ASTROSAT, India’s unique space observatory

Till date, Indian astronomers had to rely on international resources for X-ray and ultraviolet data. But that is all set to change.

September 27, 2015 05:00 pm | Updated November 16, 2021 04:11 pm IST

The payload mass is more than 60 per cent the ASTROSAT's mass. Photo: Special arrangement

The payload mass is more than 60 per cent the ASTROSAT's mass. Photo: Special arrangement

The launch of Indian Space Research Organisation’s (ISRO) ASTROSAT telescope today (September 28) will be a shot in the arm for astronomers, particularly those in India. This is the first time India is launching a space observatory.

But that is not the only reason why the ASTROSAT telescope is so special. Unlike most other telescopes, the five instruments (payloads) of ASTROSAT can observe a wider variety of wavelengths — from visible light to the ultraviolet and X-ray bands. Even in the X-ray band, it can study both low and high energy X-ray regions of the electromagnetic spectrum. Most other satellites are capable of observing only a narrow range of wavelength band.

“The capability to cover the full spectrum of wavelength simultaneously is the unique feature of ASTROSAT,” said Dr. Mylswamy Annadurai, Director of ISRO Satellite Centre in Bengaluru.

“ASTROSAT is not the first of its kind but is the best so far. It is the best all rounder in the world. It is a one-stop shop for studying astronomical sources,” said Dr. Varun Bhalerao, Post Doctoral Fellow at the Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA).

“Astronomical sources change in all time scales. So if I were to take data in optical light one day and X-ray the next day from different telescopes, each day I will be seeing something different. So can’t put the picture together. To understand all of that I must see the object in all different bands of light at the same time. The ASTROSAT telescope will allow me to do this. That is its uniqueness,” said Dr. Bhalerao.

India does have ground-based telescopes (including the Giant Metrewave Radio Telescope near Pune and the Indian Astronomical Observatory in Ladakh). But like all other ground-based telescopes, these can only detect radio waves and infrared radiation as they penetrate the Earth’s atmosphere. However, in the case of higher frequency radiations, the atmosphere tends to block most ultraviolet light and all X-rays and gamma-rays.

“The atmosphere blocks most UV light and all X-ray from the Sun. But for other stars which are very far away, the intensity of UV light and X-ray is not much and the atmosphere completely blocks all UV light and X-ray,” Dr. Bhalerao said.

Hence, a space-based observatory like ASTROSAT will be of immense value to researchers based in India. “Ground-based telescopes and the space observatory will complement each other,” Dr. Bhalerao said.

Till date, Indian astronomers had to rely on international resources for X-ray and ultraviolet data. “Without a space telescope of their own, Indian scientists have had to rely on ones operated by NASA and the European Space Agency (ESA) to study such radiation bands, which carry information about exotic neutron stars, newly born or exploding stars and the spiralling hot gases around black holes,” notes Nature.

But that is all set to change. “For the first time, we will be getting data from our own Indian X-ray and ultraviolet telescope. That makes a lot of difference,” Dr. Bhalerao said. “We need not have to go to NASA and other agencies. But we will continue to collaborate.”

It is for the first time that a majority of instruments (payloads) of the ASTROSAT had come from outside ISRO. “The combined mass of the payloads is more than the mass of the satellite,” said Dr. Annadurai. “At 850 kg, the payload mass is more than 60 per cent of the mass of the satellite.”

Generally, the payload mass is less than 10 per cent of the mass of the satellite, like in the case of Chandrayaan-1. It was less in the case of Mars Orbiter Mission Mangalyaan. “Because of the lower orbit, ASTROSAT can afford to have heavier payloads,” Dr. Annadurai explained. Though designed to orbit at 650 km above the Earth for five years, there is great likelihood that like most other telescopes, ASTROSAT too would last much longer.

Scientific objectives

The scientific objectives of ASTROSAT mission are to (1) understand high energy processes in binary star systems containing neutron stars and black holes, (2) estimate magnetic fields of neutron stars, (3) study star birth regions and high energy processes in star systems lying beyond our galaxy, (4) detect new briefly bright X-ray sources in the sky and (5) perform a limited deep field survey of the Universe in the ultraviolet region.

Explaining how the magnetic field of neutron stars will be measured, Dr. Bhalerao said: “The frequency with which electrons spiral around a magnetic field depends on the strength of the magnetic field. Whatever frequency they are spiralling they scatter light at that frequency. So in the case of neutron stars, the frequency of electron spiralling matches high-energy X-ray light.”

Dr. Bhalerao has been studying neutron stars using high-energy X-ray wavelengths with NASA’s Nuclear Spectroscopic Array (NuSTAR) satellite at the California Institute of Technology in Pasadena. He was part of the team that built the NuSTAR satellite. “To understand what is happening on neutron star, must study in both low- and high-energy X-ray,” Dr. Bhalerao said. “With NuSTAR, you need to pair up with other satellites that study lower energy X-ray. Each satellite has its own time allotment issue. Though the same process of time allotment will happen with ASTROSAT, we will get data from all energies from a single satellite.”

According to ISRO, to fulfil these objectives the ASTROSAT carries the following five payloads.

(1) The Ultraviolet Imaging Telescope (UVIT, capable of observing the sky in the Visible, Near Ultraviolet and Far Ultraviolet regions of the electromagnetic spectrum.

(2) Large Area X-ray Proportional Counter (LAXPC, is designed for study the variations in the emission of X-rays from sources like X-ray binaries, Active Galactic Nuclei and other cosmic sources.

(3) Soft X-ray Telescope (SXT) is designed for studying how the X-ray spectrum of 0.3-8 keV range coming from distant celestial bodies varies with time.

(4) Cadmium Zinc Telluride Imager (CZTI), functioning in the X-ray region, extends the capability of the satellite to sense X-rays of high energy in 10-100 keV range.

(5) Scanning Sky Monitor (SSM) is intended to scan the sky for long term monitoring of bright X-ray sources in binary stars, and for the detection and location of sources that become bright in X-rays for a short duration of time. According to Dr. Bhalerao, LAXPC is the best X-ray timing instrument so far. “Astronomical objects cannot be controlled. If you want to study something in a star, must catch it in its act. So it is important to monitor the sky,” he said about the Scanning Sky Monitor.

“For some researchers, the satellite’s X-ray detection capability will fill the gap left when NASA’s Rossi X-ray Timing Explorer satellite died in 2012, after 16 years of operations,” notes Nature. “ASTROSAT’s X-ray detectors can also cope with very bright objects that would saturate those on other satellites such as NASA’s Chandra X-ray Observatory or ESA’s X-ray Multi-Mirror (XXM-Newton) mission.”

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