The history of telescopes shows that as innovative technologies open new windows in the electromagnetic spectrum, new cosmic phenomena are discovered. The sub millimetre wave band (about 1 to 0.1 mm) at high frequencies (1000 to 1500 GHz) is one of the newly opened windows for exploring the Universe. The window is unique as it would show the star-forming regions in our and nearby galaxies in the early Universe.
Low-cost telescopes for submillimetre and shorter wavelengths demand innovative design. A spherical design with a wide field of view requires a large secondary mirror, which results in gravitational flexure. In conventional paraboloidal altazimuth-mounted telescopes, the primary mirror has to be moved to track the source in elevation. Hence a radically different design is adopted by installing a cylindrical primary. It can provide not only a wide elevation coverage but also a good field of view. The paraboloid converts a plane wavefront to a spherical one in one step. However, it has been found that the conversion need not be in one go but made in stages using more than one mirror. Making large and smooth parabolic cylinders is relatively inexpensive.
The Raman Research Institute has proposed a design which uses four mirrors, three of which are parabolic cylinders, each converting the wavefront in sequence. The fouth mirror converts the cylindrical wavefront into a spherical one. The RRI scientists have also defined the shape of the fourth mirror.
Hanle: An Ideal Place for Other Telescopes
Indian is naturally endowed with many high altitude desert sites in the Himalayas. A group of Indian astronomers evaluated the atmospheric transparency in the Hanle area for installing a facility for a telescope working in millimeter and submillimetre wavelengths. The group made use of a radiometer initially deployed atop Mt Fuji in Japan, augmenting it with some hardware and software at the Raman Research Institute, Bangalore.
After a three-year evaluation of the opacity in the area, the group found Hanle a promising site for submillimetre waves and infrared astronomy during most of the year. Given the promise of innovation in telescope design, India can be in the forefront in submillimetre wavelength astronomy. The upcoming Atacama Large Millimeter Array will have a large collecting area and high spatial resolution but a small field of view. A large single dish with modern photon-detector can be a faster array than the one at Atacama.
A Satellite for Low Frequency Radio Waves
RRI has yet another initiative. In a proposal to ISRO, RRI has outlined its plan to launch a small scientific satellite, about 100 kg including a 40 kg of scientific payload to probe the low frequency radio waves from space. The payload will carry dipoles, which will break into four separate units after the satellite is in orbit. The dipoles will drift apart in space catching the radio waves.
The proposal is timely as a key unsolved problem in astronomy today relates to the formation of the first stars and the first galaxy. How did they form, a billion years after the Big Bang, some 13.6 billion years ago? Astronomers call it a period marked by the red shift (the apparent shift of the light from the source to the red end of the spectrum) between 7 and 15, corresponding to 0.3 billion to 0.8 billion years, respectively. What happened when the fist light emerged? How did it transform the matter?
What were the feedback mechanisms at work? Moreover, if the relative separation of the galaxies is accurately known, then it would be possible to have a better idea of the spread of the galaxies and thereby evaluate the role of the so-called dark unseen energy, believed to account for 70 per cent of the content of the Universe. In other words, we need a detailed picture of the early Universe when the galaxies were forming to know how the spacing of the galaxies changed over time—which would be a clue to the gravitational forces that would reveal the nature of the dark matter.
That stage of evolution of the Universe can be detected through low frequency radio waves or infrared from the distant past, because any radiation from them in the ultraviolet region of the electromagnetic spectrum would readily be absorbed by the hydrogen in the area and would not be visible to us. The neutral hydrogen there is revealed in the 21-cm radiation, which can be detected through the 100 to 200 MHz radio window. And the light in the optical region would be red-shifted by a factor of 8, as the source is moving away and would thus be seen only in the infrared. That’s why the Americans are planning to launch the sophisticated James Webb Space Telescope that would detect the celestial sources in the infrared.