Radio waves are about a hundred thousand times longer than optical waves. But scholars and researchers in India realized than the picture of the Universe would be incomplete without the inputs at radio and other non-optical wavelengths.
In the 1960s, a radio astronomy group at the Tata Institute of Fundamental Research (TIFR), under Prof Govind Swarup, built a solar radio telescope at Kalyan near Mumbai. It consisted of 32 parabolic dishes of 1.8 m diameter. They were operated at a frequency of 610 MHz. The dishes were gifted under the Colombo plan by the Director of the Radio Physics Division of CSIRO, Australia. The solar telescope found interesting results when it was operation from 1965 to 1968.
Radio Telescope at Ootacamund
The TIFR group, led by Prof Swarup, designed and built a radio telescope at Ootacamund (Ooty) in the Nilgiris in Tamil Nadu. The telescope was commissioned in 1970 and has been in almost continuous operation ever since.
It is the first major facility for radio astronomy in India ORT, as it is briefly called, is a cylindrical paraboloid, 530 m long and 30 m wide and works at a frequency of 327 MHz. Over 1000 thin stainless steel wires, threaded through the 24-parabolic frames, constitute the reflecting surface. The telescope operates at a wavelength of about one metre. The entire surface can be rotated in unison through an angle of about 110 deg and the telescope can be steered in the east-west direction.
The telescope has an innovative design. It makes use of a hill which is higher at its northern end than at its southern end, the slope being equal to the latitude of Ootacamund (11.deg 4”). The axis of the telescope is thus kept parallel to the Earth’s axis of rotation in the north-south direction. The telescope is thus able to compensate for the Earth’s rotation by rotating about a single axis (long axis). Conventional radio telescope mounts have to rotate about two axes to compensate for the rotation of the Earth. The imaginative design has resulted in considerable cost savings.
The telescope’s proximity to the equator enables astronomers to watch the sky for nine and half hours a day. The design is deal to study distant galaxies by a method called lunar occultation. As the Moon moves across the sky, radio waves from distant sources are eclipsed by it for a while. A small point of radio source would be hidden completely behind the Moon, whereas a broad source would disappear slowly. After the radio waves are recorded, the data are sent for a comparative study through optical telescopes at Kodaikanal, Kavalur and Nainital.
Giant Metrewave Radio Telescope
In the mid-1980s, the TIFR Group designed and constructed a unique radio telescope, the biggest and the most powerful radio telescope in the world at low radio frequencies.
Its low-cost design has won worldwide commendation. Called the Giant Metrewave Radio Telescope (GMRT), it is located at khodad, about 90 km north of pune. The telescope is located sufficiently north of the equator to have a reasonably quiet ionosphere for allowing the cosmic radio waves and yet suitable to observe a good part of the southern sky, through the proliferation of mobile phones poses a problem to the sensitive instruments. TV stations and mobile phone operators in the area have been requested to take all possible steps to reduce radio interference.
The telescope consists of 30 fully steerable gigantic dishes, each 45 m in diameter in a 25 km area. Each antenna is linked to the Central Electronics Building by two optical fibre connections. The entire array can be remotely operated and real time monitoring of data quality is possible. Some really smart work has gone into the design of the telescope array. The design has an appropriate acronym: SMART! It stands for Stretched Mesh Attached to Rope Trusses, which is somewhat self-explanatory, as it indicates a light structure to save cost. The antennas have a parabolic shaped reflector, which brings all the waves that arrive along its axis to the focus of the parabola, no matter what frequency it works. In fact, GMRT operates in six frequencies ranging from 150 MHz to 1420 MHz. It has the largest collecting area in the world for observing the universe in such frequencies.
GMRT has incorporated a technique called ‘aperture synthesis’ for processing its observations. Twelve of its antennas are in a compact array (1 km x 1 km), while the other 18 antennas are spaced out along three arms, each about 14 km long. The central array can detect diffuse emission, while the arms given high resolution. At metre wavelength, the GMRT is the most sensitive aperture synthesis telescope in the world.
It has a correlator—an advanced computer—made in India, designed for the purpose of combining the signals from smaller antennas. In a parabolic reflector, all waves that arrive along the axis of the reflector (after reflection) meet at the focus of the parabola, independent of the frequency of the wave. At very low frequencies thin wires can act as antennas, related to the wavelength of the radio waves. The wires accept certain frequencies and reject others. Raman Research Institute, Bangalore has undertaken to install 50 MHz dipoles on GMRT antennas for studies in low frequencies.
GMRT is a versatile instrument for investigating a variety of radio astrophysical problems from the solar system to the edge of the observable Universe. One of its most important objectives is detection of the spectral line of neutral hydrogen expected from proto clusters or proto galaxies, before they condensed to form galaxies in the early phase of the Universe. Neutral hydrogen is detected at a frequency of 1420 MHz. As the Universe expands, the hydrogen line is detected at longer wavelengths (or lower frequencies), because of the red-shift it undergoes under the Doppler Effect. If the line is observed at frequencies of 350 and 1130 MHz, it would mean the lines have undergone red shifts of 3 and 10. It would indicate that the lines show the Universe, when it had attained only a few per cent of its present age.
The second important objective of GMRT is the study of pulsars, rapidly rotating neutron stars with extremely high density of 200 million tones per cubic centimeter! In particular, the scientists at GMRT are on the look out for extremely rapidly rotating pulsars with periods in the range of milliseconds and pulsars in binary systems. One reason for the study of these objects is that their strong gravitational forces would provide an ideal place to test Einstein’s General Theory of Relativity. Minute changes in the arrival time of the pulses would indicate asymmetries in the gravity of the source when the Universe was less than a billionth of a billionth of a billionth of a second old!
Recently, GMRT astronomers discovered a pulsar circling another massive object, revealing the most eccentric orbit ever seen in such phenomena.
GMRT’s large collecting area and the wide frequency of the coverage are useful in the study of solar and planetary radio emissions, impact of solar activity on the interplanetary medium, clouds of ionized hydrogen (where the electron in the hydrogen atom is removed) associated with young stars, and monitoring of a variety of extra-galactic radio sources as well as supernova in external galaxies.
Recent results announced in the website of GMRT give an idea of the scope of its observations. GMRT has carried out the longest-ever radio follow-up of any gamma ray burst (030329) after-glow and also of the lowest frequency detection’s ever made. The array has recorded the fading radio counterpart of the giant flare from the magnetar (SGR 1806-20).
In other achievement, GMRT defined the radio spectrum of the supernova SN 1993J on the 3200th day after its explosion on March 28, 1993. The array first detected the central compact source SgrA* in the centre of our galaxy. Near the galactic centre, the array identified the line emission for the molecule, acetaldehyde, in a giant molecular cloud complex. An X-shaped radio galaxy, a ‘head-tail’ galaxy and emission of atomic hydrogen (HI) from more than 60 galaxies of the Eridanus group, a prominent structure in the southern sky are some of the other findings.