The sun is often in the news! Ground-based telescopes and several satellites are revealing new features. The study of the Sun has become an important component of the International Heliophysical Year (2007-2009). Technology developments have enhanced the value of ground-based telescopes. The telescopes already working in India have given a lot of data and attracted considerable expertise of a high caliber to process and analyze them. Advanced theoretical studies have also progressed and gained worldwide recognition.
Many institutions in India were working on several aspects of the solar-terrestrial physics. Several universities and institutions have ionospheric and atmospheric monitoring systems. The Indian Institute of Geomagnetism(IIGM) has a network of geomagnetic observations spread across the country. The IHY provides an appropriate turning point for India to go in for a new solar telescope.
Currently there are two one-and-a-half meter solar telescopes being developed in the world: one in the Canary Islands by the Germans and the other in California. They are excepted to see the first light by 2011. A four-meter solar telescope planned by the US National Solar Laboratory is not likely to materialize soon. Thus, a two-meter solar telescope in India, if it materializes, would be the world’s largest of its kind and would make a significant contribution to solar studies.
The concept is being finalized and once the cost and manpower implications are worked out, the Indian Institute of Astrophysics (IIAP), Bangalore would prepare a detailed plan for a two-meter class National Large Solar Telescope for approval of the government. The telescope proposed would be a world-class facility that is expected to bring new and exciting scientific results.
A new solar telescope is necessary to investigate the fundamental nature of solar magnetism down to the smallest scale that can be measured from the ground, as it influences the 11- year solar cycle, solar variability that influences the space weather and activity that controls the heating of the Sun’s atmosphere. It is recognized today that magnetic activity on the Sun takes place on a scale of 40 to 50 km. For a telescope to have the capacity to resolve the scenes of this scale, it should be able to see one-tenth of an arcsecond or 70 km. (one arcsecond corresponds to 700km on the Sun and one-tenth of an arcsecond is required.) Even at this scale, one can barley resolve the scene and there is a need for registering more photons from the Sun, if one were to study the phenomenon at narrow wavelengths. Thus it has been calculated that a two-metre aperture would be ideal for detailed studies of the magnetic fields. A 2-metre telescope working at a wavelength of 500 nanometres will correspond to about 35 km on the solar surface. With adaptive optics features of this size can be resolved. But there is still a big if. That is the so-called seeing conditions which are set by nature. Site selection thus assumes critical importance.
The criteria for site selection relate to: the number of sunshine hours in a year; wind speed and direction; the volume of water vapour; microthermal fluctuations etc. Different locations in the Himalayan region are being studied. Special equipments for site characterization is being fabricated with the assistance of the National Solar Observatory, USA.
Three sites are being thought of for locating the telescope. One is of course Hanle, where there is already a two-metre optical stellar telescope functioning; another is Pangong Lake in Ladakh, where the lake’s surface would annul the temperature variations on the ground and the third site is in Devasthal near Nainital, where a 3.6-m optical stellar telescope would soon be installed.
In these days of satellite technology, it may be asked whether ground-based telescope have a major role. The answer today is a resounding yes in view of several new developments. With the development of adaptive optics, it would be possible to correct the effects of atmospheric turbulence in a ground-based system. Thus the most important reason for going above the atmosphere into space is met by a less expensive alternative. Moreover, a ground-based optical system can have heavy equipment like spectrograph.
Of course there are areas like X-rays and extreme ultraviolet where only space-borne probes work, as the Earth’s atmosphere would absorb such radiation. In view of the many positive factors, ground-based telescopes for optical astronomy are favoured. In the words of Prof. Siraj Hasan, it is a golden era now for optical telescopes on the ground. The proposed National Large Solar Telescope should have most of the recent technological innovations in detector arrays, optics, active mirrors and adaptive techniques. It would be complementary to space missions.
The proposal could not have emerged at a more appropriate time, when space-based detectors have indicated fascinating phenomena on the solar surface that call for sustained and detailed studies from the ground. All regions and layers of the solar atmosphere need to be studied. One configuration being studied would place the telescope on the top of a 20-m tower for feeding the solar image to different processing instruments at lower levels. Accessories would include a spectro-polarimeter to determine the magnetic fields on the Sun accurately; and a high-resolution spectrograph. These are to be custom-built and call for well-versed expertise at home. The National Large Solar Telescope will be a joint effort at a national level involving several institutions currently engaged in solar research.