The solar tower telescope, as are the others at the Kodaikanal Observatory, is on an equatorial mount. Under this system, one of its axes—the polar axis—is tilted to compensate for the latitude of its location. The angle between the ground and the elevated telescope equals the latitude of the telescope’s location. The telescope moves in the same east-to-west direction as do the stars. It provides a high-resolution solar image, together with spectrographs.
A solar telescope does not need huge mirrors as the Sun is quite bright. It needs magnification and hence the focal length should be higher. Moreover, a solar telescope itself does not move but has a tracking mirror to reflect the light into the tube. Accordingly, the solar tower telescope in Kodaikanal continuously follows the Sun as the Earth rotates. Three identical mirrors reflect the captured light by turn in what known as a coelostat, thereby avoiding image rotation. The light is reflected by the third mirror, kept at an angle of 45 degrees to an object lens (38 cm), which produces a 34 cm image at its focal point, 36 m away in a 60 meter long tunnel. The extended focal length is needed to get a larger image of 340 mm.
One millimeter of the image corresponds to 4,000 km on the solar surface. This is the best possible resolution one can get at Kodaikanal. The light is then directed to enter a spectrograph, which disperses it. The spectrum is obtained right below the solar image in the image plane so that an observer can watch both the solar disc and the spectrum from one place. The arrangement of the spectrograph in this manner is called Littrow mounting. The spectrograph uses a plane reflection grating which has 600 lines per millimeter. The spectrum can be registered in a CCD or a photographic plate.
In 1971 the Indian Institute of Astrophysics was established in Bangalore and Bappu moved the headquarters of the Kodaikanal Observatory to the new institute. He chose Kavalur in Tamil Nadu for stellar astronomy in view of the better sky conditions there for night-time observations. He obtained the integrated light of the Sun in the Ca-K line on a daily basis using the solar tower tunnel telescope in Kodaikanal and compared the results with the Sun-like stars observed at Kavalur.
Observations from Kodaikanal
It is interesting to recall that it was in Kodaikanal that in 1934, oxygen lines were detected in the emission spectrum from the solar chromosphere, when there was no eclipse. The important research programmes completed in Kodaikanal include: high-resolution spectroscopic studies on the pattern of outflow of gases in Sunspot filaments; study of the minimum temperature region of the solar atmosphere; five-minute oscillations in the photosphere; and shorter oscillation (5 to 90 seconds) in the corona; solar eclipse observations that lead to clues on the physics of the solar corona; mapping of weak magnetic fields, movement of such fields towards the poles during the solar minimum period, and studies on solar flares from the optical observations.
A spectro-polarimeter, developed and installed by the IIAP at Kodaikanal, gathers information on the state of ionization of individual chemical reactions in the Sun. The reactions are seen as lines in the solar spectrum. Information on the solar magnetic fields is gained from a complete description of the polarized light given off by the device. For this purpose, the incoming light is split into two beams and recorded in as many detectors.
Studies conducted over 15 years at the Kodaikanal Observatory indicate twists in the solar magnetic field, caused by the shear motions in Sunspots. The twisted magnetic fields lead to violent outbursts. The Sun seems to behave like a giant magnet, and coronal mass ejections erupt when loops of solar material lifting off the surface suddenly snap, hurling plasma into space. Plasma is formed of electrons and ions of hydrogen and helium. A coronal mass ejection (CME) typically has a billion tones of matter, speeding at 400 km a second. However, many of the CMEs move away from Earth.
Another interesting area of study that is emerging is known as Helioseismology. It is now known that the Sun oscillates in certain frequencies that can be observed on the surface. The frequencies are measured routinely by interested teams with great precision. What is described as five-minute oscillations are acoustic waves trapped below the solar photosphere.
IIA has initiated a major project to digitize the photographic images of the Sun taken in white light, calcium and H-alpha lines since 1906 at the Kodaikanal observatory. State-of-the-art digitisers have been developed for the programme. Regular work started in March 2008. The facilities include precision lens and CCD cameras cooled to a temperature of minus 100 degrees C. the data would be used to study long-term variations in the size of the Sun, photospheric and chromopheric rotation rates and their variation overtime. The data will be made available to scientists for research.
Prior to the use of CCDs, it would take ‘days’ to get profiles of the spectral lines from the photographic format, but now CCDs make it convenient to digitize and obtain the profiles instantly. In the pre-computer days, it would take 12 seconds to register the ionized calcium line, whereas in the digital format today, it can be done twice or thrice in just one second. The new CCD-based digitization will be done thoroughly so that maximum scientific data are extracted from each and every plate. It is not just a sample survey. The uninterrupted data over one century are expected emphasis nowadays on the impact of the heliosphere on the Earth. The change in the recording medium makes the observations speedily available for analysis. The long stretch of data on the Sun including Sunspots has been most useful in defining solar rotation and the migration of the spots during the solar cycle. The digitized data would be put on IIA’s website, open to any scientist or students for research studies.
The location of Kodaikanal at just one degree north of the Earth’s magnetic equator is useful for observing interesting phenomena in the ionosphere—the radio mirror in the sky—such as the equatorial electro jet. In 1955, the Observatory acquired an instrument for vertical soundings of the ionosphere. In 1993 a digital Ionosonde was commissioned. The data collected are the longest series of its kind in the country.
Today in Kodaikanal, a museum at the observatory has many interesting exhibits including some wood samples showing the effects of carbon-14 on trees, which would give an idea of solar activity and Sunspots before the age of the telescope. It is now known that carbon-14 is formed in the Earth’s atmosphere when cosmic rays are able to interact with it in times of solar minimum.
A rich legacy of a fruitful era in Indian astronomy in recent times overwhelms the visitor to the Kodaikanal Observatory. A “mute witness” to this extraordinary record is a pendulum clock preserved at the Observatory is a pendulum clock preserved at the Observatory (reportedly made by one John Shelton of England), similar to the one used by Captain Cook in his travels, still ticking!