The year 2009 is celebrated as the International Year of Astronomy to mark the 400th year of the invention of telescope by Galileo and its use as a novel tool for astronomical investigations. Since then, the telescopes have come of age and hunted the heavens at visible and invisible regions of the spectrum contributing dramatically to the advancement of our knowledge on the universe.

The Early  Dutch Discovery:- A less known Dutch spectacle-maker named Hans Lippershey was credited with creating the designs of the first practical telescope in 1608. He, however, failed to obtain a patent for his invention since it was claimed by two other contemporary spectacle-makers. The original Dutch telescope was composed of a combination of a convex and a concave lens that produced an erect image. However, the device had never been applied for looking at the sky; rather advocated for use as a spy glass for locating the pirates in the ships. 

  An Eye at  the Sky: Telescopes Revolu- tionised Astronomy:- Galileo happened to be in Venice in June of 1609 when he first heard of the wonderful Dutch discovery. After his return to Padua, he made his first telescope that magnified only 3 diameters, but subsequently achieved magnification as high as 33.

           Now, an epoch-making incident unfolded in the history of astronomy as Galileo, for the first time, pointed his telescope skyward to look at the stars. He discovered the four larger moons of Jupiter (1610) , the spots on the Sun, the phases of Venus, and the hills and valleys on the Moon. Galileo demonstrated the revolution of the Jupiter’s moons around the mother planet which established the validity of the Copernican model of   the solar system  dispelling the dogmas of the philosophers of that time. Galileo’s instrument was the first to be given the name ‘telescope’ by a Greek poet  and theologian Giovanni Demisiani at a banquet held on April 14, 1611 by Prince Federico Cesi in honour of  Galileo  .

Refracting Telescopes:- The first telescopes used lenses (called Objective lens) to collect light from distant objects which were focussed to form an image. Another lens (called the Eyepiece) then magnified the image to be viewed by the observer. The result was that a faint, distant object became larger, brighter and clearer.  The telescopes designed by Galileo used a combination of convex and concave lenses that has a very small field of view.  Johannes Kepler  improved the design by constructing a telescope by assembling two convex lenses. The first powerful Keplerian telescopes were made by Christian Huygens who discovered the brightest of Saturn’s moons Titan (1655) and the Saturn’s ring (1659). Giovanni Cassini constructed a giant 35 ft long telescope and discovered Saturn’s fifth satellite Rhea in 1672.

Reflecting Telescopes:- In 1666, Isaac Newton, built an Objective mirror and added a Secondary diagonal mirror  near the primary mirror’s focus for reflecting the image to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction. Encouraged by this success, Newton eventually made a telescope with a magnifying power of 38 diameters which he presented to the Royal Society of London in December 1672.

The Smarter and Sharper:- The Cassegrain reflecting telescope was devised in 1672 by a French astronomer Laurent Cassegrain. The telescope had a small convex hyperboloidal secondary mirror placed near the prime focus to reflect light through a central hole in the main mirror. However, for taking wide angle photographs of the sky, a special telescope called Schmidt telescope is used. It is a reflector which also has a thin objective lens placed at the top of the tube. This produces a sharp image of large portion of the sky with no blurring at the edges.

Telescopes: Giants and Hungry:- The first giant reflecting telescope was built by William Herschel in 1789 with a mirror of 49 inches size and a focal length of 40 ft. To cut down on the light loss, Herschel eliminated the small diagonal mirror from his design and tilted the primary mirror so that he could directly view the final image. In 1897, the refractor reached its maximum practical limit with the construction of a 40 inch research telescope at the Yerkes Observatory near Chicago.

Some of the very large silvered glass mirrors used in reflecting telescopes are: 36 inch Crossley Reflector (1895), 60 inch Mount Wilson Observatory Hale telescope (1908) and the 100 inch Mount Wilson Hooker telescope (1917). The year 1948 saw the completion of the 200 inch Hale reflector at Mount Palomar which was the largest telescope in the world until the completion of the massive 238 inch Large Altazimuth Telescope in Russia 17 years later.

World Beyond the Naked Eye:- The 20th century saw the construction of telescopes to explore the heavens at invisible wavelengths of the electromagnetic spectrum  ranging from radio to gamma  rays. We are no longer limited to the narrow range of wavelengths that managed to leak through the shimmering hazy atmosphere that we breathe, but for the first time we are able to see the whole universe.

Radio telescopes:- Radio astronomy began in 1931 when Karl Jansky discovered that the Milky Way was a source of radio emission. The first dedicated radio telescope was built in 1937 by Grote Reber, with a 9.6 m dish. Interest in radio astronomy grew after the second World War when much larger dishes were built including the 76 m Jodrell bank telescope (1957), the 91 m Green Bank Telescope (1962), and the 100 m Effelsberg telescope (1971).

X-ray Telescopes:  X-rays from space do not reach the Earth’s surface. Therefore X-ray astronomy has to be conducted above the Earth’s atmosphere. The early X-ray experiments were conducted on sub-orbital rocket flights which enabled the first detection of X-rays from the Sun (1948) and the first galactic X-ray sources i.e. Scorpius X-1 (June 1962) and the Crab Nebula (October 1962). The first dedicated X-ray satellite was the Uhuru (1970) which discovered as many as 300 radio sources. More recent X-ray satellites are EXOSAT (1983), ROSAT (1990), Chandra (1999), and Newton (1999).

Gamma-ray telescopes:- Gamma rays are absorbed at higher layers in the Earth’s atmosphere. Therefore, most gamma-ray astronomy is conducted with satellites. There were balloon-borne experiments in the early 1960s, but gamma-ray astronomy really began with the launch of the OSO 3 satellite in 1967 and the first dedicated gamma-ray satellites SAS B (1972) and Cos B (1975). The Compton Gamma Ray Observatory (1991) was a big improvement over previous surveys.

Ultra-violet telescopes:- Although optical telescopes can image the near ultra-violet region, the ozone layer in the stratosphere absorbs ultraviolet radiations shorter than 300 nm. So most ultra-violet astronomy is conducted with satellites. The OSO 1 satellite carried out observations in the ultra-violet region as early as 1962. The International Ultraviolet Explorer (1978) systematically surveyed the sky for 18 years using an 18 inch aperture telescope with two spectroscopes.

Infra-red telescopes:- Although most infrared radiation is absorbed by the atmosphere, infrared astronomy at certain wavelengths can be conducted on high altitude mountains where there is little absorption by atmospheric water vapour. Some telescopes such as the 3.8 m UKIRT and the 3 m IRTF, both on the summit of Mauna Kea in Hawaii are dedicated infrared telescopes. The launch of the IRAS satellite in 1983 revolutionized infrared astronomy from space. This reflecting telescope, which had a 60 cm mirror, detected 245,000 infrared sources which are 100 times more than the number previously known.

Interferometric telescopes:- Because radio telescopes operate at much longer wavelengths than the optical telescopes, their angular resolution is very poor. In a simple two-element radio interferometer, two distant dishes are hooked with wires and the signals from a source interfere producing interference fringes The resolution is sometimes 1,00,000 times better than the sharpest picture from best optical telescopes.