How Do Reflecting Telescopes Work?

Written by joan reinbold
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How Do Reflecting Telescopes Work?
(NASA)

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Overview

Reflecting telescopes are a type of optical telescope that uses one or more mirrors to reflect light, forming an image. Because reflecting telescopes use mirrors, they are also called "catoptric" telescopes.

Inspired after the invention of the refracting telescope, people such as Niccolo Zucchi, Cesare Caravaggi and James Gregory (whose published concepts later became the Gregorian telescope) began experimenting with mirrors in telescope designs. Later, Sir Isaac Newton developed what is generally known as the first practical reflecting telescope in 1668. As material used for telescopes improved and silver plating was invented, the reflecting telescope's ability to reflect images improved.

Reflecting telescopes are generally built with two mirrors, a large one called the "primary mirror" and a small one called the "secondary mirror." The primary mirror is usually placed at one end of the telescope's tube, and the secondary mirror is placed in the eyepiece's line of sight. The eyepiece contains a magnifying lens.

A principle of reflection is that when light hits a mirror at any angle, it is reflected at that same angle. This means that the reflected image is not changed.

Depending on the type of reflecting telescope, the two mirrors can be a combination of concave, convex and flat mirrors. The secondary mirror, when flat, is placed at a 45-degree angle.

To obtain an image, the telescope is aimed at an object, and the light enters the tube. The light hits the primary mirror and is reflected to the secondary mirror. It is then reflected from the secondary mirror to the eyepiece, where the image is magnified and sent to the eye.

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