Radio Astronomy in India: A historical perspective
Pune is an awesome city. My love for this city is due mainly to two reasons. First - the climate, which is pleasant throughout the year. Second, Pune (then Poona) is the birthplace of badminton, which happens to be my favorite sport. However, I didn't know much about the city until I went for a summer school at IUCAA in my final year of college. IUCAA (Inter-University Centre for Astronomy and Astrophysics) is located in the lush green Pune University campus. As the name suggests, it is an institute specializing in various branches of astronomy and astrophysics. A few weeks into the summer school and I learned how cool astrophysics is! By the end of the school, I knew that whether or not I pursue research in astrophysics, I shall never miss another chance to visit the city. The very next year, I applied for an internship and, lo and behold, I am assigned to work on a project at NCRA!
The oldest of all natural sciences, astronomy, began when human beings looked up in the sky and started wondering about the celestial objects. In the early 17th century, the first telescopes were invented. It was only during the 20th century that people started constructing instruments to observe the sky in other electromagnetic frequencies such as X-ray, gamma-ray, radio etc. The era of radio astronomy began when Karl Jensky serendipitously detected radio waves coming from the Milky Way in 1933. Later, people discovered radio emission from the sun as well as other galactic and extra-galactic sources. These discoveries paved way for the development of radio telescope facilities around the world. In India, the Kodaikanal Observatory started monitoring the radio emission from the Sun in 1952. TIFR (Tata Institute of Fundamental Research), in 1963, established a radio astronomy group under the leadership of Prof. Govind Swarup (who would later be known as the father of Radio Astronomy in India). This newly formed group set up the first Indian radio telescope at Kalyan near Mumbai. Radio telescopes can be used individually or linked together to create an array of telescopes, known as interferometer. The Kalyan telescope consisted of 32 parabolic dishes of 1.8 m diameter and was used for solar studies at meter wavelengths. Only a year after the Kalyan Radio Telescope got disbanded in 1968, the group constructed the ambitious Ooty Radio Telescope (ORT). It is a 530 m long and 30 m wide cylindrical, steerable telescope operating at wavelength of 92 cm (327 MHz). The unique feature of this telescope is that it sits on a hill whose natural slope matches the latitude of its location -Ooty. As a result, the latitudinal correction is automatically taken care of, bypassing several engineering difficulties. This ingenious idea of Prof. Swarup helped in reducing the cost of the telescope drastically. This was indeed a breakthrough for Indian radio astronomy.
Located just next to IUCAA is the NCRA (National Centre for Radio Astrophysics), which is one of the many centers of TIFR. During my stay at NCRA, I worked on some theoretical aspects of pulsar (a type of neutron star) emission. I got to know during this time that NCRA was home to the radio astronomy group of TIFR, which then went on to develop the GMRT (Giant Meterwave Radio Telescope) at Khodad (about 80 km North of Pune). GMRT is a Y-shaped array of 30 fully steerable parabolic dishes of 45 m diameter each. Conceived under the leadership of Prof. Swarup, GMRT offers an interferometric baseline of upto 25 km. For radio telescopes to achieve arc-second resolution (ability of a telescope to distinguish smaller objects in the sky) at wavelength of ~1 m, the aperture size of the telescope dish requires to be kilometers across, which is clearly not possible or practical. Interferometry is a viable solution to this problem. Since the resolution of an interferometer depends on the maximum separation between them rather than the diameter of individual radio telescopes, moving them apart increases the angular resolution. The correlation of signals from all the possible pairs of GMRT dishes (which act as interferometers) taken over several hours of observation is equivalent to a resolution obtainable with a single dish of 25 km diameter. For GMRT, the highest achievable angular resolution ranges from around 60 arcsec at low frequencies to about 2 arcsec at high frequencies. Apart from Indian astronomers, GMRT caters to the needs of astronomers from several other countries. In Nov 2020, GMRT bagged the IEEE 'Milestone' status for its contributions to exploring the universe through radio astronomy.
GMRT along with other Indian operational radio telescopes are being used to study cosmology with 21 cm line of neutral atomic hydrogen, pulsars, low frequency probing of active galaxies etc. Indian science has a history of being extensively innovative within financial constraints. The work done by Prof. Swarup and his team placed India in the world map of radio astronomy. With the advent of gravitational waves (GWs) as new probe of astronomical observations, radio astronomy will additionally play an important role in identifying the electromagnetic counterparts to GWs from astrophysical sources.
This aricle is a tribute to Prof. Govind Swarup (Mar 23, 1929 - Sep 7, 2020 )
Uddeepta Deka
A wanderer on the landscape of physics problems.