What are Telescopes?
Telescopes are optical instruments that are used to magnify and observe objects which are very far away. Telescopes were first invented by a German-Dutch Spectacle-maker named Hans Lippershey. He had two sons. They were playing with lenses and they aligned two lenses in a straight line at a specific distance and looked through it. They discovered, when they looked at a faraway object through the lens, the object appeared to be magnified and they could see objects which were very far away. The children told this to their father (Hans Lippershey). He took a cylindrical tube and placed two lenses at both ends of the tube. It was called the Dutch Perspective lens. Since objects that were very far away could be observed very clearly using this equipment, the military also started using it to observe and spy on far away enemies. When the military used this equipment it was also called spyglass. Then, Galileo Galilei improved the spyglass by adding larger lenses. He was also the first person to observe astronomical bodies using the spyglass. Then he called it the ‘Telescope’. Hence, he is known as the father of telescopes. What Telescopes do is, capture a lot of light, and then focus and magnify.
Telescopes are mainly used for sky observation. Stars emit light in different parts of the light spectrum. There are different types of telescopes and are categorized into different types based on what part of the light spectrum it observes such as Visible Light, Radio waves, Microwaves, Infrared, UV, X-rays and Gamma-Rays.
1.Visible Light Telescopes
These are the most common and most popular types of telescopes. They are used to observe the object in visible light which is the light that we can see with our eyes. With this type, we can observe the object with our naked eyes or by attaching a normal camera to it. These telescopes use mirrors or lenses.
Different types of Visible Light Telescopes
Visible light telescopes are of mainly two types. They are Refractor Telescopes and Reflector Telescopes, classified based on the optics used in them (lenses or mirrors).
Refractor Telescopes
Refractor Telescopes are the classic ones. The first Telescope invented was of this kind and it uses lenses to collect, magnify and focus light.
First, let’s see what are the parts of a Refractor telescope.
The first part is the Tripod. It is a stand that holds the telescope very steady and stable. The main advantage of having a tripod is that there won’t be any vibration in the telescope when you are looking through the telescope. For example, if you are looking at the Andromeda galaxy through a telescope, even a small vibration due to the tripod can change the telescope to pointing at a different spot in the sky. So a good sturdy tripod is very important for good sky observation.
The second part is the optical tube. It is a cylindrical tube. It is called the ‘Optical’ Tube because the optics (lenses or mirrors) of the telescope is placed inside this tube. The focal length of a refractor telescope is the distance between the primary lens and the eyepiece. The focal length of a reflector telescope is the distance between the open side of the optical Tube to the primary mirror and eyepiece. Focal length is basically the distance light travels inside the optical Tube. The greater the focal length, the less is the field of view. The field of view of the Telescope tells you how much of the sky you can see through the telescope at one given moment. So, the higher the field of view, you can see more of the sky at a time.
The third part is the eyepiece. It is a small lens in which you look through the telescope. The eyepiece is made up of lenses no matter what type of telescope it is. The diameter of the lens used in the eyepiece also changes the field of view of the telescope. Lesser the diameter of the lens in the eyepiece, the lesser the field of view.
The fourth part is the viewfinder. It is basically a mini telescope. It is made up of lenses. It is used to find what you want to look at through the telescope. Since the field of view of the telescope is very low, only a small part of the sky is visible at a time. So it will be very hard to find an object by looking through the telescope. But the viewfinder has a higher field of view than the telescope. So, it will be easier to find objects by looking through the viewfinder. So, when you align the viewfinder at what you need to look at and then when you look through the telescope, you will find that the telescope is also pointed at the same object.
The fifth part of the telescope is the mount. This one comes in between the optical tube of the telescope and the tripod. There are mainly two different types of telescope mounts. They are the Equatorial mount and Alt-Azimuth mount.
Equatorial Mount
The equatorial mount is mostly used for astrophotography or photography of astronomical bodies. This mount needs to be aligned towards the pole star (Polaris) for it to work properly. Using this mount, the telescope can be moved in two axes. They are the vertical axis (x-axis) and horizontal axis (y-axis). This mount is harder to use but its advantage is that, when this mount is correctly aligned at the Pole star, you need to move the telescope on only one axis to track an object in the sky. But, this mount also has a disadvantage- This mount works precisely only when you are viewing from the Northern Hemisphere of the earth. Because the pole star is not visible from the southern hemisphere.
Alt- Azimuth Mount
Alt-Azimuth means altitude and Azimuth. This mount is very easy to use and it does not need to be aligned towards the pole star. With this mount, we can change the altitude (vertical movement) and azimuth (horizontal movement) of the telescope. This mount is used to view multiple objects one after another rather than observing one object for a long time.
So how does a refractor telescope works?
Let’s imagine that we are viewing a galaxy. The light from the galaxy enters the optical tube from the left side as shown in this picture. The left side is open so that that light can enter. When the light enters the optical tube, first it will pass through the objective lens or the primary lens. Because both sides of the lens are curved, when light passes through the lens, it will bend. It bends the light in a particular way that light from the top part of the galaxy will bend and intersect the light from the bottom and they will pass through each other. As a result, the galaxy will appear to be upside down and left side right (i.e. inverted image) and that light will enter the eyepiece, which focuses the light. So when we view the galaxy through the eyepiece, we can see it very clearly. The distance between the primary lens and the eyepiece is called the focal length. The longer the focal length, the higher the magnification and lower the field of view.
Reflector Telescopes
These telescopes were invented by Isaac Newton. Hence, they are also called the Newtonian telescope. They use mirrors instead of lenses.
Now, let’s see what are the parts of a reflector telescope
A reflector telescope has the same parts as a refractor telescope. They are Mount, Tripod, Optical Tube, Eyepiece and a Viewfinder.
How does it work?
This is an image of the cross section of the optical tube of the Reflector Telescope. The right side of the optical tube is open so that light can enter. The left side of the optical tube is closed. The opening of the optical tube on the bottom right is the eyepiece. The 45 degree slanted blue shaded object is the secondary mirror. It only reflects light coming from the closed side (primary mirror) of the optical tube. The light which enters the optical tube from the middle will be blocked by the secondary mirror. So we won’t be able to see that light. But the light which enters the optical tube from the side of the tube will pass by the secondary mirror and then hit the curved primary mirror (the blue shaded part on the left side of the image). The primary mirror will then reflect that light to the reflective side of the small secondary mirror. The secondary mirror will reflect the light onto the eyepiece, since it is tilted at a 45 degree angle. When we look through the eyepiece we can see the light from the object we are looking at.
2.Radio Telescopes
Radio Telescopes observe in radio waves range. These telescopes are used to observe objects which are invisible to our naked eye. The wavelength of radio waves is from 1 metre to thousands of kilometres. Because of this, radio waves don’t need any glass mirrors to reflect. Radio telescopes are usually reflector telescopes, even two metal beams will act as a mirror because of their very high wavelength. If the wavelength of the radio wave is 100km then, 2 metal beams placed 100km away from each other will act as a mirror. That is why the mirrors in radio telescopes are made of metal beams. Since our eyes can’t see radio waves, we can’t see anything when we look through a radio telescope. So, cameras which observe and take pictures in radio waves are used instead of our eye. Some examples of radio telescopes are the Arecibo telescope, FAST telescope (Five-Hundred-Meter Aperture Spherical Telescope), etc…
3. Microwave Telescope
Microwave Telescopes observe in microwaves range of the spectrum. The wavelength of microwave is from 1millimeter to 1meter. For microwaves to reflect, they need a surface which has no gaps and if there are gaps, they shouldn’t be larger than 1mm. These telescopes also use Microwave detecting sensors or cameras to observe the sky in Microwaves.
4. Infrared Telescopes
Infrared Telescopes observe the sky in the infrared rays or thermal radiation. The wavelength of infrared is from 780 nanometers to 1 millimetre. Infrared rays require a very smooth surface to reflect. Since infrared is used to map thermal radiation, everything with a high surface temperature will emit infrared light. From humans to stars everything emits some amount of infrared light. At this range of EM spectrum, we can observe stars that are invisible to the naked eye. These telescopes also use infrared detecting sensors and infrared cameras to view objects on this wavelength. Some examples of infrared telescopes are Wyoming Infrared Observatory (WIRO) and Stratospheric Observatory for Infrared Astronomy (SOFIA) and Hubble Space Telescope (HST).
5. Ultra Violet Telescopes
Ultraviolet Telescopes or UV telescopes observe the sky in the Ultra Violet Rays. The wavelength of Ultra Violet is 100 nano meters to 400 nanometers. Since infrared has a very short wavelength it needs a very smooth mirror with much smoother surface than the mirrors which we use in homes. Since UV is also invisible to the naked eye, these telescopes use UV detecting sensors and UV camera to detect infrared rays. Some examples of UV telescopes are Hubble Space Telescope (HST), Ultra Violet Imaging Telescope (UVIT).
6. X-ray Telescope.
X-ray telescopes observe the sky in the X-Ray region of the spectrum. The wavelength of X-ray is 0.01 nanometers to 10 nanometers. Since X-ray has a very short wavelength, it needs an extremely smooth mirror which is smoother than the ones used in UV telescopes. These telescopes use X-ray detecting sensors and X-ray cameras to detect X-rays. Some examples of X-ray Telescopes are Chandra X-ray Observatory, Nuclear Spectroscopic Telescope Array (NuSTAR).
7.Gamma Ray Telescope.
Gamma Ray Telescopes observe the sky in Gamma Rays. The wavelength of Gamma rays are less than 0.01 nanometers. Since Gamma rays have a wavelength shorter than the gamma ray, it needs the smoothest mirror possible. They use Gamma Ray detecting sensors and Gamma ray cameras to detect Gamma rays. An example of Gamma ray Telescope is The Fermi Gamma ray Telescope.
Have A Look at a Related Article: Webb Space Telescope
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