Telescopes types: optical schemes

Telescopes that we are used to seeing in astronomical observatories and neighbors’ back yards are not the only types that exist. There are also infrared telescopes and radio telescopes.

  • The infrared telescopes are used in astronomy to study the thermal radiation of space objects. As you know, all types of matter, both liquid and solid, heated to a certain temperature, emit infrared spectrum energy. The wavelengths emitted depend on the heating temperature: the lower the temperature, the longer the wave and the lower its radiation intensity.
  • The radio telescopes are widely used in astronomical research to study electromagnetic radiation of various astronomical objects. Their construction uses special antennas that allow you to pick up a wide range of frequencies. Radio telescopes look like huge satellite dishes.
  • Optical telescopes are the telescopes that are used in amateur astronomy. Due to the special design of optical lenses and mirrors, optical telescopes allow us to see distant planets, stars, and galaxies with our own eyes. We’ll delve into them today.

Optical telescopes are classified by their type of optical scheme. There are three basic types of telescopes:

  • Refractor
  • Reflector
  • Catadioptric

Each of them has its advantages and disadvantages.

Refracting telescopes

An optic scheme of a refracting telescope

Refractors. These telescopes use only lenses in their objectives (no mirrors). They are easy to recognize: usually, they have a long and thin body, tapering from an objective to an eyepiece. They are the easiest to use and do not require adjustment. Because of the design, their aperture is small and does not allow you to observe dim objects. They are better for the Moon, planets of the Solar system and, maybe even binary stars. Refractors can be used as a very powerful spyglass, they are very suitable for beginners and children. Though they are more expensive than reflectors of the same aperture, however, they have several advantages:

  • Their closed telescope tube does not need regular care;
  • There is no air flow inside;
  • You don’t need to adjust them;
  • The image is saturated and contrast.

The disadvantages of most refractors are:

  • A low aperture (if it is high, then distortions increase too);
  • A large tube length;
  • They suffer from chromatic aberrations – the appearance of colored aureoles around bright details.

Achromatic refractor scheme

An apochromatic refractor uses two or three lenses, which are made of ultralow dispersion glass or of fluorite.

Apochromatic telescope scheme

Reflecting telescope

An optic scheme of a catadioptric telescope

Reflectors are designed based on mirrors, without lenses in their optical scheme. They are more broad and short than refractors, and their optical tube is open. They require periodic adjustment and maintenance (dust settles on the mirrors). The most inexpensive and most common reflector among lovers of nebulas is Newton’s optical scheme. They create an image with the help of a parabolic or a spherical mirror. Such telescopes are free of chromatic aberration (color distortion), which often affects refractors. The main disadvantage of reflectors is the effect of central shielding, which is a blackout in the center of the image. Large reflectors also have disadvantages as heat flows inside the tube and they need a long time to stabilize before starting observations. Such telescopes have a good aperture and therefore are great for the deep sky. If you want to take a deeper look into the sky, and you want the best value for your money, then this is the best option for you! Choose a reflector with the largest possible lens. You can also take a look at catadioptrics.

Catadioptric telescope: how it works?

Catadioptric (or mirror-lens) telescopes are made with a combination of lenses and mirrors. Such telescopes usually have compact closed tubes and are relatively light weight. This type was invented to correct the optical distortions of the other two types of telescopes. If you have the skills to use it, your catadioptric telescope can serve well both for the deep sky and for planets. However, they are the most expensive.

Each of these types of telescopes can be built in different ways and have different distortions.

The same should also be said about solar telescopes.

Chromospheric Solar Telescopes

You probably know that the main task of any telescope is to get as much light as possible. Actually, the ability to collect light efficiently makes it possible to transmit images of planets and stars. The more powerful the telescope, the greater its aperture, therefore it is strictly forbidden to look into a regular telescope at the Sun without a solar filter: this will lead to blindness! If you point the telescope at the Sun, you can light a cigarette from the eyepiece (of course it will also probably be the last seconds of your eyepiece’s life). Astronomers have a dark joke: you can look at the Sun through your telescope only two times – with your left eye and the right one. And do not forget that the optical finder is also a telescope, only a small one!

The special telescope with a protective filter system was invented for exploring the Sun. It is based on a refractor optical scheme and allows observing in a narrow spectrum – near the H-alpha hydrogen line, thence the Sun through it looks like the color of a carrot. The chromospheric solar telescopes can show many details on the Sun’s limb and its surface: solar prominences, flares, solar wind, etc. The strong built-in filtration system blocks UV and infrared radiation, so you can just stop worrying and start your exploring!

These special solar telescopes are expensive, and they are usually acquired only by people seriously involved in the solar activities and research studies. The rest of the astronomers prefer solar filters. They are cheap and can be attached to a regular telescope. Thus, you also will able to see the Sun, just not in detail. Yet you should make sure that you have attached the filter very well!


As mentioned above, each of the three types of telescopes can be constructed with different optical schemes. The design of the telescope largely determines its image quality.


  • Galileo Telescope
  • Kepler telescope
  • Achromat
  • Apochromat


  • Newton's system
  • Gregory system
  • Cassegrain system
  • Ritchey-Chretien system
  • Herschel (Lomonosov) system
  • Nesmith system
  • Schmidt
  • Korsch


  • Maksutov-Cassegrain
  • Schmidt-Cassegrain
  • Volosov
  • Maksutov-Newton
  • Gregory-Maksutov
  • Klevtsov
  • Super-Schmidts
  • Hamilton
  • Richter-Slevogt
  • Baker-Nunn Camera
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