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Telescopes


How do telescopes make distant planets and stars appear so big? Read on to find out.


Have you ever used a telescope to look at the night sky? Even if you have not, you must have seen the detailed pictures of the moon, planets, and stars that were taken with telescopes.


What exactly is a telescope?

The word telescope mainly refers to an optical telescope. It is an instrument that is used to view objects that are far away. Seeing them through a telescope seems to make them appear closer and bigger. The world telescope comes from the Greek words: tele meaning 'far' and skopein meaning 'to see'.


How do telescopes work?

To understand how telescopes work, it is necessary to know how our eyes function. We are able to see objects because the light falling on these objects is reflected into our eyes.

If you see a small object, like a postage stamp, held at a distance, you will hardly be able to see its details. This is because not enough light is reflected from the postage stamp onto your eye, and because this reflected light does not occupy enough space on the retina of your eye.

However, you will be able to clearly see the postage stamp held at the same distance, through a telescope. This is because the design of a telescope includes a mechanism to catch more light from the stamp and bring that light to a point or focus so that a clear image is formed. Different types of telescopes achieve this in different ways:

  • Refractor telescopes use a lens known as the objective lens, while reflector telescopes use a curved mirror known as the primary mirror.

  • A lens called the eyepiece then takes this brighter light and spreads it over a greater part of your retina. The eyepiece works like a simple magnifying glass, by spreading the light from a small object over a bigger portion of your eye.
Thus, a telescope consists of an objective lens or a primary mirror that catches more light and forms a brighter image of an object, and an eyepiece lens that magnifies a part of this image onto the eye.


What are the different kinds of telescopes?

Optical telescopes come in a wide range. There are the little telescopes that you can hold in your hand. At the top of the range, there are giant telescopes like the Hubble Space Telescope, a complex and expensive piece of machinery that actually orbits the earth.

Telescopes are mainly divided in three types:

Refractor Telescopes

This type of telescope was first invented by Hans Lippershey in Holland. It was generally used by the navy and the military, before the famous scientist, Galileo, pioneered its use for astronomy.

In this type of telescope, a lens (or a combination of lenses) called the objective lens is used to gather the light and bring it to a point of focus. It is known as refractor telescope, because the lens refracts (bends) the light to a point of focus behind it.

Refractor telescopes are good for viewing details in the planets and stars. However, because large objective lenses are difficult to make, refractor telescopes usually have smaller lenses with diameters ranging from two to six inches. The smaller lens of the refractor may make it difficult to observe dimmer objects, such as galaxies or nebulae (luminous clouds of gas and dust) through it.


Reflector Telescopes

Reflector telescopes are among the common telescope designs used today. This telescope was first created by the scientist Isaac Newton, in about 1680. This type of telescope uses a curved mirror called the primary mirror instead of the objective lens, to gather light. Mirrors are easier to make than lenses, and so at the same cost as a refractor, you can usually buy a reflector telescope with more light gathering capacity.

The primary mirror in a reflector is positioned at the bottom of the tube that forms the body of the telescope (unlike the objective lens in a refractor telescope which is at the upper end). Because the light entering the telescope is reflected by the primary mirror back into the tube, reflectors have a small flat mirror called the secondary mirror. The secondary mirror, in the Newtonian design, is placed opposite the primary mirror at an angle of 45 degrees. At this angle, the mirror deflects the image out through the side, into the eyepiece.

Another design, called the Cassegrain design, places the secondary concave mirror at 180 degrees opposite the primary mirror and the eye piece directly behind it.


Catadioptric Telescopes

Catadioptric telescopes are also called compound telescopes. They are a combination of refractor and reflector telescopes because they use both, mirrors as well as lenses. It was first developed by German astronomer Bernhard Schmidt, in 1930. The Schmidt-Cassegrain Telescope, which is a kind of catadioptric telescope, is quite popular today. It makes use of a secondary mirror, which reflects light through a hole in the primary mirror into the eyepiece.


Apart from optical telescopes, are there any other kinds of telescopes?

In recent years, computers have revolutionised stargazing through telescopes. For instance, software has been created to help the telescope automatically point to, or track, specific astronomical objects as desired by the user. These telescopes are referred to as 'GoTo' telescopes by amateur astronomers.

Also, apart from optical telescopes that gather and focus light rays that belong to the visible part of the electromagnetic spectrum, radio, x-ray, and gamma-ray telescopes observe waves forming part of the electromagnetic spectrum that is not visible to the human eye. They are important tools for modern astronomers to study electromagnetic emissions from space objects that will help them learn more about our universe.


Have you used a telescope? Do you own a telescope? What kind of telescope do you own? For what do you use it? To share your views and opinions, click here.


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