Observing Space: Telescopes PDF

## 18. Observing Space: Telescopes ### Can you recall? 1. What is the difference between the sky and space? 2. What is meant by space observation? Why is it important? From early days, man has been curious about the sun as well as the moon and stars seen in the night sky. Using his boundless imag...

## 18. Observing Space: Telescopes ### Can you recall? 1. What is the difference between the sky and space? 2. What is meant by space observation? Why is it important? From early days, man has been curious about the sun as well as the moon and stars seen in the night sky. Using his boundless imagination, he tried to understand the sky as observed by the naked eyes. He noticed that the position of the stars changed with time and had something to do with the occurrence of seasons. The knowledge of the cycle of seasons was necessary for agriculture, sky watching began to prove useful to him. The position of the constellations was also useful to sea goers for navigation. Man began to make determined efforts to find answers to questions which arose out of his sky watching. But he did not have any equipment to get a closer view of the stars and planets in the sky. Today, 400 years after Galileo's use of the telescope, tremendous progress has been made in telescope technology and in space science and technology on the whole. This great leap in technology has helped to construct for us an astounding picture of our universe. Space science and technology are not only important for research purposes, but also to help provide us with many of the comforts and facilities we enjoy in our everyday life. A telescope is used to observe space, but will one telescope be sufficient for us to observe space completely? Why do we need different telescopes for the purpose? Are telescopes installed even in space? In this chapter, we are going to study the science behind many such questions. ### Different Forms of Light Light is an electromagnetic wave. Every wave has a characteristic wavelength. Our eyes can only see that light which has wavelengths between 400 nm to 800 nm. Such light is called visible radiation. However, there are electromagnetic waves of wavelengths other than the visible ones most of which we cannot 'see' as our eyes are not sensitive to them. ### Study The Following Table: | Type of Radiation | Wavelength | |---|---| | Radio waves | Longer than about 20 cm | | Micro waves | 0.3 mm - 20 cm | | Infrared waves | 800 nm - 0.3 mm | | Visible light rays | 400 nm - 800 nm | | Ultraviolet rays | 300 pm - 400 nm | | X-rays | 3 pm - 300 pm | | Gamma rays | Shorter than 3pm | Of all the above types, our eyes are only capable of seeing the visible radiation. Thus, we use the visible radiation telescopes, i.e. optical telescopes, made from regular lenses or mirrors to see the visible radiation coming from the space. However, many heavenly bodies emit radiations other than the visible light. Thus we need different types of telescopes like the X-ray, gamma-ray and radio telescopes to receive such radiation and to study their sources. ### An Introduction to Scientists - In 1608, spectacle maker and researcher, Hans Lippershey discovered that seeing through two lenses kept one behind the other, seems to bring objects closer to us. He thus made the first telescope. Galileo made a telescope in 1609 and used it for space observations. He realized that there are many more stars than what could be seen with naked eyes. Using his telescope, he also discovered the moons of Jupiter, the black spots on the sun etc. * **Galileo Galilei** ### Telescopes #### Optical Telescopes Most optical telescopes are made with two or more lenses as shown in figure 18.1. To collect the maximum amount of light coming from a heavenly object, the objective lens should be made as large as possible. Using the light collected by the objective, a smaller lens, called the eyepiece, produces a large image of the source. Light rays change their direction as they enter a lens from the atmosphere and again when they enter the atmosphere after passing through the lens. This is called refraction. Hence such telescopes are called refracting telescopes. We shall study image formation by lenses in the next standard. Even though such a telescope is useful for space observations, it presents certain difficulties. 1. As we saw above, if we wish to obtain a bright image of a source by collecting the maximum possible light from it, the objective lens must be made as large as possible. However, it is very difficult to make very large lenses. Also, large lenses are very heavy and tend to get distorted. 2. As the objective and eyepiece are placed at the opposite ends of the telescope, the length of the telescope also increases with increase in the size of the lenses and the telescope becomes difficult to manage. 3. The images formed by lenses have errors of colours. This is called chromatic aberration. To overcome these difficulties, telescopes are made using concave mirrors. As light rays get reflected by mirrors in these telescopes, they are called reflecting telescopes. In order to get a bright image of a source, large mirrors are necessary (so that they can collect a large amount of light from the source), but it is easier to make large mirrors as compared to making large lenses. Also, big mirrors can be made by combining several smaller pieces. The weight of a large mirror too is less than that of a lens of the same size. The images formed by mirrors do not have errors of colour. Only by using these large telescopes, can we see far away stars and galaxies, which we could never see using our naked eyes. The reflecting telescopes are mainly of two types: Newtonian and Cassegrain. As shown in figure 18.2, light rays coming from space are reflected by the concave mirror. Before these reflected rays converge at the focus, they are deflected again by a small plane mirror. As a result, they get focused at a point lying on the perpendicular to the axis of the telescope’s cylinder. They pass through the eyepiece and we get a magnified image of the source. The construction of a Cassegrain type of telescope is shown in figure 18.3. The Cassegrain telescope also uses a concave mirror. However, here light rays, after reflection from the concave mirror, are reflected back towards it by a small convex mirror. They pass through a hole at the centre of the concave mirror and then through the eyepiece situated at the back of the mirror. The eyepiece gives us a magnified image of the source. #### **18.1 A Refracting Telescope** *Image illustrating a Refracting Telescope* #### **18.2 The Newtonian Telescope** *Image illustrating a Newtonian Telescope* #### **18.3 The Cassegrain Telescope** *Image illustrating a Cassegrain Telescope* **In India, we have several telescopes with concave mirrors of 2m diameter that have been in use for many years. The biggest optical telescope in India, having a mirror of 3.6 m diameter is situated in the Aryabhatt Research Institute of Experimental Sciences, Nainital. This is the largest optical telescope in Asia.** ### Radio Telescope Many heavenly objects emit radio waves in addition to visible radiation. We cannot see this radiation with our eyes. Hence, a special type of telescope is used to receive these rays. It is called a radio telescope. It is made from one or more dishes of a particular parabolic shape. As in optical telescope, the incident radio waves are reflected by these dishes and converge at the focus. A radio receiver is placed at the focal point. The information gathered by this receiver is passed on to a computer which analyses it and constructs an image of the source. A large radio telescope called the Giant Meterwave Radio Telescope (GMRT) has been erected at Narayangaon near Pune. It uses radio waves having wavelengths of about a metre, coming from planets and stars to study those heavenly bodies. This telescope is actually a collection of 30 dishes, each having a diameter of 45 m. It is called a giant telescope as the arrangement of the 30 dishes over an area which measures up to 25 km across, is made in such a way that it works as a single dish having a diameter of 25 km. This means that the GMRT gives the same data that we would have got from a telescope having a single dish of 25 km diameter! GMRT has been made by Indian scientists and engineers at minimum cost. It is a world standard research facility. Scientists study the solar system, solar winds, pulsars, supernova, interstellar hydrogen clouds etc. with the help of the GMRT. Scientists from all over the world come to India to make use of this facility. #### **18.4 A. Radio Telescope (photograph) B. The structure of a radio telescope.** *Image illustrating a Radio Telescope* ### Telescopes in Space Visible light and the radio waves emitted by heavenly bodies in space can pass through the earth’s atmosphere and reach the earth's surface. So, optical and radio telescopes can be erected on the surface of the earth. However, these earth-based telescopes present some problems in making good quality observations. The visible light coming from a heavenly body has to pass through the earth’s atmosphere to reach the earth’s surface. During this journey, some of the light is absorbed by the atmospheric gases and molecules. The atmospheric pressure and temperature changes alter/change the turbulence in the atmosphere, in turn disturbing the light rays. This weather can cause difficulties in observing the heavenly bodies. Also, because of Sunlight, we cannot use optical telescopes during the day. During the night too, city lights and cloudy weather can cause difficulties in observing the heavenly bodies. To reduce these problems, optical telescopes are situated on top of mountains, at uninhabitated places. However, if we want to get rid of all the above problems completely, we should place the telescope above the earth’s atmosphere, in space itself. These problems do not exist in the space and thus the image obtained by space telescopes would be bright and very clear and will remain at one place. Scientists have turned this idea into reality. - In 1990, the National Aeronautics and Space Administration launched into space an optical telescope called the Hubble telescope. It has a mirror of diameter 94 inches and is orbiting the earth at a height of 589 km from it. This telescope is still working and has helped to make important discoveries. - In 1999, the National Aeronautics and Space Administration launched an X-ray telescope named Chandra, in space, to study X-rays coming from heavenly objects. Special mirrors which can reflect X-rays were used in this telescope. Chandra has given us very useful information about stars and galaxies. The telescope is named after the famous Indian scientist Subramanian Chandrashekhar. ### Indian Space Research Organization (ISRO) Bengaluru This institute was established in 1969 with the aim of developing technology for making and launching of artificial satellites. Till date, ISRO has successfully launched a large number of satellites. ISRO’s programme is foremost among the successful programs undertaken by independent India. India's progress in space science has played a big role in national and social development. The INSAT and GSAT series of satellites support our telecommunication network, television broadcasting and meteorological services. It is because of them that telephone, television and internet services are available everywhere in the country. The EDUSAT satellite in this series is used exclusively for education. The IRS satellite series is used for the monitoring and management of natural resources as well as disaster management. ### Astrosat - In 2015, Indian Space Research Organization (ISRO) launched an artificial satellite called Astrosat, in space. This satellite has ultraviolet and X-ray telescopes and detectors. Most of the parts used in this satellite are made in India. It is a unique system having different kinds of telescopes on a single satellite. Indian scientists are studying various aspects of the Universe using the data obtained with these telescopes. #### **Find out** - Collect more information about telescopes that work in space apart from the Hubble and Chandra telescopes. #### Exercises 1. Fill in the blanks with the proper 2. Study the figure and answer the