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An optical telescope is an instrument which magnifies distant images. It consists of two optical elements: a lens and an eyepiece or an eyepiece and a mirror, joined with an optical tube. The image obtained with a telescope is inverted and magnified. An optical telescope allows you to receive an exact (both in terms of spatial details and brightness distribution), the brightest possible image of the tested part of the sky or an astronomical object.


The main optical parameters of each telescope are:
- a diameter of aperture - responsible for resolving power of equipment and observational range
- focal length - a distance between the focus of an optical system and the main point of this system
- focal ratio - the ratio of the focal length of the telescope to its aperture
- magnification - depends on the eyepiece, which is replaceable; a change of an eyepiece changes magnification

Depending on whether refraction or reflection is used to gather light, telescopes can be divided into refractors and reflectors (as well as complex telescopes consisting of both lenses and mirrors).

Refractors (lens telescopes) use refraction of light by bending the light passing through another object, such as glass. In a refractor, an image is created by a lens, which bends the light into a single point of focus on a focal plane. Refractors are ideal instruments for observing the planets and the Moon.
Reflectors (mirror telescopes) reflect light off a curved mirror. The light is gathered on the primary mirror, then reflected to the secondary mirror, which then reflects it to the eyepiece. Usually these telescopes are very big, allowing an observation of even the smallest objects. First reflectors were made in the seventeenth century, shortly after the construction of a spyglass. Since then, they are being continually improved.
The most popular types of reflective telescopes are:

The Newtonian Telescope - designed in 1668. Its primary mirror was parabolic and the secondary mirror was flat, placed at a 45 ° angle to the optical axis. This type of a telescope has a simple design and you can build it yourself. It is suitable for observing all objects. Its main disadvantage is a small field of vision.



The Cassegrain Telescope - designed in 1672. The innovation of this telescope, compared to the Newtonian telescope, was the introduction of a secondary hyperbolic mirror. The beam was directed into the hole located in the middle of the primary mirror, where the eyepiece was placed. Small size is this telescope?s advantage, but it results in a small field of vision and a large coma.



The Ritchey-Chrétien - There are also telescopes which have both lenses and mirrors They are usually modifications of the Cassegrain telescope, for example the Schmidt-Cassegrain telescope or the Cassegrain-Maskutova telescope. They have an extra lens placed before the primary or the secondary mirror which is called a corrector plate.