The Kodaikanal Solar Observatory (KoSO) has been observing the Sun for over a century.

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• Kodaikanal Solar Observatory (KoSO) has been observing the Sun for over a century
• KoSO has captured images of sunspots and recorded changes in the Sun’s behavior
• Solar physicists at the Indian Institute of Astrophysics (IIA) and Aryabhatta Research Institute of Observational Sciences (ARIES) have digitized 1.48 lakh solar images captured since 1904

A Brief History of Kodaikanal Solar Observatory

• KoSO is one of the world’s oldest observatories studying the Sun.
• Norman Pogson, astronomer and Government Astronomer of the Madras Observatory, proposed the idea of taking pictures of the Sun using a 20-inch telescope.
• The Madras Observatory was set up as the private effort of an official of the British East India Company in 1786.
• The decision to establish a solar observatory was taken in 1893, and Kodaikanal in present-day Tamil Nadu was chosen for its high altitude and dust-free environment.
• The Solar Physics Observatory opened on April 1, 1899, and was later named KoSO.
• The Bhavnagar Telescope, named after the Maharaja of Bhavnagar, was one of the more famous instruments at KoSO during the early decades of its operation.
• A 15cm telescope was used to capture solar images onto a photographic film or plate.
• Solar magnetic plages and prominences were recorded since 1911, taken on photographic films and plates.

Solar Observations, One Every Day: How They Are Taken

• White light images of the Sun have been captured every day since 1904 using a 6-inch telescope
• Visible light images reveal sunspots on the surface of the Sun.
• One image is taken daily around 8 am, which has been a fixed routine for over a century now
• Each observation accompanies the corresponding date and time, which is key for calibration purposes later.
• These plates or films are sent to the darkroom and developed either the same day or the next day
• Once the film has been developed, the date and time of observation are written on the plate and entered in the logbook.
• These plates or films are kept in an envelope with the handwritten date and time of observation and stored carefully in humidity-controlled rooms.

Arrival of New Technology and the Process of Digitization

• Between 1904 and 2017, all solar observations were traced onto photographic films and plates
• A new telescope mounted with CCD cameras has taken over and, since 2017, continued to observe the Sun.
• Digitization of the records was initiated in 1984 by Prof J C Bhattacharyya, and others continued the effort.
• In 2018, digitized solar observations for the period 1921-2011 were made available to the scientific community.
• Raw and calibrated data for the period of 1904 to 2017 were added, and the digitization process is nearly complete.
• KoSO is now home to a digital repository of a whopping 1.48 lakh solar images adding up to 10 terabyte of data.
• These include 33,500 white-light images (showing sunspots), 45,000 images of the Ca II K spectral line (which reveals plages), and 70,000 H-alpha photographic plates that show prominences.

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