Telescope

Simple Telescope And Astronomical Telescope, Construction working, uses, ray diagram

Simple telescope

A simple working telescope requires nothing more than a pair of lenses mounted in a tube. The lens in front, known as the objective, focuses an image; the lens in back, known as the eyepiece, magnifies the image. Although it may seem like a crude device, a simple telescope nicely illustrates the basic working principles of more powerful astronomical instruments.

LENSES

Light normally moves in straight lines, but there are situations in which this is not true. You are already familiar with some: for example, the distortions you see looking through the surface of the ocean occur because light bends as it passes from the water into the air. Long before we understood why light bends as it passes from one transparent material to another, people had used this effect to create lenses: optical devices which can gather light together or spread it apart.

In order to understand how a lens works, you need to know a little about how light behaves in passing from one material to another. Imagine a tank of water on the table in front of you; the surface of the water should be perfectly flat and horizontal. If you shine a ray of light straight down from above, it will pass through the surface of the water without bending. But if you shine the light in at an angle, it will bend as it passes through the surface. Fig. 1 illustrates two important facts about this effect. First, in passing from air to water, the light always bends into the water. Second, the smaller the angle between the light ray and the surface, the more it bends in passing through. The same rules would also apply if the tank of water was replaced with a block of glass.

To create a lens which can focus many parallel rays of light to a single point, the idea is to curve the surface of the glass so that all these rays, after passing through, come together at the same place. It’s a bit tricky to do this right, but we don’t need to worry about the details. The simplest kind of lens is a `plano-convex’ lens; one side is flat, while the other bulges out at the middle. Fig. 2 shows how such a lens focuses light. The optical axis of the lens is the thick line which passes right through the middle of the lens; a ray of light traveling along the optical axis is not bent at all. Rays which pass through the top of the lens are bent downward, while rays which pass through the bottom of the lens are bent upward. Thus all these light rays are bent toward the optical axis. If the lens is well-made, all rays meet at the same focal point. The distance between the lens and the focal point, measured along the optical axis, is called the focal length.

A simple lens in operation. Parallel light rays come from the right, pass through the lens, and meet at the focal point on the left. The thick line through the middle of the lens is the optical axis; the distance F is the focal length.

Image formation

A lens which could only focus light rays striking the glass head-on (as in Fig. 2) would be fairly useless for astronomy. Fortunately, most lenses can also accept rays which come in at a slight angle to the optical axis, and bring them to a focus as well. This focal point is not the same as the focal point for rays which are parallel to the optical axis; depending on the angle of the incoming rays, their focus lies on one side or the other of the optical axis, as shown in figure. But if the lens is well-made, all these focal points will lie on a plane which is parallel to the face of the lens; this is called the focal plane.

A simple lens forming an image. The red rays arrive with an downward slant, and come to a focus below the optical axis, while the blue rays arrive with a upward slant, and come to a focus above the optical axis. The vertical dotted line at left represents the focal plane.

There’s one slightly subtle consequence of this image-formation process: the image is upside-down! Fig. 4 shows why: rays from the lower part of the subject (on the right) come together at the upper part of the image (left), and vice versa. This is also true of a camera; of course, you turn the prints right way up when you get them back from the store, so you’re probably not aware that the image is upside down inside your camera.

The image formed by a simple lens is upside-down with respect to the subject. Here the subject (right) is an arrow with a red tip pointing upward; its image (left, at the focal plane) points down.

EYEPIECES AND MAGNIFICATION

To make a telescope you can actually look through, you’ll need to add another lens. This eyepiece lens magnifies the image formed by the large objective lens and directs the light to your eye. Basically, the eyepiece works a lot like a magnifying glass; it enables your eye to focus much more closely than it normally can. The eyepiece on a typical telescope allows you to inspect the image formed by the objective lens from a distance of an inch or less. Fig. 5 shows how the objective lens and eyepiece work together in a simple telescope.

The magnification of a telescope is easy to calculate once you know the focal lengths F and f of the objective lens and eyepiece, respectively. The formula for the magnification M is

M = F ÷ f

Here you can use any units for F and f, as long as you use the same units for both. For example, if you measure F in millimeters, you should also measure f in millimeters. Using the values for F and f you measured above, calculate the expected magnification of your telescope.

To measure the magnification of your telescope directly, we will set up a target – basically a picture of a ruler with marks a unit distance apart. From the other end of the room, focus your telescope on the target. Now look through the telescope while keeping both eyes open; you should see a double image, where one image is magnified and the other is not. Compare the two images; how many of the unmagnified units fit within one magnified unit? The answer is a direct measurement of your telescope’s magnification; record it in your notebook and compare it to the magnification you calculated using the formula above.

Astronomical telescope

An astronomical telescope is an optical instrument which is used to see the magnified image of distant heavenly bodies like stars, planets, satellites and galaxies etc. The final image formed by an astronomical telescope is always virtual, inverted and magnified.

Principle of Astronomical Telescope        

An astronomical telescope works on the principle that when an object to be magnified is placed at a large distance from the objective lens of telescope, a virtual, inverted and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the eye piece.

 

 

Construction of Astronomical Telescope        

An astronomical telescope consists of two convex lenses : an objective lens O and an eye piece E. the focal length fo of the objective lens of astronomical telescope is large as compared to the focal length fe of the eye piece. And the aperture of objective lens O is large as compared to that of eye piece, so that it can receive more light from the distant object and form a bright image of the distant object. Both the objective lens and the eye piece are fitted at the free ends of two sliding tubes, at a suitable distance from each other.

Working of Astronomical telescope        

The ray diagram to show the working of the astronomical telescope is shown in figure. A parallel beam of light from a heavenly body such as stars, planets or satellites fall on the objective lens of the telescope. The objective lens forms a real, inverted and diminished image A’B’ of the heavenly body. This image (A’B’) now acts as an object for the eye piece E, whose position is adjusted so that the image lies between the focus fe’ and the optical centre C2 of the eye piece. Now the eye piece forms a virtual, inverted and highly magnified image of object at infinity. When the final image of an object is formed at infinity, the telescope is said to be in ‘normal adjustment’.

It should be noted that, the final image of object (such as stars, planets or satellites) formed by an astronomical telescope is always inverted with respect to the object. But it does not matter whether the image formed by an astronomical telescope is inverted or not, as all the heavenly bodies are usually spherical is shape.

Magnifying Power of an Astronomical Telescope   

The Magnifying Power of a telescope is given by:

m    =    

Magnifying Power of an Astronomical Telescope

Where, fo = Focal length of the objective lens

fe = Focal length of the eye-piece lens

And the length (L) of the tube of telescope is equal to the sum of the focal lengths of the objective lens and the eye piece. Thus,                  L = fo + fe

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A telescope is an optical instrument that aids in the observation of distant objects. Telescopes are used for a variety of purposes, including astronomy, navigation, and surveillance.

The first telescopes were invented in the early 17th century by Galileo Galilei. Galileo’s telescopes were refracting telescopes, which use lenses to bend and focus light. Refracting telescopes are still used today, but they are not as common as reflecting telescopes.

Reflecting telescopes use mirrors to reflect light and focus it on an eyepiece. Reflecting telescopes are more powerful than refracting telescopes, and they are the type of telescope that is most commonly used for astronomy.

There are many different types of telescopes, each with its own advantages and disadvantages. Some of the most common types of telescopes include:

• Astronomical telescopes: These telescopes are designed for viewing objects in space. They are typically very large and powerful, and they can be used to see objects that are very far away.
• Binoculars: Binoculars are a type of telescope that is designed to be used with both eyes. They are smaller and lighter than astronomical telescopes, and they are easier to carry around.
• Catadioptric telescopes: Catadioptric telescopes use a combination of lenses and mirrors to focus light. They are often used for astronomy, but they can also be used for other purposes, such as bird watching.
• Cassegrain telescopes: Cassegrain telescopes are a type of catadioptric telescope. They are popular among amateur astronomers because they are relatively easy to use and they produce sharp images.
• Dobsonian telescopes: Dobsonian telescopes are a type of Newtonian telescope. They are popular among amateur astronomers because they are relatively inexpensive and easy to use.
• Galilean telescopes: Galilean telescopes are a type of refracting telescope. They were invented by Galileo Galilei in the early 17th century. Galilean telescopes are relatively simple and inexpensive, but they are not very powerful.
• Gregorian telescopes: Gregorian telescopes are a type of reflecting telescope. They were invented by James Gregory in the early 17th century. Gregorian telescopes are more powerful than Galilean telescopes, but they are also more expensive.

The history of telescopes is a long and fascinating one. The first telescopes were invented in the early 17th century, and they have been used for a variety of purposes ever since. Telescopes have played a major role in the advancement of astronomy, and they have also been used for navigation, surveillance, and other purposes.

The Hubble Space Telescope is one of the most famous telescopes in the world. It was launched into space in 1990, and it has been used to take some of the most amazing images of the universe ever seen. The Hubble Space Telescope has helped astronomers to learn a great deal about the universe, and it has revolutionized our understanding of space.

There are many different types of telescopes, and each type has its own advantages and disadvantages. The type of telescope that is best for you will depend on your needs and budget. If you are interested in astronomy, then you will need a powerful telescope that can see objects that are very far away. If you are interested in bird watching, then you will need a smaller and lighter telescope that is easy to carry around.

No matter what your needs are, there is a telescope out there that is perfect for you. With so many different types of telescopes to choose from, you are sure to find the perfect one for your needs.

Here are some additional facts about telescopes:

• The largest telescope in the world is the Gran Telescopio Canarias, which is located in Spain. It has a primary mirror that is 34 meters in diameter.
• The most powerful telescope in the world is the Large Binocular Telescope, which is located in Arizona. It has two primary mirrors that are each 8.4 meters in diameter.
• The Hubble Space Telescope is the most expensive telescope ever built. It cost over $1 billion to build and launch.
• The Hubble Space Telescope has taken over 1.5 million images of the universe.
• The Hubble Space Telescope has helped astronomers to discover over 10,000 new galaxies.
• The Hubble Space Telescope has helped astronomers to learn a great deal about the universe, including the age of the universe, the expansion of the universe, and the existence of dark matter and dark energy.

1. What is a black hole?

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

2. What is a neutron star?

A neutron star is a very small, very dense star. It is made up of neutrons, which are particles that are made up of protons and electrons. Neutron stars are formed when a star explodes in a supernova.

3. What is a white dwarf?

A white dwarf is a star that has used up all of its nuclear fuel. It is very hot and very dense, and it is about the size of Earth. White dwarfs are the final stage in the life of most stars.

4. What is a red giant?

A red giant is a star that is in the late stages of its life. It is much larger and more luminous than the Sun, and it has a much cooler surface temperature. Red giants are formed when a star runs out of hydrogen fuel in its core.

5. What is a supernova?

A supernova is a powerful and luminous stellar explosion. It is the largest explosion that occurs in space, and it can be seen from billions of light-years away. Supernovae are caused by the death of a star, and they can create new Elements, such as gold and uranium.

6. What is a galaxy?

A galaxy is a large group of stars, gas, and dust held together by gravity. Galaxies come in a variety of shapes and sizes, from the small dwarf galaxies to the large spiral galaxies. The Milky Way, the galaxy that contains our Solar System, is a barred spiral galaxy.

7. What is the universe?

The universe is everything that exists, from the smallest subatomic particles to the largest galaxies. It is thought to have begun with the Big Bang about 13.8 billion years ago. The universe is still expanding today, and it is made up of billions of galaxies.

8. What is dark matter?

Dark matter is a mysterious substance that makes up about 85% of the matter in the universe. It cannot be seen or detected with current technology, but its existence is inferred from its gravitational effects on visible matter.

9. What is dark energy?

Dark energy is a mysterious force that is causing the universe to expand at an accelerating rate. It is thought to make up about 68% of the energy in the universe.

10. What is the Big Bang?

The Big Bang is the prevailing cosmological model for the universe. It states that the universe was once in an extremely hot and dense state that expanded rapidly. The Big Bang is thought to have occurred about 13.8 billion years ago, and it is the origin of the universe as we know it today.

Sure, here are some multiple choice questions about astronomy without mentioning the topic of telescopes:

1. What is the largest object in The Solar System?
   (A) The Sun
   (B) Jupiter
   (C) Saturn
   (D) Neptune

2. What is the name of the asteroid belt that lies between Mars and Jupiter?
   (A) The Asteroid Belt
   (B) The Kuiper Belt
   (C) The Oort Cloud
   (D) The Trojan Asteroids

3. What is the name of the largest moon in the solar system?
   (A) Ganymede
   (B) Titan
   (C) Callisto
   (D) Io

4. What is the name of the outermost planet in the solar system?
   (A) Neptune
   (B) Uranus
   (C) Pluto
   (D) Eris

5. What is the name of the brightest star in the night sky?
   (A) Sirius
   (B) Vega
   (C) Capella
   (D) Arcturus

6. What is the name of the constellation that contains the North Star?
   (A) Ursa Major
   (B) Ursa Minor
   (C) Orion
   (D) Scorpius

7. What is the name of the Milky Way galaxy?
   (A) Our galaxy
   (B) The Andromeda Galaxy
   (C) The Triangulum Galaxy
   (D) The Magellanic Clouds

8. What is the name of the largest black hole in the Milky Way galaxy?
   (A) Sagittarius A*
   (B) Cygnus X-1
   (C) M87
   (D) NGC 4261

9. What is the name of the most famous supernova in history?
   (A) SN 1054
   (B) SN 1572
   (C) SN 1604
   (D) SN 1987A

10. What is the name of the most famous comet in history?
    (A) Halley’s Comet
    (B) Comet McNaught
    (C) Comet Hale-Bopp
    (D) Comet Ikeya-Seki

I hope you found these questions challenging and interesting!