Optical, Infrared and Radio Telescope Facilities in India

In the past 2-3 decades a number of ground based telescopes have been established in India. These telescopes cover the electromagnetic spectrum in the optical, infrared and radio wavelength bands. The telescopes have been set up by national laboratories such as PRL, TIFR, IIA, RRI etc., as well as by Osmania University. Indigenous development of sophisticated telescopes within limited resources has been the main pursuit in some of these projects. Major incentive is, however drawn from the ability to conduct world class research in astronomy, astrophysics and cosmology.

In this report a summary of the facilities established and those in the pipeline are mentioned along with some estimate on the funds being utilised for building and operating such facilities. FIGURE-1shows some of the major telescope facilities which are already operational. It may be noted that in some areas like solar observations and stellar metre-wave radio observations India has the best facilities in the world. Particularly the recently established Giant Metrewave Radio Telescope (GMRT) is the largest telescope in the world for its wavelength range.

The Vainu Bappu Observatory (VBO) is the main optical observatory of the Indian Institute of Astrophysics (IIA) and is located in the picturesque neighbourhood of the village of Kavalur in the state of Tamilnadu. Facilities of VBO include the following:

• The Vainu Bappu Telescope (2.3m)

• The Zeiss (1m) Telescope

• The 0.75m Telescope

• The Fabry Perot Interferometer

Technical Details

Available Instruments

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Technical Details

Available Instruments

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2.3 The 0.75 m Telescope

Technical Details

Instruments Available

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2.4 The Fabry-Perot Interferometer

A long-term programme to study the Earth's equatorial thermosphere has been initiated at the VBO, through the deployment of a pressure-scanned Fabry-Perot Interferometer (FPI), for high resolution spectroscopy of the [OI] 630nm airglow emissions from the night sky. The FPI has been built around an optically-contacted etalon of 100mm effective diameter made up of FP plates at 630nm and spaced 10mm apart. A narrow-band (0.3nm) temperature-tuned interference filter is used to isolate the 630 nm emission line.

The Infrared Telescope at Mount Abu is the first major facility in India specifically designed for ground based infrared observations of celestial objects. The telescope is located near the hill resort of Mount Abu in the state of Rajasthan. The observatory is at an altitude of 1680 metres adjacent to Gurushikhar - the highest peak in the whole of central India. Further the low amount of precipitable water vapour (1-2 mm during winter) at Gurushikhar makes it a good site for the infrared telescope observations. The site has been found to be good (about 150 cloud free nights per year) for astronomical obsevations. The specifications for surface finish of the telescope mirrors has been one-tenth of 589 nm line of sodium so that the telescope can be effectively used for imaging studies at both optical and infrared wavelengths. The optical system of the telescope consists of 1.2 m paraboloid primary mirror that can be used in conventional Cassegrain and Coude configurations using hyperboloid secondary mirrors. The standard Cassegrain configuration utilises a non-vibrating secondary mirror with an effective focal ratio of f/13.

For infrared observations, to detect faint objects in the presence of sky brightness gradients a provision for nodding the primary mirror has been made. In addition there is provision for a vibrating secondary mirror with an effective focal ratio of f/45 for modulation of star signal. The second optical configuration consists of f/45 focal ratio Coude system for very heavy focal plane instruments like high resolution spectrographs.

The Indian Astronomical Observatory, the high-altitude station of IIA, is situated at an altitude of 4500 metres above mean sea level to the north of Western Himalayas. Atop Mt. Saraswati in the vast Nilamkhul Plain of Changthang Ladakh region (4250 m above msl) of Jammu & Kashmir State. The site is a dry, cold desert with sparse human population. The cloudless skies and low atmospheric water vapour make it one of the best sites in the world for optical, infrared, sub-millimetre, and millimetre wavelengths.

A 2-m optical-infrared telescope is planned to be installed at the Observatory by the middle of 2000. This telescope will be remotely operated from Hosakote, using dedicated satellite links. A 0.3-m Differential Image Motion Monitor, a 220-GHz radiometer and an Automated Weather Station have been installed to facilitate continuation of site characterisation. The infrastructure developed by IIA at IAO, Hanle, has paved way towards initiating many new projects in astronomy as well as other paradigms of science.

The Kodaikanal Observatory of the Indian Institute of Astrophysics is located in the beautiful Palani range of hills in Southern India. It was established in 1899 as a Solar Physics Obser-vatory and all the activities of the Madras Observatory were shifted to Kodaikanal. The telescope facilities include:

5.1 20 cm Refractor

A 20 cm refractor at the Observatory is used occasionally for cometary and occultation observations. It is also sometimes made available to visitors for night sky viewing.

5.2 Solar Telescope

A Grubb Parson 60 cm diameter two-mirror fused quartz coelostat mounted on 11 m tower platform directs sunlight via a flat mirror into a 60m long underground horizontal `tunnel'. A 38 cm aperture f/90 achromat forms a 34 cm diameter solar image at the focal plane. The telescope has an option to mount a 20 cm achromat which provides an f/90 beam to form a 17 cm image.

A Littrow type spectrograph is the main instrument at the telescope. A 20 cm diameter, 18m focal length achromat in conjunction with a 600 lines/mm grating gives 9 mm/A dispersion in the fifth order of the grating. Together with the 5.5 arcsec/mm spatial resolution of the image, it forms a high resolution set up for solar spectroscopy. Recording of the spectrum can be done photographically or with a Photometrix 1k x 1k CCD system.

The converging solar beam from the objective can be diverted to a high dispersion spectroheliograph with Littrow arrangement using a 3.43 m achromat.

5.3 Full Disc Imaging

A 15 cm aperture English mounted refractor by Lerebour and Secretan, acquired in 1850 and remodelled by Grubb in 1898 to serve as a photo heliograph, is in use since the beginning of this century to obtain 20 cm white light pictures of the Sun on a daily basis, sky permitting. Twin spectroheliographs giving 6 cm diameter full disc photographs of the Sun in Calcium K and hydrogen alpha lines are in regular use. A 46 cm diameter Foucault siderostat feeds light to a 30 cm aperture f/22, Cooke triplet lens. The two prism K spectroheliograph was acquired in 1904 and the H alpha grating spectroheliograph was operational in 1911. Since 1912, prominence pictures over the full limb are also being obtained in K by blocking the solar disc. These observations and the white light pictures are obtained around 200 days a year. Light from the 46 cm siderostat is diverted to a 15 cm Zeiss achromat objective which provides an f/15 beam and a 2 cm image. A prefilter and a daystar Ca K narrow band filter are used together with a Photometrix 1k x 1k CCD to record the K filtergram.

UPSO has six telescopes of apertures 15, 25, 38, 52, 56 and 104 cms located at Manora Peak. Nainital (1951 m above msl). Of these the 25 cm one is an off-axis telescope with an associated spectrograph and a Ha patrol unit and used exclusively for solar observations. The last four telescopes are regularly used for stellar observations

7. Udaipur Solar Observatory, USO (PRL)

Udaipur Solar Observatory is situated on an island in the middle of the Fatehsagar lake (24.5 N, 74 E). The sky conditions at Udaipur are quite favourable for solar observations. Since the observatory is situated amidst a large mass of water, air turbulence which occur due to ground heating by sun's rays is decreased. This improves the image quality and seeing (average between 1-2 arc seconds). The main objective of obtaining the high spatial and temporal resolution observations of solar photospheric and chromospheric activity is to understand the various dynamic phenomena occurring on the surface of the sun. The USO has a 12¢ solar spar telescope operational since 1975 for observations of solar eruptions in white light and in Ha line.

At Japal-Rangapur Observatory (695 m asl) near Hyderabad a 48² telescope was commissioned in 1968. It can be used as a Newtinian system, a folded Cassegrainan (Nesymth) system, a Coude system or a Baker System. It also has an 8² Astrograph and a Meinel spectrograph. 

NCRA has set up a unique facility for radio astronomical research using the metre-wavelength range of the radio spectrum. Known as the Giant Metrewave Radio Telescope (GMRT), it is located at a site about 80 km north of Pune. GMRT consists of 30 fully steerable parabolic dishes of 45m diameter each spread over distances of upto 25 km. GMRT is one of the most challenging experimental programmes in basic sciences undertaken by Indian scientists and engineers. Recently established GMRT is the world’s most powerful radio telescope operating in the frequency range of about 50 to 1500 MHz.

The metre wavelength part of the radio spectrum has been particularly chosen for study with GMRT because man-made radio interference is considerably lower in this part of the spectrum in India. Although there are many outstanding astrophysics problems which are best studied at metre wavelengths, there has, so far, been no large facility anywhere in the world to exploit this part of the spectrum for astrophysical research.

The site for GMRT, about 10 km east of Narayangaon town on the Pune-Nasik highway, was selected after an extensive search in many parts of India, considering several important criteria such as low man-made radio noise, availability of good communication, vicinity of industrial, educational and other infrastructure and a geographical latitude sufficiently north of the geomagnetic equator in order to have a reasonably quiet ionosphere and yet be able to observe a good part of the southern sky as well.

GMRT is an indigenous project. The construction of 30 large dishes at a relatively small cost has been possible due to an important technological breakthrough achieved by Indian Scientists and Engineers in the design of light-weight, low-cost dishes. The design is based on what is being called the ‘SMART’ concept i.e., Stretch Mesh Attached to Rope Trusses.

The ORT is operated by the NCRA, Pune and the Radio Astronomy Centre (RAC), Ooty both of which are sub-groups of the TIFR, Bombay. The ORT was built in the early '70s by RAC. The ORT is a 530 m x 30 m parabolic cylinder which operates in the frequency band 320-335 MHz. Its effective collecting area is about 8500 m² and became operational in 1970. The long axis of the parabolic cylinder is made parallel to the axis of earth's rotation by locating the telescope on a hill slope with an inclination of 11° which is equal to the latitude of Ooty. By rotating the cylinder about this axis, the ORT can track any astronomical source in the declination range -40 to +40° for about 9 hours. The telescope was conceived, designed and fabricated fully indigenously. It is uniquely suited for making studies of the sizes and structures of radio sources by the method of lunar occultation.

The RRI and the IIA in a joint collaborative programme operate this decametre-wave telescope located at Gauribidanur (13.5° N and 77.5 ° E) about 100 km from Bangalore. This telescope operating at 34.5 MHz, is essentially a meridian-transit instrument although a limited tracking capability is available. The telescope consists of 1000 dipoles arranged in the form of the letter `T'. The dipole orientation being along the east-west direction the instrument is sensitive only to the east-west component of the polarisation. The usable bandwidth is about 10 MHz centred at 32 MHz. The maximum available effective collecting area is about 18000 m². The beam can be steered electronically in the N-S direction by ± 45° .

This telescope, which became operational in 1979 has been used for continuum survey of the accessible sky and for studies of supernova remnants, giant HII regions, as well as of the radio emission from the undisturbed Sun and solar bursts. Using the tracking facility, observations of low-frequency radio recombination lines and emission from many nearby pulsars have been made. Currently, pulsar observations with a new and more sensitive receiver are in progress.

RRI has developed this 10m diameter dish telescope for operation down to 1mm wavelength. High resolution molecular spectroscopy to study space molecules is being pursued at the facility.

REFERENCES

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