18. Observing Space: Telescopes

The NASA X-ray telescope Chandra (1999) is named after the Indian-born astrophysicist S. Chandrasekhar. The 3.6 m telescope at ARIES, Nainital, installed at Devasthal, is the largest optical telescope in India. GMRT near Pune studies radio waves of about one metre wavelength. Galileo used his telescope in 1609 to discover the moons of Jupiter and spots on the sun.

Difficulties:

• Light from a heavenly body passes through the earth’s atmosphere; some of it is absorbed, so the intensity of the light reaching the telescope decreases.
• Changes in atmospheric pressure and temperature cause turbulence, which makes the light rays change their path slightly, so the image shakes (twinkles) and is not steady or sharp.
• Because of sunlight, optical telescopes cannot be used during the day.
• At night, city lights and cloudy weather make observation difficult.

How they are overcome:

• Optical telescopes are set up on the tops of mountains, in uninhabited places. There the air is thinner and cleaner, there is less turbulence and fewer clouds, and there are no city lights.
• To remove these problems completely, telescopes are placed above the atmosphere, in space. Space telescopes, such as the Hubble telescope, give bright, very clear and steady images at all times.

• Lens: a refracting telescope — a large convex objective lens collects light and a small eyepiece lens gives a magnified image.
• Concave mirror: a reflecting telescope — the concave mirror is the main mirror, which collects and focuses the light.
• Concave mirror + plane mirror: the Newtonian telescope — a small plane mirror turns the reflected rays to the side, into the eyepiece.
• Concave mirror + convex mirror: the Cassegrain telescope — a small convex mirror reflects the rays back through a hole in the centre of the concave mirror, to the eyepiece behind it.

It is a Newtonian telescope, a type of reflecting telescope.

Main parts: concave mirror (primary mirror), plane mirror kept at 45° (secondary mirror), eyepiece, and the tube of the telescope.

It uses a concave mirror as the main mirror and a small plane mirror to turn the rays towards the eyepiece.

The Cassegrain telescope, which uses a concave main mirror and a small convex mirror. (A radio telescope also uses a curved, parabolic dish.)

Light rays coming from a heavenly body enter the open end of the tube and fall on the large concave mirror at the other end. The concave mirror reflects them and makes them converge. Before these reflected rays meet at the focus, a small plane mirror kept at 45° to the axis deflects them sideways, so they are focused at a point on a line perpendicular to the axis of the tube. The rays then pass through the eyepiece fixed on the side of the tube, which gives a magnified image of the source. Because a large mirror collects a lot of light, the image is bright, and since it is made by reflection, there is no error of colours.

Galileo’s telescope (1609) was a refracting telescope made of two lenses fixed at the two ends of a long tube:

• Objective lens: a convex lens at the front end, facing the object. It collects the light from the distant object and makes it converge.
• Eyepiece: a lens at the other end, near the eye. Galileo used a concave (diverging) lens as the eyepiece, which gives an upright, magnified image.
• The distance between the two lenses could be adjusted by sliding one tube inside another, to focus the image.

With this simple telescope, which magnified about 20–30 times, Galileo discovered the moons of Jupiter and the spots on the sun, and saw that there are many more stars than can be seen with the naked eye. The telescope itself was first made by Hans Lippershey in 1608.

• A radio telescope is made of one or more large dishes of a particular parabolic shape, which act like the concave mirror of an optical telescope.
• Radio waves from heavenly bodies fall on the dish, are reflected, and converge at its focus.
• A radio receiver is placed at the focal point to receive these waves.
• The weak signal is strengthened by an amplifier and passed on to a computer, which analyses the information and constructs an image of the source.

Example: the Giant Metrewave Radio Telescope (GMRT) at Narayangaon near Pune has 30 dishes, each 45 m in diameter, spread over an area 25 km across, and works like a single dish 25 km in diameter.

• At high altitude the layer of air above the telescope is thinner, so less light is absorbed and the images are brighter.
• There is less turbulence and dust, so the image does not shake.
• Mountain tops are often above the clouds, giving more clear nights for observation.
• Uninhabited places have no city lights to brighten the sky, and no smoke or pollution.

So the observations are much clearer. That is why the 3.6 m telescope of ARIES is at Devasthal in the Himalayas near Nainital.

X-rays coming from heavenly bodies are absorbed by the earth’s atmosphere and do not reach the earth’s surface. Only visible light and radio waves can pass through the atmosphere. A telescope on the earth would therefore receive no X-rays to study. X-ray telescopes are placed in space, above the atmosphere; examples are NASA’s Chandra X-ray telescope and the X-ray instruments of India’s Astrosat. They also use special mirrors that can reflect X-rays.

For each, find its location, altitude, main telescope, what it studies and the institute that runs it.