Largest Solar Telescope in the World – Kalpana Palkhiwala



The sun’s atmosphere is an ideal place to study and test many magneto-hydro-dynamic (MHD) processes controlling turbulent plasma. Some of the finest solar features can be resolved with state-of-the-art telescope. Ladakh, the cold desert in the country, will have the world’s largest state-of-the-art solar telescope on the earth soon for this purpose. This National Largest Solar Telescope (NLST) will be set up at high altitude Pongong Tso Lake Merak near Line of Actual Control on the Sino-Indian divide. It will be unique globally because the largest solar telescope, the Mc-Math-Pierce Solar Telescope, in the world at present is 1.6 meters. It is situated in Kitt Peak national Observatory at Arizona in the United States. The NLST will have the reputation of being the world’s largest telescope till 2020. It is expected that a larger telescope may become operational in the US by 2020-2021.

NLST is a Gregorian multi-purpose open telescope. It will be on-axis with the provision of carrying out night time stellar observations using a spectrograph. It will be able to study particles spread across 50 kilometers of the sun. It is expected to resolve features on the sun of the size  of about 0.1 arcsec. The focal plane instruments are to include a high resolution polarimetric package to measure polarization with an accuracy of 0.01 %, a high spectral resolution spectrograph to obtain spectra in 5 widely separated absorption lines simultaneously and high spatial resolution narrow band image in various lines.

Modalities

The telescope will be fitted with a 2 meter reflector which will enable scientists to carry out cutting edge research to understand the fundamental processes taking place on the earth. The design is by an international company which also designed the 1.5 meter telescope located at Tenarife Island in Spain. All the telescope’s instruments will be developed by the Indian Institute of Astrophysics (IIA) and will be remotely operated through the master control facility at Bangalore. The link will be through a satellite which will be provided by the Indian Satellite Research Organisation (ISRO). Night-time observations will be made using an instrument which will be built in collaboration with the University of Hamburg, Germany.

Scientific Objectives

Scientists will study the sun’s microscopic structure, and long term changes in the earth’s climate and environment. This will provide useful data to carry out research in order to minimize or remove disruptions to communication network and satellite due to periodic solar-winds.

The telescope will address the fundamental question about the nature of solar magnetism. It will aim to resolve flux tubes and measure their strength; address the development of magnetic fields on the sun which are responsible for almost all the observation phenomena on the sun. They include solar dynamo, solar cycle and solar variability that determine and control space weather.

Other scientific observations are: –

Magnetohydrodynamics(MHD) waves by resolving small structures and determining periods of oscillations which may be responsible to transport the energy to upper atmosphere of the sun:

  • Dynamic evolution of small scale structures by making high cadence observations;
  • Evolution of active regions and their role in triggering solar flare, prominences filament eruptions, CMEs, etc;
  • Thermodynamics of the chromospheres by making the observations in the infrared wavelengths; and
  • Weak and turbulent magnetic field measurements using Hanle effect which are as important as strong magnetic fields.

All these data will be gathered by making observations with high spatial resolution using adaptive optics, high spectral resolution, high temporal resolutions, multi-wave length capability of imaging and spectroscopy focal plane instruments, high photon flux and sensitivity of the detectors and using the infrared part of the spectrum for observations.

The telescope will utilize an innovative design with a low number of reflections to obtain a high throughput and low instrumental polarization. High order adaptive optics is integrated into the design that works with a modest Fried’s parameter of 7 cm to give diffraction limited performance. The telescope will be equipped with a suit of post focus instruments including a high resolution spectrograph and a polarimeter.

Selection of the Site

To install the telescope, the Indian Institute of Astrophysics studied two other sites-Hanle, Leh and Devasthal near Nainital in Uttarakhand but eventually opted for Merak in Ladakh.The cloudless skies and low atmospheric water vapour have made it one of the best sites in the world  for optical, submilimeter and millimeter wavelengths.

The site has been selected after carefully studying various scientific and environmental aspects. The site characterization has been done using the sun photometer, S-DIMM and SHABAR techniques to determine the seeing condition. SHABAR or Shadow Band Radiometer is a simple yet elegant way to probe the turbulence profile of the lower atmosphere. It observes scintillation of an extended object such as the sun or the Moon with an array of photo detectors. S-DIMM is Solar Differential Image Motion Monitor.

The Himalayan regions provide certain atmospheric conditions required for such telescope functioning. It provides a large number of clear hours for making observations with very good visibility. The water vapour in the field is also low which helps making observations in infrared wavelength for high accuracy of magnetic field and velocity measurements. The lake side provides better seeing condition. Because of lake water, there is extremely low water vapor content and this is unaffected by the monsoon.

The NLST project is a major multi-party initiative involving IIA, ISRO, Aryabhatta Research Institute of Observational Sciences, Tata Institute of Fundamental Research (TIFR) and Inter University Center for Astronomy and Astrophysics (IUCAA). The project will involve an investment of over Rs 250 crore, most of which will go in procuring equipment.
(PIB Feature.)

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