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What is a Reflector Telescope?
Reflector Telescopes Reflect Light to a Common Point
Reflector telescopes use one or several mirrors to reflect light. The mirror, which is the main optical device in a reflecting telescope reflects light, creating an image at the focal plane. This mirror is usually a parabolic mirror that directs all incoming light into a common point.
The first reflecting telescope was invented in the seventeenth century by Issac Newton. At that time, the reflecting telescope was the only optical design for observing the stars; however, it had a defect known as spherical aberration. For this reason, other people attempted to make other types of reflecting telescopes that could deal with this problem.
Before the invention of the first reflecting telescopes, there was another type of telescope known as the refracting telescope. A refracting telescope uses a lens that refracts light to a common point, but this type of telescope suffered from severe chromatic aberration. that is to say, the light wavelengths changed direction when passing through the lens, producing color deffects.
How Does a Reflector Telescope Work?
A reflector telescope consists of a primary mirror which acts as the main optical element in a telescope. The primary mirror reflects light to a point at a given distance from the mirror. The distance from the mirror to the focal plane is known as the focal length. The focal length depends on the curvature of the primary mirror. In a reflector telescope sometimes a secondary mirror is placed close to the focus to deviate the light in another direction for visual observation.
The primary mirror is usually of a parabolic shape with a thin layer of aluminum, silver or any other type of reflective surface. The use of parabolic mirrors in reflector telescopes prevents an optical defect known as chromatic aberration; however, reflective mirrors may cause other types of aberration, such as coma, field curvature, astigmatism and distortion. To get rid of these aberrations, some reflecting telescope designs utilize a set of modified mirrors and correcting lens.
Types of Reflector Telescopes
After the invention of the reflector telescope in the 17th century, there have been other designs that have been invented by other telescope enthusiasts. The principal characteristic of these telescopes is that they all include a primary mirror and one or more secondary mirrors, and all of these mirrors in conjunction reflect light to a common focus for visual observation.
The Newtonian Telescope
The principal components in a Newtonian telescope include a parabolic primary mirror which reflects light to a common focus. A secondary mirror, which is flat, is placed some distance before the focal plane and its main task is to reflect the light out of the telescope. An eyepiece helps an observer view the image formed at the focal plane. Amateur telescope designers sometimes choose this as a design to make at home due to the simplicity of its construction.
The Gregorian Telescope
The Gregorian telescope uses a parabolic primary mirror with a hole in the center. The light that enters the telescope tube is reflected by this mirror to a secondary, usually, concave mirror, which reflects the light back to the primary mirror and out of the telescope through the hole in its center. The image formed can be viewed with an eyepiece by an observer.
The Cassegrain Telescope
This type of telescope uses a parabolic primary mirror and a secondary mirror with a hyperbolic shape. The light is reflected by the primary mirror to the secondary mirror. The secondary mirror reflects the light back to the primary mirror. The light then travels out of the telescope through a hole at the center of the primary mirror. The Cassegrain telescope is usually very compact, although, with a long focal length due to the folding and diverging of the light rays.
The Ritchey Chretien Telescope
This type of telescope is similar to the Cassegrain telescope, except that this telescope uses two hyperbolic mirrors. one as the primary and the second as the secondary mirror of the telescope. This design creates long focal lengths due to the mirror configuration; eliminates many optical aberrations and is widely used in research astronomy.
The Nasmyth Telescope
This type of telescope uses a primary parabolic mirror and a secondary hyperbolic mirror. The light travels from the primary mirror´s surface to the secondary, which then reflects the light back to the primary mirror; however, a third flat mirror, placed between the primary and secondary, reflects the light out of the telescope through a hole on its side where the image formed can be viewed with an eyepiece.
Why are Reflector Telescopes Used in Astronomy?
The reflector telescope is used in astronomy due that the light reflected off its surface is free of chromatic aberration, whereas in other telescopes, such as the refractor telescope, the light, which is refracted when entering the lens, causes chromatic aberration, Although building refractor telescopes is not impossible, it takes time and money to build big refractive telescopes.
The spectrum of light in a reflector telescope is more ample and can give sharper images than those of a refractor telescope, where some wavelengths are absorbed when entering its surface. In a reflector telescope only one surface is coated with a reflective surface, and the other surface can be used to build a mechanical support for large reflector telescopes.