An Introduction To The Basic Concepts Of Telescopes.

Today, we know how our neighboring world's look like, we know how beautiful this universe is, we have seen the magnificent supernovas and we have our first image of a black hole with us. All thanks to the telescopes! Without any doubt, telescopes have become the most employed instruments in the field of Astronomy & Astrophysics. Anyone who is inclined to study this field, is bound to encounter a telescope at one point or the other. So, the third article of our series, "The Basics of Astrophysics" , is dedicated to this wonderful device.

Telescopes have indeed helped us to find out a lot about our place in this universe. In earlier days, pirates and sea captains used to carry the telescopes that only magnified their vision about four times and had a very narrow field of view. However, today’s telescopes are huge arrays that can view entire quadrants of space. A telescope acts as our extended eye and helps us to observe the things which the unaided human eyes are unable to see.

Technically speaking, a telescope is an optical instrument that makes distant objects appear magnified by using certain arrangements of lenses and curved mirrors. The term usually refers to optical telescopes, but they have undergone an enormous transformation since their invention in the 17th Century. Consequently, nowadays we have telescopes that can work in a wide range of wavelengths ranging from radio to gamma-rays . The main purpose of a telescope is to gather light or other radiations emitted by a remote object and then to bring that light or radiation to a focus, where the image can be observed, photographed or studied.

History of Telescopes

Hans Lippershey
Hans Lippershey

The first telescope was patented about 400 years ago in 1608 by a Dutch eyeglass maker Hans Lippershey. Although Lippershey did not receive his patent, news of his new invention spread like a wildfire across Europe. Lippershey's basic design consisted of a convex  objective lens and a concave  eyepiece. Hans' device was capable of magnifying objects up to three times their original size.

When Galileo heard about Hans Lippershey's telescope in 1609, He immediately started building his own design without even seeing Hans' device. He was able to make significant improvements in the performance of his telescope and achieved a magnifications of around 20 times. Moreover, Galileo became the first person to turn the telescopes towards the heavens above and consequently discovered Jupiter's four largest moons in 1610.

Galilean Moons
Galileo's ink renderings of the moon which were first telescopic observations of a celestial object
Image Credits - NASA

Types of Telescopes

Telescopes are of different types. There are many classifications: on the basis of their lens and mirror arrangement, on the basis of radiations they are sensitive to, on the basis of surrounding conditions they can withstand and many more. But there are three basic types of telescopes from which all the advanced versions are derived. These three types are as follow:

Refractor Telescope - The first telescope ever developed was a refractor type. The refractor telescope uses a lens to gather and focus light. The glass lens is at the front of the telescope and light is bent (refracted) as it passes through the lens. Mostly, the beginners use refractor telescopes as they are easier to use and require less maintenance as compared to the other type.

Refractor Telescopes

Diagrammatic description of working of a Reflector telescope
Image credits- reddit.com, HowStuffWorks, telescope-guide

Reflector Telescopes- The reflector telescopes use a mirror to gather and focus light. All celestial objects are so far away that all of the light rays coming from them reach the Earth as parallel rays. Since the light rays are parallel to each other, the reflector telescope's mirror is constructed to have a parabolic shape. The parabolic-shaped mirror focuses the parallel light rays to a single point. All modern research telescopes and large amateur ones are of the reflector type because of their advantages over the refractor ones.

Reflector Telescopes
Diagrammatic description of working of a Reflector telescope
Image credits- Liverpool Astronomical Society

Compound/Catadioptric Telescopes- Compound telescopes can be considered as hybrids of the reflecting and refracting telescopes. They combine the best of both types.

Compound Telescopes

Diagrammatic description of working of a Compound telescope
Image Credits- Abrams Planetarium

Basic Terminology Related To Telescopes

Objective Lens - The lens in front is known as the objective lens or primary lens. It collects light from the distant object and brings it to a point or focus.

Aperture - The diameter of the primary mirror or lens is known as its aperture. More is the aperture, brighter is the image formed. A good backyard telescope for has an aperture of 80 mm to 300 mm. Whereas, some big billion-dollar professional telescopes have mirrors with an aperture of 10 meters.

Focal length -When light falls onto a mirror or through a lens, it is then directed to come to a focus at a plane some distance away. This distance between the centre of a lens or curved mirror and its focus is termed as focal length.

Eyepiece -Eyepiece refers to a small tube that contains the lenses needed to bring a telescope's focus to a final image in the eye. Telescopes usually come with at least two eyepieces: one for low power and a second for a higher power view.

Magnifying Power - It tells us the amount by which a telescope increases the apparent size of objects. It is determined by dividing the focal length of the telescope by the focal length of the eyepiece. So more the focal length of the eyepiece, lesser will be the magnification but brighter will be the image.

Author's Message

I hope this article has given you a basic flavor of what a telescope is! If you are an astronomy enthusiast, make sure you look through a telescope at least once. It will be a surreal experience and will definitely motivate you to study the subject more passionately!

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