What Is A Visible Light Telescope? (Types, History & More!)

Telescopes are among the most important devices ever created by humanity. From professional astronomers to rank amateurs, there’s one for every possible viewing situation.

There’s also a lot of jargon around telescopes. One of the first and most important terms to understand is a “visible telescope,” since it’s by far the most common, especially for home use.

What a visible light telescope is & does

A visible light telescope, also known as an optic telescope, has three functions. It serves to gather the light of an object, to focus the light in an image, and to magnify it. There are three main variants: the reflecting telescope (which uses mirrors), the refracting telescope (which uses lenses), and the catadioptric telescope (a hybrid of the other two). visible light telescopes are distinct from several non-optical types that can observe non-visible light such as radio waves, ultraviolet and infrared waves, gamma-ray and X-ray waves.

Below, we’ll take a closer look at how these work. We’ll then discuss what you can see with a visible light telescope, how they came to exist, and some examples of the biggest and most impressive ones around.

How Visible Light Telescopes Work

For hundreds of years, telescopes have been altered and upgraded, but their working principles are the same. A visible light telescope collects and concentrates light.

It uses a convex lens or a concave mirror for this process. The bigger they are, the more light it collects, and thus the more distant and fainter objects become observable. Although the human eye has convex lenses, they are very small, and as such, we receive a very small amount of light.

After the light is collected, the telescope focuses that light and creates an image. Because of the shape of the lenses and the mirrors, the light rays meet at a single point, known as the focal point. This focal point is critical for a clear image.

If the light rays don’t meet, the image will be very blurry. The next step is to record that image. Many years ago, people used drawings to record the images they saw through a telescope.

Now, astronomers use CCDs, the Charge-Coupled Device, the same technology used in digital cameras.  CCDs are composed of pixels, and they can see objects that are 1000 million times fainter than the eye can see. If you wonder what makes a good visible-light telescope, let’s find out together:

  • Resolution: This is the telescope’s ability to see as detailed as possible an object.
  • Magnifying power: Magnification depends on focal length, the distance from the center of a concave mirror, or convex lens to the focal point. If the telescope focuses on a smaller piece of the sky, the magnification is increased. To discover larger pieces of sky, you must operate your telescope at the smallest magnification possible.
  • Instrument quality: Astronomical instruments include spectrometers, photometers, spectroheliograph, and CCDs. How effectively they spread the light, the quantity of light they capture, and their images’ quality are the factors we judge them by.
  • Light-collecting ability: The larger the mirror and lenses are, the more light they will collect, and the better we can see faint objects.

Next, let’s review the three main types of light telescopes, and each of them functions differently.

3 Types of Visible Light Telescopes You Should Know About

In visible-light astronomy, we use these three kinds of telescopes:

  1. Reflecting telescopes: To form the image, they use mirrors. You can see deep-sky objects such as galaxies, nebulas, distant stars, and more with them.
  2. Refracting telescopes: Due to ease of usage and cheaper prices, they are used mostly by amateurs to see bright objects like the planets or the Moon. They use lenses instead of mirrors.
  3. Catadioptric telescopes: Are a combination of the first two types, using both mirrors and lenses. With them, you can see just about anything. However, they are more complex to use.

Optical aberrations occur in each type of telescope. They are responsible for light being spread out over a large space region, rather than a single focused point. Here are the types of aberration for every telescope.

  • Chromatic aberration:  occurs in refracting telescopes. The lenses are unable to focus all colors to the same point. This means the colors are shown on edges separating light and dark parts of the image.
  • Off-axis aberrations: happen to reflecting telescopes, which means optical inaccuracies near the edge of the field in view.

It is important to understand that an aberration occurs when the lenses or another optical system fails to focus the light to one point, and it’s over a region. The image created will be distorted or blurred. Each type of aberration has its nature of the distortion.

Because of many catadioptric telescope designs, aberrations may include a little bit of both of the two previously mentioned types of aberrations. There are many catadioptric designs, each with some trade-offs, which we’ll cover in detail another time.

Do All Telescopes Use Visible Light?

We can answer that question with a definite “no”!

As their name implies, visible light telescopes are only for optical astronomy, i.e., observation of the light frequencies that we can actually see.

Conversely, invisible-light astronomy use the electromagnetic radiation spectrum.

A few examples are the radio waves, ultraviolet and infrared waves, gamma-ray and X-ray waves. In wavelength, visible light ranges between 380 to 750 nanometers. Let’s look at how telescopes were invented, especially how the visible light telescope came to be, and gauge their evolution!

What Can You See With a Visible Light Telescope?

All of these different types of telescopes gather light in different ways. If you wish to start stargazing, observing the Moon and the planets is the best way to start, and this can be done with a refractor telescope. This might be the best option for a beginner.

When it comes to deep-sky objects, such as galaxies and nebulae, a reflector telescope will do the trick! While the catadioptric telescope is used for both. So let us see how telescopes work and why some are best for viewing close objects, and others are more suited for faraway celestial objects.

A Quick History Of Visible Light Telescopes

For thousands of years, we have watched the stars and the night sky with awe, and today, even more so than ever. In ancient times astronomy was limited to the naked eye. The Greek ancients are responsible for many names we are still using today.

Take, for example, Ptolemy. He listed the first 48 Greek constellations, in his 2nd century Almagest, including some of the famous zodiacal constellations. However, his discoveries will later be outshined by the invention of the telescope.

The Invention of the Telescope

The most famous inventor of the optical telescope is a German-Dutch spectacle maker named Hans Lippershey. He invented the telescope in 1608. Even though he was the first to apply for a patent, it is not clear if he was truly the first who created the telescope.

In the following year, Galileo Galilei made a telescope with 3X magnification, but it’s not clear if it was made based on the descriptions Hans Lippershey was trying to patent. However, a 30x magnification improved version was made by Galileo, which is known as the Galilean telescope. Its common name was the spyglass.

At the time, Galileo was one of the few who proved that telescopes are useful to observe the sky by showing to the Venetian lawmakers one of his early telescopes on the 25th of August 1609.

The Visible Light Telescope

In March 1610, Galileo released his first telescopic astronomical observations. Its title was Sidereus Nuncius ( Starry Messenger). His invention’s benefits were many, as merchants purchased telescopes and found them very useful at sea.

Galileo then discovered the four major moons of Jupiter, which were later named the Galilean Moons. By analyzing their motion and the Moon phases, he demonstrated that not everything revolves around the Earth.

Today, because the telescopes are so available for everyone, amateur astronomers still practice visible-light astronomy. Outside of the Earth’s atmosphere, telescopes provide the best quality pictures and data.

They are called Space telescopes and are used by national agencies around the world. These telescopes provide clearer observations and better viewing quality as the image is not disrupted by the atmosphere, plus the ability to make observations at any time, not only at night.

The Largest Optical Telescopes Ever Built

If you are curious which are the largest telescopes ever built, here is a short chronological list:

  • Hooker telescope: From 1917 to 1948, the Hooker telescope was the biggest. It had an aperture of 2.54 meters. It was built at an altitude of 1742 meters on Mount Wilson, California.
  • Hale: Until 1976, the larger telescope was the Hale in San Diego County. It had an aperture of 5.1 meters and was built at an altitude of 1713 meters.
  • BTA-6 telescope: is located in southern Russia, on the north side of the Caucasus Mountains, in Zelenchukskaya District. It has a 6 meter (20 ft) aperture, and it was considered the biggest telescope until 1993.
  • The W.M. Keck Observatory: has two telescopes with an aperture of 10 meters each. The second one was finished in 1996 and was considered the largest in the world. They’re on Mauna Kea, the U.S. state of Hawaii, at an elevation of 4145 meters (13,600 ft).
  • The Gran Telescopio Canarias: is the biggest telescope globally since 2009. It’s situated on the island of La Palme, Spain. Its aperture is 1-4 meters (410 in) at an altitude of 2267 meters.

But that’s not the end of the list. In the next few years, some other giant telescopes will meet the light of the Universe for the first time. For example:

  • The Iranian National Observatory planned its first light in 2020.
  • In 2021 the James Webb Space Telescope is planned to launch in March.
  • The European Extremely Large telescope will meet its first light in 2024. It will have an aperture of 39.3 meters (1,550 in).


The invention of the visible light telescope/optical telescope changed the perspective of our world forever. Now, every one of us can gaze into the sky with these instruments and view the Moon, planets, distant galaxies and stars, and why not, even discover new celestial objects out there.

Just remember, with a refractor telescope, you can see the moon, planets; with a reflector telescope, you can view deep-sky objects, and with the catadioptric telescope, you can do both. The universe is your to conquer, and it’s all thanks to the invention of the visible light telescope.