Telescopes types: optical schemes

There are three different types of telescopes: reflector, refractor and catadioptric. Each type has its own advantages and disadvantages and suitable for different cases. This article is going to help you to understand, which is suitable for you. So, let`s see, what`s better: telescope reflecting or refracting, or catadioptric, maybe?

Refracting telescopes

An optic scheme of a refracting telescope

A refractor, or a lens telescope, is the oldest type of telescopes. It was invented in 1609 by Galileo Galilei and was updated in consequence. Firstly, a refractor used two lenses: convex as an objective and concave in an eyepiece, that caused spherical and chromatic aberrations. This scheme was supplemented by a second lens of different sort of glasses in objective to avoid visual distortions. Now, an achromatic refractor it`s a classic at astronomy: it gives a good image and doesn`t take too much money. If you want the best quality of image, then you should pay attention to apochromatic refractor. In this case, the objective has already three ultralow dispersion glasses and provide a clear image that is free of aberrations.

How refracting telescope works?

Any type of telescopes works like our eye: it gathers the light and focuses it in one point – in an eyepiece. The lighter an object – the better the image. Different design types are intended to reduce the distortions of the image. A refractor design is the oldest and easiest at the same time. It consists of an objective lens, which collects the light, and an eyepiece.

An achromatic refractor has in its objective two lenses of different sort of glasses, that helps to reduce chromatic aberrations.

Achromatic refractor scheme

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

Apochromatic telescope scheme

Advantages of refracting telescopes:

  • Their lenses are covered by a telescope`s case, so they are protected of dust
  • Their lenses are set up during production, so you do not need to be engaged in an adjustment
  • They give contrast and colorful image
  • Reliability thanks to the simplicity of a design
  • They are good for solar system planets, for the Moon and binary stars

Disadvantages of refracting telescopes:

  • Powerful refractors are more expensive than two other types
  • Long focal length causes bulky design
  • Not good for deep sky
  • Chromatic aberrations

Reflecting telescopes

An optic scheme of a reflecting telescope

In 59 years after Galileo`s discovery, Isaac Newton designed the first reflecting telescope, based on the scheme of Scottish astronomer James Gregory (his book “Optica Promota”, 1663 year). A reflector uses a mirror instead of lenses to gather light. After the hit in an objective, a beam of light goes to the opposite mirror and turns back to the eyepiece.

Types of reflecting telescopes and how do they work

There are several most famous reflecting telescope designs:

Cassegrain. The main big mirror is concave or parabolic. It transfers light to a small convex secondary mirror. The system suffers from a coma.

Cassegrain reflector scheme

Gregorian, which uses the concave parabolic main mirror and another one – small, concave elliptic. An eyepiece is built in the back of the pipe.

Gregorian telescope scheme

Newtonian. The secondary flat mirror is located near the focus point. As it`s turned diagonal, it leads the light outside. This type has a recognizable design, where the eyepiece sticks out of a pipe side.

Newtonian telescope scheme

Ritchey-Chrétien. An updated Cassegrain system. The main mirror is hyperbolic.

Ritchey-Chretien telescope scheme

The Newtonian scheme remains the most popular among reflecting telescopes.

Advantages of Reflecting telescopes:

  • Powerful reflectors are cheaper than two other types
  • Good for the deep sky (big aperture makes dim objects available)
  • They are compact (a mirrors system allows the long focal length to be fitted into the short case)
  • They give bright image without chromatic aberrations

Disadvantages of reflecting telescopes:

  • Low contrast image
  • The open design of a case causes hit of dust and dirt on mirrors
  • Need in a long time for temperature stabilization
  • Need in an adjustment time to time

Catadioptric telescopes

An optic scheme of a catadioptric telescope

It`s a mix of a refractor and a reflector: there are both mirrors, and lens in their optical train. This method allows creating relatively low-cost telescopes with a large objective. But, more important – they give a high-quality image. There are two most common types of catadioptric telescopes: Schmidt-Cassegrain and Maksutov-Cassegrain.

Catadioptric telescope: how it works?

The system of Schmidt-Cassegrain is based on spherical mirrors in a short closed case. A full-aperture correctional plate of Schmidt removes spherical aberration. The telescope has a wild field of view (up to 6 degrees), that is good for observation views. Residual distortions are the curvature of the field and a lump.

In a Maksutov-Cassegrain system, the special meniscus is responsible for the correction of distortions. It allows reducing almost all aberrations and able to form a better image than Schmidt-Cassegrain. Minuses of the system are a long time for thermostabilization and great weight.

Advantages of the catadioptric telescopes:

  • Powerful catadioptrics are cheaper than two other types
  • Good for both: deep sky and the planets
  • There are models with a closed case – protection against dust
  • Compactness

Disadvantages of the catadioptric telescopes:

  • Long time for thermostabilization
  • High cost
  • They need an accurate adjustment, so are better for professionals

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