Notes-Class 9-Science-Chapter-18-Observing Space : Telescopes-Maharashtra Board

Topics to be learn :

Forms of light
Telescopes
Types of telescopes
Telescopes in space
Indian Space Research Orgnaisation (ISRO)

Sky : A visible portion of the atmosphere and outer space seen by simple eyes, without any equipment from the earth is known as the sky.

Space : An infinite, boundless, limitless three dimensional expanse in which the solar system, stars, celestial bodies, galaxies and endless Universe exist is known as space.

Both sky and space lack a definite boundary.
However, the sky is a very tiny part of space.

Space observation : ‘A study of planets, stars, celestial bodies and galaxies with the help of satellites and telescope; situated either on the earth or in the space and attempting to understand the extent of universe, its structure, etc. is known as space observation.

Importance of space observation :

Importance of space observation :

Space observation gave us the information of stars, planets, comets and other celestial bodies around the earth.
It is because of space observation, we learnt about different forms of light (radiation) and the infinite extent of the universe.
Modern equipment like telescope and satellites were sent into space to satisfy our quest of knowledge to know the universe.
A new branch of knowledge as astronomy emerged from this very quest which led further to social, national and international cooperation.

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Need of sky observation and equipment :

Need of sky observation and equipment :

Man has been curious about the sun, the moon and other stars.
Man tried to understand the sky with sheer observation of sky by simple naked eyes.
With time, man’s realisation that the position of stars and constellations, their changing position, its impact on seasons, its utility in navigation made him determined to study the sky deeply.
Initially, man did not have any equipment to get a close view of planets, stars and the space beyond.
However, in 1608 Hans Lippershey made the first lens. He also made the first telescope by fitting two lenses at the either ends of a tube/ pipe.
Later, Galileo also made his telescope in 1609 and used it for space observation. Galileo, with the help of his telescope, discovered many more stars, four moons of Jupiter and sun spots.
Post Galileo, in the last 400 years man has made tremendous progress in telescopes technology and space science. This has helped us to construct an astounding picture of the universe. Besides, space technology has also helped us in improving comforts and facilities in our everyday life.

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Different forms of light : Light is an electromagnetic wave.

The light which our eyes can see and known as visible radiation, is a very tiny part of electromagnetic spectrum.
Light has electromagnetic waves and wavelength is a characteristic of light.
Visible light/ radiation has a wavelength of 400 nm to 800 nm.
However, there is a light having different electromagnetic wavelength and we cannot see such lights as our eyes are not sensitive to these types of lights.

Wavelength of light rays (radiation) :

Light rays with different wavelength and its characteristic :

Information about light rays with different wavelength and its characteristic is given in the table below :

Type of radiation	Wavelength	Characteristics
Radio waves	Longer than about 20 cm	Wavelength is longer than the wavelength of visible light.
Micro waves	0.3 mm – 20 cm	Wavelength is longer than the wavelength of visible light.
Infrared waves	800 nm – 0.3 mm	Our eyes cannot see these types of light rays.
Visible light rays	400 nm – 800 nm	Light visible to our eyes
Ultraviolet rays	300 pm – 400 nm	Wavelength is shorter than wavelength of visible light.
X-rays	3 pm – 300 pm	Wavelength is shorter than wavelength of visible light.
Gamma rays	Shorter than 3pm	Even this light is not visible to human eyes

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Need and structure of telescope :

Need and structure of telescope :

Our eyes can see only visible light which has a wavelength between 400 nm to 800 nm.
Thus, with the help of optical telescope 1-8-visible radiation telescope made from regular lenses and mirrors, we can do sky observation.
However, many heavenly bodies emit radiations other than the visible light. We need different types of telescopes to capture these radiations.

The structure of the telescope is shown in the diagram below :

Types of telescopes :

Types and sub-types of telescope are as follows :

(i) Optical telescope :

Refracting telescope
Reﬂecting telescope : (a) Newtonian telescope (b) Cassegrain telescope

(ii) Radio telescope

Both types of telescopes are installed on the earth as well as in space.

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Types of optical telescopes :

Most of the optical telescope are made with lenses. But some optical telescopes are made using concave mirrors. Two main subtypes of optical telescope are as follows :

Refracting telescopes :

Refracting telescopes :

These telescope are made with two or more lenses.
Light rays change their direction while passing through the lens. This is also called refraction. Hence, telescopes made with the lenses are called refracting telescopes. .

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Limitations of Refracting telescopes :

Limitations of Refracting telescopes :

In order to get a bright and clear image, lenses used must be as large as possible.
However, making such large lenses is very difficult. Besides, such large lenses become heavy and tend to get distorted.
The length of the telescope also increases with larger lenses. Such telescope become difficult to manage.
Images formed by the lenses have errors of colours due to 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.

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Reﬂecting telescope :

Reﬂecting telescopes are designed as such to overcome the limitation of refracting telescope.
Light rays get reﬂected by concave mirrors in these telescopes. Hence, these are called reﬂecting telescopes.
It is easier to make large mirrors in compare to making large lenses. Big mirrors can also be made by combining several smaller pieces of mirrors and thus, the weight of such concave large mirror is too less than that of lens of the same size.
Images formed by mirrors do not have errors of colours.

Types of reﬂecting telescope :

Two sub-types of reﬂecting telescope are as follows:

(1) The Newtonian telescope : In this, one concave and one plane mirror is used. Concave mirror is used to receive the light coming in from the source and reflect it to the plane mirror. Plain mirror then deflect these rays for focusing to pass through eyepiece.

(2) The Cassegrain telescope : In this telescope, one concave and one convex mirror are used.

Concave mirror is used to receive the light coming in from the source and reﬂect it to convex mirror.
Convex mirror reﬂects light rays back towards a concave mirror which pass through a hole at the centre of concave mirror. An eyepiece situated at the back of concave mirror is used to see the image.

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Optical telescopes having a mirror of 3.6 m diameter situated in Aryabhatta Research Institute, Nainital is the largest optical telescope in India and 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.

Many heavenly bodies like stars, planets, Galaxies, emit radiation other than visible light like X-rays, gamma rays, infrared rays, etc. Radio Telescopes are capable to receive and produce images from such rays.
Radio telescopes are made from one or more dishes of a particular parabolic shape.
As in optical telescope, the incident radio waves are reflected from the concave part of the parabolic dish and converge at the focus.
A radio receiver placed at the focal point gathers such waves and later image is constructed with the help of computers.
Giant Meterwave Radio Telescope (GMRT) is erected at Narayangaon near Pune. This telescope studies the light rays having the wavelength of about a meter coming from heavenly bodies. This large radio telescope is actually an interconnected 30 parabolic dishes. Each dish is 45 m in diameter. These 30 dishes are spread in 25 square kilometer area in such a way that it works as a single dish as if having a diameter of 25 sq. km.
GMRT has enabled scientists to study Solar System, Solar winds, Pulsar, Supernova and interstellar hydrogen clouds.

Salient features of radio telescopes :

Salient features of radio telescopes :

Radio telescopes have the ability to receive all forms of electromagnetic radiation other than visible light.
Radio telescopes do not use any lens and mirror.
Radio telescopes are made from Ra dish_ of a parabolic shape.
A giant radio telescope can be formed by linking mini radio telescopes together.
Radio telescopes are useful in any atmospheric conditions like cloudy sky and even at the night.

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Limitation of telescopes erected on the earth :

Limitation of telescopes erected on the earth :

Visible light and radio waves emitted by heavenly bodies in the space passed through the atmosphere before reaching the earth surface. During this journey, some light is absorbed by atmosphere and intensity of the light reaching the earth surface decreases.
Besides, temperature and air pressure causes the atmospheric turbulence and thus, light rays changes their path and shakes the position of the image.
City lights during night and bright sunlight during day also put limitations on using optical telescopes on the earth.
To reduce the intensity of these problems, optical telescopes are situated on mountain top, away from inhabited places.
However, limitations caused due to atmosphere still persist.

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Telescope in space :

To get rid of problems in situating telescope on the earth, scientists have successfully placed the telescope in the space. Images obtained by the telescope placed in the space are bright and clear.
In 1990 National Aeronautics and Space Administration (NASA) of USA launched an optical telescope ‘Hubble’ in space. The lens of this telescope is 94 inches in diameter and orbiting the earth at a height of 589 km above in space.
In 1999 NASA launched the X-ray telescope ‘Chandra’ in Space. This telescope has an ability to grasp (receive) X-rays from heavenly objects. Chandra telescope has given us useful information about stars and galaxies. The telescope is named after the famous Indian scientist Subramanian Chandrashekhar.

Contribution of ISRO :

Contribution of Indian Space Research Organisation : This institute was established in 1969 with the aim of developing technology for making and launching of artificial satellites.

ISRO’s contribution is significant in space research of independent India.
ISRO has independently developed the technology to launch the satellites in space and till date ISRO has successfully launched many satellites.
ISRO’s programme in space science has played a big role in national and social development.
ISRO’s space research and satellites have a great inﬂuence and utility in telecommunications, television broadcasting, meteorological services, internet services, educational programme resource monitoring and management and disaster management.

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Astrosat : In 2015, ISRO launch an artiﬁcial satellite Astrosat. This satellite has ultraviolet and X-ray telescopes and detectors. Most of the parts used in the satellites are made in India. With the help of Astrosat, Indian scientists are studying various aspects of the universe.

Active telescopes in space :

Active telescopes in space :

List of some of the important active telescopes in space are as follows :

Spitzer Space : launched in 2003
Integral : This X-ray telescope is launched by European Space Agency in June 2012.
Spektr-RG : Russian–German high-energy astrophysics space observatory which was launched on 13 July 2019.
Wise : NASA launched this infrared telescope in December 2009.
Odin : Sweden launched this microwave telescope in February 2001.
Agile : is an X-ray and gamma ray astronomical satellite of the Italian Space Agency (ASI). launched in Apr 2007
Spektr-R: ASC-LPI launched this radio wave telescope in May 2011.
Lisa Pathfinder : European Space Agency launched this telescope in December 2015. This telescope studies the gravitational waves.
Fermi Gamma-Ray : Space Telescope, launched in 2008
OAO2 : The Orbiting Astronomical Observatory 2 was the first successful space telescope, launched on December 7, 1968.

All the scores are active even today. Examples of each type of the telescope is given above. There are hundreds of such telescopes functioning in space today.

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