, essentially telescopes are looking into the past.

Optical Telescope Properties

The important features and properties of a telescope are:

  • The objective: this is the element that gathers light. In refracting telescopes this is a lens. In reflecting telescopes this is a mirror, also called the primary mirror. The larger the objective the more light is collected.
  • The aperture: this is the light-collecting area of the telescope. The size of the aperture is determined by the diameter of the objective lens or primary mirror. This determines the resolution of the telescope; a larger aperture can be used to look at smaller objects.
  • The eyepiece: this is a small lens which allows the image from the telescope to be viewed.
  • The focal length: this is the distance over which light is focused. A system with a short focal length has a greater optical power than one with a longer focal length.
  • The magnification: this is the amount by which an object is enlarged. It is determined by the focal length of the objective and the focal length of the eyepiece.

Optical Telescope Types

As mentioned above there are three primary types of optical telescopes. Refracting telescopes were the first telescopes to be built. They gather and focus light using lenses. Reflecting telescopes use mirrors to gather and focus light. While catadioptric telescopes use a combination of lenses and mirrors.

Refracting Telescopes

The earliest kind of optical telescope was a refracting telescope. These use one or more lenses to refract the incoming light.

Diagram showing the operation of a refracting telescope.

refracting telescope

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Frequently Asked Questions

Why are telescopes used?

Telescopes are used to make distant objects appear larger. They magnify distant objects with mirrors, lenses or a combination of the two.

What are interesting facts about telescopes?

The first telescopes were used to watch for incoming ships. Merchants wanted to be the first to see incoming ships so that they could sell their products first.

Why is the telescope important?

The telescope is important because it has a wide variety of uses. Telescopes can be used in areas such as astronomy, aviation, navigation and in military applications.

What is an optical telescope used for?

An optical telescope is used to help see distant objects easily. They gather and focus visible light to create magnified images of objects.

How does an optical telescope work?

A telescope uses mirrors or lenses to reflect or refract light to a single point, called the focal point, forming an image. An eyepiece is then used to view the image. They can be used to make very far away objects appear larger.

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