Evolution of the Astrosat idea

Published : Sep 16, 2015 12:30 IST

THE saga of the evolution and realisation of Astrosat is worth recounting in some detail. According to P.C. Agrawal, former TIFR scientist and the main person behind the development of the Astrosat instrument Large Area X-Ray Proportional Counters (LAXPC), after the success of the first X-ray astronomy satellite experiment, called the Indian X-ray Astronomy Experiment (IXAE), many people, including Satish Dhawan, the main architect of the Indian space programme, apparently felt that it was time ISRO started thinking of bigger space astronomy experiments. The IXAE was a piggy-back payload aboard the Indian Remote Sensing Satellite IRS-P3, launched by the PSLV in March 1996.

In the same month, Agrawal drew up a proposal to the TIFR titled “X-ray astronomy instrument for Indian satellite to survey in hard X-ray band and for spectral and temporal studies of sources in 2-80 keV”. This proposal aimed at enhancing the capability of the gas-filled proportional counter-based detector that successfully performed in IXAE but covering a larger area with better resolution. The then TIFR director, V. Singh, sent it to ISRO for consideration. Agrawal then spoke to K. Kasturirangan, the then ISRO Chairman, about his ambitious proposal. Apparently, Kasturirangan enthusiastically supported the idea and suggested conducting a workshop at the ISRO headquarters in Bengaluru to discuss the scope of the mission. In fact, Kasturirangan seems to have actively supported and driven the project right from the beginning.

In his own words, “Astrosat as a major mission for astronomy was prompted by ISRO and came from the group of scientists from ISRO/ISAC, TIFR, IIA and later IUCAA,” Kasturirangan said in an email response to this correspondent. “This was driven to the level of feasibility by ISRO, which provided an early grant of Rs.45 crore for the development of complex instruments to the different laboratories. Since the entire conceptualisation, planning/definition and a good amount of development of the mission was done during the time I was Chairman of ISRO, and being also a person actively involved in X-Ray astronomy research, I could bring my own knowledge to bear on the conceptualisation and progress of the design and development of this satellite,” said Kasturirangan.

At the two-day workshop that was hosted by ISRO on July 30-31, 1996, other people were also asked to make proposals. According to Agrawal, Kameshwar Rao, former scientist at the IIA, made a proposal to include a UV telescope, which had the backing of the then IIA Director Ramnath Cowsik. K.P. Singh of the TIFR made a proposal for a soft X-ray telescope (SXT). There were other proposals for an infrared detector and a hard X-ray detector, which were apparently not accepted. “What was felt at the end of the meeting was that proportional counter-based instrument should be there because whatever mission we undertake, the major instrument should have proven technology and should be able to make it. Then we should also have some new technology-based instrument for the future. LAXPC was chosen because we were confident that we can make it after the IXAE success.”

In X-ray astronomy, timing and spotting (source location) studies, which LAXPC is designed to do, are very important, said Agrawal. “At that time, NASA was about to launch its RXTE mission, which turned out to be highly successful. We thought we can improve upon the NASA instrument by making bigger and deeper detectors. The LAXPC’s spectral response went up to 80 keV instead of 20 keV of RXTE, which worked mostly in 2-20 keV band because it has a shallow counter,” Agrawal said.

Cross section for absorption of X-rays is energy dependent and goes as inverse cube of the X-ray energy. As you go to harder X-rays, or higher energies, the probability of absorption decreases. So if you have greater depth, the probability increases. As against the 9 cm-deep detection volume in RXTE, the depth of each of the three LAXPCs in Astrosat was planned to be 15 cm. Also, at that time, RXTE had the largest effective photon collection area of 6,500 cm. So it was decided then that the effective area of LAXPC would be more than that. In the final satellite configuration, LAXPC had an effective area of about 8,000 cm at 5-20 keV X-ray energy.

Following the July 1996 workshop, Kasturirangan constituted two working groups in August 1996, with Agrawal and Koteswara Rao as chairmen. Kasturirangan’s official order had also suggested that after individual deliberations, the working groups should hold discussions to define the scope of the mission. According to Agrawal, the discussions began in December 1996 and went on for nearly two years. In January 1997, the working group recommendations were sent to experts to ensure that the whole astronomy community in the country was involved. Following this, a report made to the Chairman, ISRO, in 1998, recommended that three instruments be selected for the first mission—the LAXPC, the SXT and a new technology instrument to be developed in the country for future imaging studies.

The report also recommended that it should not be a purely X-ray oriented mission but should have a broader wavelength coverage. Also, considering that UV was an area not targeted by many missions except the successful International UV Explorer (IUE) mission, which worked for 18 years until 1996 in geostationary orbit, the report suggested that a UV telescope could be the third instrument. “We thought that if we include UV astronomy, it would broaden the horizon, extending the energy coverage from a few eV to 80 keV, nearly five orders of magnitude,” Agrawal said. “Also, a UV telescope is closer to optical astronomy in terms of techniques, data analysis and so on. So it was an opportunity for Indian astronomers in optical astronomy also to participate,” he added. “To this, the Scanning Sky Monitor (SSM) was added, basically an X-ray sky monitor to look for variable sources and alert the other instruments for observing them over the broad wavelength band.” A similar all-sky X-ray monitor had been part of RXTE as well.

On the basis of the report’s recommendations, Kasturirangan suggested that Agrawal write a formal proposal. Agrawal began to write the proposal in 1998, but it had to be kept in abeyance for nearly a year when he was asked to assume the directorship of the National Centre for Radio Astrophysics (NCRA) of the TIFR in Pune, after the demise of its director V.K. Kapahi. The proposal was finally submitted in 2000. Kasturirangan had the proposal reviewed by senior astronomers of the country and a meeting was called on September 19 and 20, 2000, at the TIFR to discuss the proposal. So, it has taken exactly 15 years since the formal proposal for the Astrosat project was made and deliberated upon for it to be realised.

The TIFR meeting was attended by all the leading lights of astronomy in the country, which finally recommended that the project be taken up as it would be good for science and the future of astronomy in India. Given the work that has been carried out in astronomy groups across the country towards realising the satellite and its instruments, and the science that it promises, it would seem that a successful Astrosat launch would indeed be good for science and astronomy in the country.

In 2001, a small Astrosat Configuration Committee was constituted by the then ISAC Director P.S. Goel to arrive at the payload specifications, satellite bus definition, etc. The committee had several rounds of discussions and decided on the final configurations of the payloads and the satellite. In 2002, pending the approval of the project by the Space Commission and the Cabinet, the ISRO Council provided some seed funding for the development of the payloads. In 2003, the Committee appointed Koteswara Rao as the project director. A project report was also submitted to the Space Commission at that time, which gave its approval to the project. In 2004, soon after the United Progressive Alliance (UPA) government assumed power, the Cabinet gave the go-ahead and sanctioned the projected cost of about Rs.178 crore. It was one of the first decisions of the UPA government. It then became a formal ISRO project. “That was the beginning and we targeted a launch around 2008-09 at that time. However, because it kept evolving as we began to understand the technology difficulties, we are ready for launch only now, in September 2015,” said Koteswara Rao.

R. Ramachandran

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