The Star-Seekers

The stars, that seem to shine silently in the sky, have a story of their own to share with us through the light (EM waves) that reaches us as messengers from them. And, telescopes are those magic eyes that enable us to witness those breathtaking astronomical events, be it an explosion of a star into a supernova or the discovery of an entirely new planetary system, nebulae or craters of moon or the mighty storms of Jupiter.

From Galileo to The Hubble, space observation has come a long way. At present, there are different types of telescopes that help us study the outer space, and here is a small article about them.

The oldest and the most popular kinds of telescopes are optical telescopes that usually work in the visible spectrum of light. These are further classified into three types:

Refraction (dioptric telescope) type: The refractor is the classic telescope design which uses a lens as its objective to form an image. These are resistant to misalignment. But, chromatic aberration (the objective lens behaves just like a prism causing annoying colour fringes in the image, some filters or achromatic doublets may have to be used for correction) and difficulty in construction of large refractors are some of its drawbacks.

Fig 1. Refractor type telescope

Reflection (catoptric telescope) type: Unlike refractors, reflector telescopes use mirrors instead of lenses! Curved (parabolic or concave) primary mirror collects light and focus it for observation. Also identified as Newtonian Reflectors, these devices spare you from the worries of chromatic aberration, since all wavelengths reflect off the mirror in the same manner. The downside of this device is that constant adjustments (collimation) and alterations must be made to enhance smooth operation and images also suffer from coma (comet like appearance of objects towards the edge of field of view).

Fig 2. Reflector type telescope

Catadioptric cassegrain: these types of telescopes aim at consolidating the goodness of refractors and reflectors into one and use combination of lenses, mirrors and corrective plates (to reduce optical aberrations) to form the image. Schmidt Cassegrain and Maksutov Cassegrains are some of the coveted variations.

Fig 3. Schmidt-Cassegrain telescope

 But, the space is being constantly flooded with electromagnetic radiation. And, the need to analyze and understand a lot more than what our eyes can see, gave birth to radio telescopes, gamma ray telescopes and many others.

A radio telescope is similar to any other commonly used radio receivers, but is much bigger and more sensitive. These comprise of a large dish, receiver, detectors and analyzers to create a visual picture of the signals it receives. These telescopes tend to be placed in remote locations away from the cities in order to reduce interference with radio waves from earth. An array of radio telescopes scattered at different locations (like very long baseline interferometry) helps create a very large virtual aperture resulting in higher resolution images.

Fig 4. FAST, The largest filled-aperture radio telescope (Guizhou Province, Southwest China) with a diameter of 500m!

Some electromagnetic radiation gets attenuated in the earth’s atmosphere making it necessary to place the telescopes that collect such data; like gamma ray telescopes (detect radiation from supernovae, black holes, quasars…), X-ray telescopes, UV telescopes and microwave telescopes (detection of cosmic microwave background, galaxy clusters…) at a higher altitude or in space observatories. Telescopes nowadays also include particle detectors and gravitational wave observatories too. After all, telescopes are those which let us study ‘celestial events’, even those that may have happened thousands of light years away.

Fig 5. NASA’s Chandra, X-ray observatory

 Be it a small telescope in your backyard or large ones like that of ground based or space observatories, they let us enjoy the chaos and celebration in the universe and bring us closer to unravelling the many unresolved mysteries of nature.

-Namratha G Naik

3rd year, Electrical and Electronics

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