Astronomical Telescopes

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Although the telescope was invented by Hans Lippershey in Holland, it was Galileo in Italy who was the first to put it to use for astronomical observations. Basically, a telescope makes use of curved reflecting surfaces and/or lenses to bend the light rays from a distant source in such a way that its clear and magnified image is formed closer to the^ observer. Even the human eye forms images on the retina by the same optical principles; but it has limitations regarding how clearly it can see and how faint an object it can detect.

When you look at a tree in the distance you cannot make out its individual leaves. A telescope 'brings the tree closer' and enables you to see the leaves clearly. The telescope therefore increases the clarity of the object under study. In astronomical jargon we say that the telescope has improved the 'resolution'.

In the same way a large telescope can collect and focus light more effectively and therefore help us to see things that the human eye cannot see. Thus, by exposing a photographic plate to the light coming through a telescope for several hours, the astronomer is able to get photographs of faint nebulae which are otherwise invisible to the human eye.

Galileo was able to discover the four nearest satellites of the planet Jupiter by studying the planet through his newly acquired telescope. He also discovered sunspots—the dark patches on the bright disc of the Sun—that are not visible to the naked eye.

Today's telescopes are far bigger and vastly superior to the one Galileo used. The largest telescope using visible light is in Russia, although the largest working telescope is the Hale Telescope at Mount Palomar in southern California in the USA.

The diagram illustrates how a reflecting telescope works. The dotted lines are rays of light from a distant star which are reflected by the large concave mirror M and .then brought into focus to form a bright and clear image at I.

This illustration of the galaxy was obtained by exposing the photographic plate for several hours. The galaxy is not visible to the human eye.

This telescope has a main mirror with a diameter of five metres. It is now planned to make even larger 'next generation' telescopes.

Technical problems make the building of very large telescopes difficult. Extreme precision is needed in the entire system if the astronomer is to trust the image that is formed by the telescope. (Have you seen your image in a mirror with an uneven surface?) To get this kind of precision the mirror has to be ground very carefully and very fine. Moreover, if the mirror 'dish' is very large it tends to sag under its own weight. The mirror also gets distorted by temperature changes during .night and day. These effects are small for small size mirrors. So, nowadays many astronomers prefer building smaller mirrors which are linked together.

The multi-mirror telescope (MMT) at Mount Hopkins in Arizona, USA, is the first telescope of this kind. It uses high precision electronics to combine the six images formed by its six component mirrors into an even clearer and brighter single image.

The light collected by the MMT's six mirrors is therefore equal to the light collected by an ordinary telescope with a single mirror of about 4.5 metre diameter. We can call it the effective diameter of the telescope. The next generation telescopes will have effective diameters from 8-16 metres.

Indeed, whatever model is chosen for a future telescope, electronic devices will play a dominant role in processing the

The Hale Telescope at Mt. Palomar, California.

The Hale Telescope at Mt. Palomar, California.

information that is brought by the light from an astronomical object through the telescope. That is why the new 2.3 metre Vainu Bappu Telescope at Kavalur in south India houses an electronic computer as an indispensable astronomical accessory.

How can a computer help the astronomer? It can help him guide the telescope accurately in the direction of the distant star or galaxy; it can form an image of the object On the computer terminal; it can control the various instruments attached to the telescope to make various technical measurements and present them in the required way, and so on.

The multi-mirror telescope at Mt. Hopkins, Arizona.

The new telescope at Kavalur, south India.

The new telescope at Kavalur, south India.

Today's astronomer, however, does not confine himself to telescopes using visible light. Modern technology has provided him with other resources also.

Astronomical images can be formed on the screens of computer terminals linked to the telescope. COSMOS is one such computer facility.

Astronomical images can be formed on the screens of computer terminals linked to the telescope. COSMOS is one such computer facility.

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