M4L5: Ground based telescopes

Okay, so the last lecture was all about telescopes in Space. Can we install such telescopes on ground?

The atmosphere of the Earth is entirely opaque to radiation at UV, X-ray, or gamma-ray wavelengths (which is a good thing for human vitality). In order to directly detect such radiation, which comes from a celestial object, one must therefore place the detector above the atmosphere. This include sounding rockets or telescopic satellites.

NASA has recently launched a Focusing Optics X-ray Solar Imager, or FOXSI, a sounding rocket mission to stare directly at the Sun and search for nanoflares that erupt from the surface of the sun. These nanoflares are miniature explosions are invisible to the naked eye. FOXSI uses hard X ray telescope. NASA plans to include soft X ray telescope, in future FOXSI mission. FOXSI rockets are smaller, cheaper and faster to develop than large-scale satellite missions. NASA is using this technique for the second time.

Gamma rays (and their relatives in the X-ray and UV regimes) cannot be observed directly from the ground. After travelling for millions or billions of years through the empty reaches of space, when they reach the Earth's atmosphere, they quickly collide with atoms and molecules -- BAM! Bye-bye, gamma ray. However, when this Gamma ray, disintegrates, it creates a charged particle and a photon. The frequency of these photons is usually under UV band or faint blue color band. Thus, Cherenkov radiation is faint bluish in color. Now since, these showers of gamma radiation give birth to other charged particles, like electrons, positrons and neutrinos, scientists use this Cherenkov radiation in experimental particle physics to detect and identify other particles like muons and neutrinos. This technique is used to make neutrino detectors and is used by Japan to make a Super Kamiokande neutrino detector

When the Cherenkov radiation generated by these charged particles is traced backwards, it helps to determine the source and intensity of the cosmic or gamma rays that was responsible for giving birth to Cherenkov radiation. So, can we use it to make telescopes?

A similar kind of experiment is also happening in Antarctica. The name of that experiment is ICE CUBE NEUTRINO LABORATORY

The Tata Institute of Fundamental Research (TIFR) and Bhabha Atomic Research Centre (BARC) are developing a new gamma ray telescope. This telescope will be located near the High-Altitude Gamma Ray (HAGAR) array at Hanle in Ladakh which houses the Indian Astronomical Observatory operated by Indian Institute of Astrophysics, Bengaluru.

It is designed, developed and manufactured by the Electronics Corporation of India Limited (ECIL), Hyderabad and weighs about 180 tonnes.

The imaging camera to be used in the telescope is different from convention ones. It used pixels made of what are called Geiger-mode avalanche photo-diodes (G-APDs), while conventional telescope cameras are based on photomultiplier tubes (PMTs).

It will work in tandem with MACE (Major Atmospheric Cerenkov Experiment) - a 21-metre diameter gamma ray imaging telescope which is also under installation at Hanle. While MACE will operate in discovery mode looking at candidate sources of gamma ray or very faint objects, the new telescope will keep an eye on blazars. Whenever there is any flaring activity in any of them, it will alert MACE which will then shift its focus to the active blazer.

The Indian Astronomical Observatory has also installed the high energy gamma ray telescope (HAGAR) in Ladakh. There are total 7 telescopes, each with 7 mirrors. MACE telescope is installed next to HAGAR as seen from the picture.