Gamma-ray astronomy gives a perspective of the non-thermal universe which complements panoramic views from other spectral windows. The subjects of very high energy gamma-rays are the most energetic objects and phenomena in the Universe. Gamma ray pulsars, supernova remnants and active galactic nuclei are some of the major sources of high energy gamma rays. These powerful cosmic sources and violent events are studied in order to understand nature at its ultimate limits.
High energy gamma-ray astronomy can be carried out from ground using a technique called atmospheric Cherenkov light. When the gamma-rays hit the atmosphere, they generate a cascade of fast electrons and positrons, called extensive air showers. These charged fast particles cause the emission of Cherenkov light as they travel down the atmosphere. This feeble Cherenkov light can be detected on the ground on clear moonless nights using large area mirrors.
TIFR established an array of 24 telescopes (called PACT) for gamma-ray astronomy using the atmospheric Cherenkov technique was established at Pachmarhi. Another array of 7 telescopes (called HAGAR) was recently built at Hanle in Ladakh at an altitude of 4300 m. This project, started as a TIFR-IIA collaboration currently includes groups from BARC, IIA and TIFR. A telescope placed at high altitude helps in reducing the energy threshold of the detectable gamma-rays. HAGAR will observe the energy region of about a 100 GeV or more, while the energy threshold of PACT is about 850 GeV. The telescopes used in these arrays were designed and fabricated indigenously.
More details about HAGAR, HEGRO can be obtained from these links.