What Is A Catad​ioptric Telescope?

Today’s amateur astronomer has more choices than ever before when it comes to choosing a telescope. Whereas backyard astronomers and stargazers in the past were limited to old-school technology like reflector telescopes or refracting telescopes, today we have available all of the varieties of those, as well as the more-recently-invented catadioptric telescope. But all of the varieties of telescopes can lead to confusion. A lot of people avoid catadioptric telescopes because they simply don’t know what they are.

In this article we’ll give you some background on the catadioptric telescope, the history of the technology they contain, and discuss some of their benefits and problems. We hope you’ll be able to use this additional information to make an informed decision when it comes time to choose a telescope.

[For more information on all types of telescopes, check out our Telescope Buyers Guide]

Catadioptric Telescopes: A Brief Explanation

The word “catadioptric” contains within it an explanation of what it is. “Catoptric” is a term applied to those telescopes that use mirrors, i.e., reflector telescopes. “Dioptric” is a term that refers to those telescopes that use lenses, i.e., refractor telescopes. Hence, the fusion term “catadioptric” indicates a system that combines both lenses and mirrors, which is exactly what a catadioptric telescope is.

In short, a catadioptric telescope is a combination of a refracting telescope and a reflecting telescope. It has a compound or lens/mirror combination that offers a combination of the benefits of the refracting and reflecting telescopes.

The technology used in a catadioptric telescope is often also found in many devices requiring the focusing of light or images, including headlights, search lamps, microscopes, and telephoto lenses.

[Check out one of our favorite catadioptric telescopes in our Celestron NexStar review]

History of the Catadioptric Telescope

When Was the Catadioptric Telescope Invented?

The history of the catadioptric system goes back to the beginning of the 19th century, which in terms of telescope technology means it is a relatively new development. In the 1820’s, Augustin-Jean Fresnel developed catadioptric lighthouse reflectors using specifically-shaped lenses and mirrors to focus the light so that it would remain true and visible for a greater distance for the protection of ships at sea.

Finding that this technology was so effective in focusing true and straight light beams over long distances, Leon Foucault saw a further application of this technology and in 1859 used a similar arrangement to build a microscope that avoided the distortions that are often encountered at higher magnifications.

Building on the technology, in 1876 French engineer Alphonse Mangin added a concave glass reflector with a silver coating on the back that resulted in a correction of the aberrations that sometimes result from a spherical mirror and thus created a truer reflection. This addition came to be known as a “Mangin mirror”, a device commonly integrated into today’s catadioptric telescopes.

How A Catadioptric Telescope Works

The catadioptric telescope is an optical telescope that uses a combination of specifically-shaped lenses and mirrors to form an image and transmit it to the eyepiece and, in turn, the viewer.

An optical telescope is one that focuses and collects light, mainly from the visible portion of the electromagnetic spectrum to create a magnified image for direct viewing. The power of an optical telescope to gather light and show small details is dependent upon the diameter (“aperture”) of the telescope’s primary mirror that gathers and focuses the light (the “objective”). The larger the objective, the greater the ability of the telescope to gather and focus light, and the greater its ability to show small details in the field of view.

Because of its combination of mirrors and lenses, the catadioptric telescope allows the folding of the light within the barrel of the telescope. The combination of lenses and mirrors also compensates for the weaknesses of each element. That is, the use of mirrors with additional correcting lenses eliminates the spherical aberrations encountered with mirrors.

Here's how a catadioptric telescope works:

How a catadioptric telescope works

1. Light enters through the end of the telescope through a correcting lens. The presence of this lens corrects the aberrations often encountered as a result of later steps of the process.

2. The light is then reflected off the primary mirror at the other end of the telescope.​

3.  Light strikes the secondary mirror in the center of the correcting lens.​

4.  Light is reflected back toward a hole in the primary mirror and into the eyepiece.​

5.  When the telescope is focused, the primary mirror moves, not the eyepiece.​

Catadioptric Telescope Advantages and Disadvantages

Benefits of the Catadioptric Telescope

  • Folding of the light within the barrel of the telescope allows for a shorter telescope that is more easily transported or carried without reducing focal length.
  • Combination of lenses and mirrors compensates for the weaknesses of each element individually.
  • Use of shaped mirrors creates a true beam of light that creates more detailed images of the field of view.

Limitations of Catadioptric Telescopes

  • Possible slight loss of light due to the positioning of the secondary mirror in the middle of the primary light-gathering mirror.
  • Image shift or jump in focus because of movement of the primary mirror during focusing.

What's the Best Catadioptric Telescope?

We always thing it's important to match your telescope to your particular needs and objectives, so it's not really possible to say what the best catadioptric telescope is without knowing what you're specifically looking for.

However, that being said, there is one catadioptric telescope in particular that we really love, and that won't cost you too much.

The Celestron NexStar 6se is a really great scope that can do a lot of things. It's got really good optics that will give you a lot of flexibility to view all sorts of objects, and it's got a stable, computerized mount to boot.

Just identify a series of reference points for the NexStar 6se, and it will be able to automatically go to any one of thousands of objects.

[Looking for more information about the Celestron NexStar 6se? Check out our full review here.]

Final Thoughts

If you’re looking for a new telescope, a catadioptric telescope is a great option. Especially for those astronomers looking for something that takes up a little less space, or one that improves on ordinary refractor/reflector setups, the catadioptric telescope is a great option.

[For some great telescopes that won't cost you an arm and a leg, check out our best telescopes under $200.]