Watchers of the sky
D. Indumathi, The Institute of Mathematical Sciences, Chennai
From ancient times, the skies have held a fascination for people. In the absence of electricity and on new moon nights, the night sky must have been a fascinating, everchanging kaleidoscope to people long ago. While most were content just to watch, some were curious about patterns that seemed to repeat over seasons.
Soon there were lists of constellations, tales of comets and stars and astronomy had begun. But some people were still not content. They wanted to examine the distant stars better and more clearly. And so telescopes were born.
Early attempts
The earliest known working telescopes date back to the beginning years of the 17th century.  Hans Lippershey, Zacharias Janssen and Jacob Metius all claimed the invention as theirs.
A combination of a convex and concave lens helped to bring the distant objects closer. Galileo Galilei, Johannes Kepler and Christiaan Huygens were all soon building powerful but extremely large and unwieldy telescopes by the middle of the 17th century. A photo of a lens made by Huygens about 350 years ago is shown on the front inside cover.
Today, telescopes not only include optical telescopes such as these old ones, but any instrument that surveys the sky in any part of the electromagnetic spectrum, whether it is visible or invisible radiation.
The idea of using a combination of mirror and lens to make a reflecting telescope (the objective is a mirror instead of a lens) was investigated soon after the invention of the refracting telescope.
The maximum physical size limit for refracting telescopes is about 1 meter, dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. The largest reflecting telescopes currently have objectives larger then 10 m.
The 20th century saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. The first purpose built radio telescope went into operation in 1937. Since then, a tremendous variety of complex astronomical instruments have been developed.
Optical Telescopes
The famous 100 inch (2.5 m) Hooker telescope at Mount Wilson Observatory near Los Angeles, California was one of the early reflecting telescopes of the 20th century. This is the telescope that Edwin Hubble used to measure galaxy redshifts. This led to the discovery that the Universe is expanding and not static, as was originally thought. The expansion of the universe is one of the landmark discoveries made by telescopes about the nature of the world we live in.
The world's highest observatory for optical & infra-red astronomy is the Indian Astronomical Observatory at Han Le in Ladakh, North India. It is dry, cold desert with sparse human population.  The cloudless skies and low atmospheric water vapour make it one of the best sites in the world for optical, infrared, sub-millimetre, and millimetre wavelengths.  A 2-m optical infrared telescope called the Himalayan Chandra Telescope is installed at the observatory. This telescope is remotely operated from Hosakote in Karnataka, using dedicated satellite links.  In addition to night observations, the site is also being characterised for solar observations.
Radio Telescopes
Radio telescopes are directional radio antennas that often have a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Multi-element radio telescopes are used to improve the collecting capacity.
The Giant Metrewave Radio Telescope (GMRT), located near Pune is the world's largest array of radio telescopes at metre wavelengths. It is operated by the National Centre for Radio Astrophysics, a part of the Tata Institute of Fundamental Research, Mumbai. (See photo on back cover).
Astronomers from all over the world regularly use this telescope to observe many different astronomical objects such as galaxies, pulsars and supernovae, the sun and solar winds.
High energy particle telescopes
High-energy astronomy requires specialised telescopes to make observations since most of the particles being observed penetrate right through most metals and glasses.
X-ray telescopes are composed of ring-shaped 'glancing' mirrors made of heavy metals that are able to reflect the rays.
The second High Energy Astrophysics Observatory (HEAO-2), launched by NASA on 13 November 1978, was renamed the Einstein Observatory after orbit was achieved. It was the first fully imaging X-ray telescope put into space, with an angular resolution of a few arcseconds, a field-of-view of tens of arcminutes, and a sensitivity about 1000 times greater than any mission before it.
Gamma-ray telescopes use a special mask that casts a shadow.  The patterns of the shadow the mask creates can be reconstructed to form an image.  Such a gamma ray telescope called HAGAR is being constructed at Han Le.
X-ray and Gamma-ray telescopes are usually on Earth-orbiting satellites or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum. For instance, the ASTROSAT astronomy mission will be launched in 2010. It will be loaded on a satellite in a 650-km, near-equatorial orbit by the Indian launch vehicle PSLV from the Sriharikota launch centre.
Its main scientific focus will be on studying cosmic ray sources, and searching for objects that emit X-rays such as Aactive Galactic Nuclei (AGN), supernovae, galaxies, etc.
In other types of high energy particle telescopes there is no image-forming optical system.  Cosmic-ray telescopes usually consist of an array of different detector types spread out over a large area. A neutrino telescope consists of a large mass of water or ice surrounded by an array of sensitive light detectors known as photomultiplier tubes.
These high energy detectors usually record the kinds of particles that are emitted from various astrophysical sources rather than the source (star) itself.
Gravitational Telescopes
Finally, gravitational wave telescopes are telescopes that try to detect any gravitational waves from massive objects in the Universe.
Today the goal is certainly to try and understand our Universe using every signal possible, from visible to invisible light and from light waves to gravitational waves!
Compiled from several sources