Astrosat

Giant leap for ISRO

Print edition : October 02, 2015

A.S. Kiran Kumar. Photo: K. Murali Kumar

The solar panel deployment test with the flight-ready satellite. Photo: ISRO

A.S. Kiran Kumar, Chairman, ISRO, on why Astrosat, India’s astronomical satellite to study objects in the deep sky, is a unique and challenging project that will fulfil the long-felt needs of the scientific community.

A.S. KIRAN KUMAR, Chairman, Indian Space Research Organisation (ISRO), and Secretary, Department of Space, is a calm and soft-spoken person. Frontline met him in his office on August 2, 2015, at the ISRO headquarters in Bengaluru, to discuss the coming Astrosat mission. A Polar Satellite Launch Vehicle-XL is to put Astrosat into orbit from the Satish Dhawan Space Centre, Sriharikota on September 28, 2015. “We are now targeting Astrosat. It has an X-ray telescope, far UV [ultraviolet] telescope and so on. Astrosat is a unique satellite because it will make observations in different wavelengths from a common platform,” said Kiran Kumar.

Astrosat is India’s astronomical satellite with telescopes, meant for studying objects in the deep sky. It can make observations in ultraviolet, optical, visible, low and X-ray wavelengths simultaneously. It will study stars, quasars, pulsars, supernova remnants, black holes and active galactic nuclei.

Kiran Kumar is an accomplished space scientist and engineer with a four-decade-old career in ISRO in the satellite payloads and applications domain. Before he took over as ISRO Chairman on January 14, 2015, he was Director, Space Applications Centre (SAC), ISRO, Ahmedabad. He was instrumental in evolving the successful strategy for steering India’s Mars spacecraft to the red planet and its insertion into the Martian orbit on September 24, 2014. He made important contributions to India’s space programme by building satellites for earth observation (remote-sensing), communication, navigation and forecasting weather, and the missions to the Moon (Chandrayaan-1) and Mars. He evolved the earth observation strategy for ISRO’s satellites, encompassing land, ocean and atmosphere. This has yielded enormous amount of valuable information on the health of crops, the onset of drought, mapping of wasteland, destruction of forests, salinity in the soil, the melting of snow in the Himalayas, glaciers, water resources, schools of fish in the sea, etc.

Kiran Kumar obtained his Physics (Honours) degree in 1971 and his master’s degree in Electronics in 1973, both from Bangalore University. He received his M.Tech. degree in Physical Engineering from the Indian Institute of Science, Bengaluru, in 1975. Excerpts from the interview:

How did the idea of building the Astrosat take shape?

This happened during Dr Kasturirangan’s [former ISRO Chairman] time and earlier. ISRO, right from the beginning, has been incorporating scientific missions and providing opportunities to various institutions in the country to have their own platforms. It is in this context that work was started by the IIA (Indian Institute of Astrophysics), Bengaluru, the IUCAA [Inter-University Centre for Astronomy and Astrophysics], Pune, the Tata Institute of Fundamental Research (TIFR), Mumbai, and others. This is an excellent opportunity for teams in these institutions to build their instruments. That way, it is a good thing that TIFR has been able to build the soft X-ray telescope.

Payloads in Astrosats are all heavy payloads. They weigh hundreds of kilogrammes. So the whole process of generating enthusiasm and giving an opportunity to institutions to put an observatory in space has been at the backdrop of ISRO’s [Astrosat] mission right from the beginning. Professor U.R. Rao and Dr. Kasturirangan were the stalwarts of the earlier scientific missions.

In what way are space-based telescopes better than ground-based telescopes?

The atmosphere above the earth is not stationary. It is dynamic. It is changing. As a result, whatever light and wavelength comes through, it gets collected at the ground telescope and images get formed. They are subject to changes. This results in what is called the “seeing effect”. So, your ability to improve the resolution gets affected. Once you go outside the atmosphere, it is just vacuum. There is no intervening medium that corrupts the path of the light or wavelength that comes through. As a result, you get a much better resolution, both in terms of temporal and spatial resolution. It is like haziness. Suppose you have an atmosphere which is hazy. Whatever picture you take will be affected by the intervening medium. On ground-based telescopes, you have this difficulty. So, in space, even a telescope with a smaller diameter is equivalent to a bigger one [on the ground]. In terms of atmospheric correction, it is always better in space.

Of course, there are many technologies such as adaptive optics which are used to improve [the resolution of ground-based telescopes]. Yet, they are additional efforts. But space gives you the opportunity to look at the objects of interest for longer periods of time because most of the starlight is of very low intensity. So detectors have the capacity to collect [light] during long periods of integration. As the time increases, the variability in the atmosphere also changes. That, in turn, impacts the quality of the images and the data that you get. So space has an advantage.

At what stage was the decision taken to extend the coverage to ultraviolet and visible?

It was a planned activity right from the beginning, and also even in terms of technology we need to get into this.

What preparations are under way for the launch of Astrosat from Sriharikota?

Right now, the satellite is in Sriharikota. It has gone through post-transportation checks. We are targeting the launch itself for September 28. So all the preparations are going on; the launch vehicle stacking and the various phase checks are proceeding smoothly.

Is there a Plan B in case the weather conditions are not conducive?

Generally, in September, there is not much of a problem. The only conditions under which we will be forced to make changes are thunderstorms or high wind velocities in the higher reaches. These are the two conditions in which there will be a shift of the launch date. Otherwise, we do not foresee any problem. In previous launches, weather conditions have not resulted in the change of the launch date.

What are the particular tests that the satellite is undergoing now?

Before the satellite was transported from Bengaluru to Sriharikota, it had completed thermo-vacuum, vibration, and acoustic and post-acoustic tests. Certain deployment tests were also done here. It was transported in a special container to Sriharikota. After it reached the Sriharikota Range (SHAR), it has undergone different tests.

One different aspect of this satellite compared with other satellites is that it has got a far ultra-violet (FUV) wavelength telescope, which requires very stringent control of contamination. Due to contamination, the performance of the telescope could get affected. That is why these telescopes also have what we call as “lids”. These lids cover the telescope’s front aperture, that is, the entrance to the telescope. And in the place where they are opened and the place where they are tested—what we call SP1 and SP2—the maintenance of cleanliness, ensuring [minimum levels of] particle fallout and contamination levels etc. are very important. Because of this, a lot of preparatory work is required in this satellite compared to other satellites.

Right now, at the satellite level, all the tests have been completed. We do not do very elaborate tests on the payloads themselves at Sriharikota. At the same time, some calibration sources and some illumination patterns are available. The tests using them have demonstrated that everything is satisfactory.

Will you do another round of calibration at Sriharikota?

No. We do not do any calibration there, but we make sure that the functionality [of the payloads] is there. These being soft X-rays and photon counting type of detectors, whatever is available in the atmosphere also get detected. They give an indication of the performance of the telescopes.

What about the effect of ‘outgassing’ from the satellite and instrument parts?

That is not an issue on the ground. Outgassing from the surroundings [of the instruments] takes place once you put the satellite into orbit and you get into vacuum conditions. So, if there is an object which is at a higher temperature and it is in vacuum, it releases whatever material it has absorbed and that will get deposited on the colder parts. We ensure that there are no such objects in the line-of-sight [of the telescope mirrors]. Anyway, since we have lids, the mirrors get protected.

During the period when the satellite is mated with the launch vehicle and it is standing at the launch site [pad], even the air circulation, which is controlled to maintain the temperature and humidity, is subjected to a high level of contamination removal, using special filters. We keep monitoring it continuously using small samples for particle fallout and other contamination. So we have a certain level which is permissible. So far, we have been able to make sure that it is much better than the requirement.

In this particular case, the instruments being sensitive, are the requirements very stringent?

The X-ray [telescopes] and other things, in this sense, are not sensitive. Only the far UV telescope is prone to contamination. Also, any reflecting surface has a lot of scattering.

If you take a photograph with a lens which has a lot of scattering, the background is very poor and the contrast comes down. As we go to smaller and smaller wavelengths, the effect of scattering increases. In fact, even for the far UV, the mirror which had to be developed required less than one angstrom type of roughness, and this required special, super-polishing. So the [far UV] telescope realisation had its quota of developing new technologies, polishing techniques, etc. It was quite a challenging task because most of the time we have been doing observations in the visible, infrared and higher wavelengths. This is the first time we are going up to far UV. So this is where one has to take special precautions.

I believe that LEOS (Laboratory for Electro-Optic Systems, ISRO, Bengaluru) has done a lot of work…

Yes. LEOS was responsible for making the telescopes and the assemblies. The fabrication and technology work was done at LEOS.

You have not previously flown UV telescopes?

This will be the first time we are doing it.

So what special challenges did you face?

Much of the detection etc. had to be done in vacuum and so calibration also needed to be done in vacuum. Beyond that, we had to make sure that contamination control was well handled. Otherwise, the basic principles are similar to what we were doing earlier.

Why do you need two separate telescopes for two different bands in the UV and visible wavelengths?

Since the detectors are different [for different wavelength bands], if you want to split and make all the wavelengths covered in the same focal plane, accommodating a large number of wavelengths and filters becomes difficult. There are filter wheels in this. Filter wheels are the instruments which decide what particular wavelength has to be detected by the detector beyond the telescope. So, accommodating them in a single telescope becomes an issue.

Another issue is contamination control. If you want to cover wavelengths from the far UV to the visible —the entire wavelength region—simultaneously, there are trade-offs in the coatings that are required on the mirrors, their efficiencies, etc. Based on that, this design approach was taken.

Can you expand on the role of LEOS in this mission?

In this particular satellite, there is a two-mirror telescope with a primary mirror of 380 mm diameter. For this telescope, you have to generate a particular “aspheric” surface for the mirror. Aspheric means it is not a spherical surface but a deviation from the spherical surface. Then it has to be polished to better than one angstrom in terms of roughness. Depending on its roughness, light gets scattered. That scattering is what we want to avoid. So super-polishing of the mirror was done. Since we had not done the polishing to such levels earlier, we had to generate new polishing techniques, which were developed at LEOS.

When you have an optical surface and typically suppose λ [lambda] is the wavelength at which it operates, deviations from the mathematical surface required are of the order of small fractions of lambda. If you are operating, for example, at 130 nanometre, λ/20 or λ/100 kind of finish is required. So, if I have an aspherical surface of 380 mm diameter, if I say λ/10 is the deviation that is required for the deviation from the mathematical profile, for 130 nanometre wavelength, the profile has to be matched to an accuracy of 13 nm. That is also one of the critical polishing techniques required. On the other hand, for visible light, which is around 450 nm, the same λ/10 would mean an accuracy of only 45 nm. As you get into finer and finer aspects, the difficulty level increases. So LEOS has done a wonderful job of achieving this super-polishing technique. λ/10 is the minimum required for both the mirrors and roughness is another part.

In the telescope itself, the mirrors are assembled by using mirror fixation devices because these mirrors should not get distorted while they are assembled. Any distortion of the mirror produces loss of performance. The telescope has to retain its performance while it goes through vibrations, shocks and other environmental conditions in space. All these issues had to be tackled. They [LEOS] have done a good job of achieving this performance.

You were earlier with the Space Applications Centre (SAC), Ahmedabad. What is SAC’s involvement in the project?

LEOS was able to do the telescope because of the telescopes built earlier up to 700 mm diameter. They were fabricating mirrors. The technologies for mirror fixating devices were evolved at SAC. Then SAC was involved in review mechanisms, supporting and testing. All the telescopes make use of Invar material, which has a very low coefficient of expansion. All this has been behind what has been achieved now. So you can say that there is SAC’s heritage here in Astrosat.

What about the electronics?

At one time, there were some issues with electronics from Canada and other places. SAC was, therefore, asked to look into the possibility of developing the electronics for detectors and the detector-processing electronics as a backup. However, finally this was not required. Corrective actions took place in Canada.

Even as late as August 2014, there were some problems.

The SAC team was tasked to do parallel development.

What was the problem with the Canadian instrument?

Actually, the detector package itself had some difficulty in meeting the vibration requirements and had other performance issues. Finally, they were able to resolve all the issues. It has gone through environmental tests satisfactorily.

Why did you initially have to go to the Canadian Space Agency?

In terms of electronics and detectors, we do not have them in the country. Only now the SCL [Semi-conductor Complex Ltd] makes CCDs [charge-coupled devices] which were used in the IMS [the Indian Mini Satellite]. That was a linear array. Then the SCL went through a process of fresh installation of 180 nanometre technology. So during that time nothing much could be done there. We also need to develop capabilities in that sector. These technologies are not available in the country.

These are all 180 nanometre CCDs?

These are not 180 nanometre CCDs. The CCDs themselves operate in different wavelength regions —shorter wavelengths. At shorter wavelengths, the penetration depths are different, the doping levels that have to be done [are different], and the sensitivities that have to be done [are different]. Since the Canadians have already built the electronics and the detectors for many such instruments earlier, they have the experience. So we had to depend on them. We have to depend on international agencies for many of the technologies that India has not developed yet.

Has the Canadian CCD been used in earlier missions?

They have been used earlier.

You planned Tauvex with Israel. It would have had a similar kind of concept.

Both in terms of sensitivity and field, Tauvex was different. It was not as sensitive as this [Astrosat], both in terms of field and resolution. This has about 1.8 arc-second resolution. That was much coarser.

Astrosat’s proportional counters have the heritage of IXAE [Indian X-ray Astronomy Experiment], used in IRS-P3. How much of technology used in the Astrosat mission is from IXAE?

LAXPC has a very large collecting area. Nothing like it will be developed till 2020. In most of the areas, the scientific teams and the instrument-building teams have been able to build better instruments than those that exist elsewhere today. To that extent, it is a significant achievement.

How do you see its performance in spite of the delay? There are various astronomy missions coming up in the near future like Astro-H. In terms of capability, how will Astrosat compare with them?

In spite of the delay in each of the areas, Astrosat has got something unique. Even compared to these [upcoming] missions, Astrosat will have better capability.

The cost of Astrosat is about Rs.178 crore. How did ISRO manage that without any cost overruns even though there has been significant time overrun?

In satellites, many of the components are common. The new things whose cost would have been different are the payloads themselves and their development. Much of the work there lies in the intensity of the value addition. So, all the facilities that were required to be established [for the development of the satellite] got established quite some time ago. There was no real overflow [of cost] in this. I think it is the prime reason [for the low cost of the Astrosat mission]. Hardware is similar for most of the satellites. There is not much change in that.

Payloads were the only ones different here. There much of the actual transfers to various institutions took place much earlier. Maybe we had to spend a little extra with the Canadian Space Agency because of the problems they faced. Even then, it was not such a large amount to impact on the total cost.

It was on a contract basis?

Whatever needs to be done externally we do only on a contract basis. All international agencies, even though they may say many things, are always on contract. For instance, when we do the deep space network connectivity, it is a contract.

But here in the Astrosat mission, these institutions are called collaborative agencies.

Yes. But still, they receive a payment.

Will they be given preference in astronomical observation because they were part of the team?

They were part of the team only to the extent of providing specific inputs.

What about providing time to foreign groups to make use of Astrosat in general?

Currently, we are looking at the requirements of Indian scientists for observations. Depending on the kind of collaborators and discussions, we will offer something in the future. You are right. In the international community too there is a tremendous interest in astronomical observations as can be seen from many of the discussions we have had. They are keen on participating in this mission and working with the data that we would get from this and we may offer them in the future.

Right now, we have not fixed anything. First, we are making sure that we provide specific opportunities to the Indian scientific community. Of course, another major change is that we are going to make an announcement on providing observational opportunities to Indian scientist and student communities for specific projects. This is another new thing in this particular mission.

Some years back, there was some talk of setting up an Astrosat Science Centre at the IUCAA.

We now have very good connectivity with NKN [the National Knowledge Network]. So this data will be made available there. What we are looking at is enabling students and researchers to access the data and work there.

They also intend to provide some computational capabilities and some modelling information. So both the Outreach Group and the IUCAA itself are coming up with specific action plans which will be supported. We would like to do that in more and more places also. Our ISSDC [the Indian Space Science Data Centre, Byalalu, Bengaluru] is anyway going to be there. In addition to that, we will also make data available at the IUCAA.

Earlier too, what we were been trying to test during Chandrayaan is what we call Payload Operations Centre. The basic things in terms of operations, reception, controls and initial data processing will all be done here at the ISSDC, but the Principal Investigators will have access to data both in terms of monitoring the satellite performance and tasking the operations being carried out at the ISSDC by making specific requests.

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