Astronomy

The Vainu Bappu Observatory

vainu-bappu-2-3m

Even before India’s independence, a committee on Astronomy, headed by the eminent scientist, Prof. M.N. Saha, drew up a plan for two large telescopes in the country. The famous Nobel Laureate. C.V. Raman supported the idea. Though the Kodaikanal solar observatory continued to be useful, there was a need to set up a telescope for stellar astronomy at a place that has the maximum number of clear nights, ideal for observing the sky.

After an extensive search, a place near a small village, Kavalur (Pronounced Kaa-va-loor), 180 km from Bangalore amidst a sandalwood forest in the Javadu hills in Tamil Nadu was chosen for locating the new observatory. It is at 78º49’6 East longitude, and 12º34’6 North latitude, its altitude being 725m above sea level. The site has clear nights, suitable for optical astronomy, from mid-December to late May, when south India is free of monsoons. One can see from Kavalur the centre of our galaxy, the Milky Way as well as the sky above the northern and southern hemispheres.

Astronomer Bappu’s initiative in locating the place succeeded, when the Tamil Nadu government gave 40 hectares near Kavalur on a long-term lease for setting up the observatory. Today, the area is fenced off with wires that keep away intruders including elephants.

The observatory started work in 1968 with a 38-cm telescope, followed by a 75-cm telescope. In 1972 a 102-cm Carl Zeiss telescope was installed. It gave a boost to the capability of the observatory. It could observe half a degree of the sky, which is about the diameter of the Moon. This telescope has today an indigenously polished mirror with some new subsystems.

Within a few weeks of its installation, the new telescope hit the headlines. It discovered visible evidence of an atmosphere in Ganymede, a satellite of Jupiter. Until then, it was thought that only Titan, the largest satellite of Saturn had an atmosphere. First years later, in 1977, the telescope was again in the news: it discovered a ring system around Uranus.

An elated Vainu Bappu began to draft ambitious plans. He envisaged a much bigger telescope, for, the large a telescope’s aperture, the more light it collects. Bappu envisaged a primary mirror measuring 2.3 m (234 cm) in diameter. Moreover, he decision that the new reflector telescope should be fully indigenously designed and fabricated. It was a bold decision, implemented with determination. Only the ‘mirror blank’ was imported from Germany, as it should suffer virtually no thermal expansion in the ambient temperature of Kavalur, which varies from 15º to 40ºC. All other works, including optical and mechanical components were made at the Indian Institute of Astrophysics (IIAP), Bangalore.

The target was quite a challenge. The polishing of the mirror—a difficult task—was done to perfection. Not only did IIA build the optics, it also set up a facility to coat the mirror with high-purity aluminium, according to international standards. The largest optical telescope in south-east Asia began to take shape in the early 1980s. Unfortunately, Bappu died in 1982. Optical astronomy in India suffered a big setback.

The tradition of scholarship and perfection that he created continued to inspire his colleagues. The new facility was formally commissioned in 1986. The late Prime Minister, Rajiv Gandhi, aptly dedicated it to the nation and named the telescope and the observatory after Bappu. The Vainu Bappu Telescope is now a National Facility for Optical Astronomy.

By then (1984), the telescope had made yet another discovery: an outer ring of Saturn. The astronomers were delighted. Their vigil was amply rewarded, when shortly after the inauguration, the telescope observed the supernova 1987A in the Large Magellanic Cloud and later the Supernova-1993J in the M81 galaxy. In 1988 the telescope discovered a minor planet between Mars and Jupiter, which was named after the famous Indian mathematician, Ramanujan.

The 2-3-m aperture telescope has a mirror, which weights 3.5 tonnes. It is on an equatorial mount. In other words, one of its axes is in line with the Earth’s rotational axis to compensate for the latitude of Kavalur. The telescope is covered by a split dome, which is opened during observation. The computer-controlled telescope can be titled and commanded to track an object automatically. The circular platform on which the observer stands can be raised to provide easy access to a small “finder” telescope, designed to correct the focus of the main telescope on the object imaged. The captured light at the prime focus is collected by a charge-coupled detector, an array of 1024 by 1024 pixels. The scale of the image is 27.1 arc seconds per millimeter (the number of seconds of arc in the sky corresponding to 1mm in the focal plane of the telescope). One arc second is equal to 700 km across the sky. An imaging camera is at the prime focus of the telescope, with a spectrograph.

Further improvements have been made to the facilities. In one technique perfected by IIA astronomers, the collected light is sent across an optical fibre to a sophisticated device, called Eschelle spectrometer, which sends the light from the prime focus into a spectrograph slit for dispersal into fine segements of spectral lines. This way one can observe very faint objects in the distant sky, measured to the 10th or 11th magnitude. It is a significant contribution of IIA astronomers and scientists.

Capturing starlight is not just a matter of focusing the telescope on a star. Light from a star has to be handled in a very sophisticated manner with modern device. The light has to be converted into a spectrograph for analysis. As almost 90 per cent of the information about a star, such as its elemental abundance, its atmosphere and dynamic behaviour, can be detected from a star’s spectrum, high-resolution spectroscopy has become important.

Conventionally, light from a telescope is focused away from it at a particular height (at its ‘elbow’ or coude in French) and sent directly to a spectrometer. In this way, much of the received light is lost. Bigger optics would of course be helpful but it was found that they cannot just be added to the telescope. A stable base located away from the telescope was found necessary, as the telescope rotates to focus on the star

At the Vainu Bappu Telescope at Kavalur, light from the 2.3-m telescope’s prime focus (which gets the light gathered by its primary mirror below) is taken to an eshelle spectrometer kept on a separate base, so that it can be free from the telescope’s vibration.

The Eschelle spectrograph has enhanced the capability of the Vainu Bappu Telescope (VBT). Indian experts have mastered this sophisticated technology which is working well. It is an in-house development based on the best technologies in the world.

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