 Okay we are live now. So good evening everybody and a very warm welcome to this special lecture by Professor Jairam Chengalur from the National Center for Radio Astronomy Pune which is a national center of the Tartan Institute of Fundamental Research. Professor Chengalur is the Dean of MCRA. Professor Chengalur obtained his B.Tech in Electrical Engineering from IIT Kanpur in 1987 and then did his PhD at Cornell University in 1994 and thereafter he worked in the Netherlands at the famous Astron Institute before joining the MCRA in 1996 where he has been since for the last 24 years. Professor Chengalur is one of the foremost researchers in radio astronomy internationally and in the country as well and he's a fellow of the Indian Academy of Sciences, the National Academy of Sciences and the Indian National Science Academy and today Professor Chengalur will be tracing the history of radio astronomy in India the development of it which was pioneered by none other than Professor Govind Saru. You may have heard of his name of course I'm sure who passed away recently unfortunately and Professor Chengalur will be taking us through this journey of radio astronomy which is now a very successful enterprise in India. Professor Chengalur I invite you to start your talk. Thank you Anvesh. So you know we've been charged with talking about Govind Saru and the development of radio astronomy in India and you know I am going to try and be true to that title that was given to me so I feel there are some you know some things I'd like to say before I start the talk itself. So you know the first clarification I'd like to make is that I'm not trying to give a general introduction to the development of radio astronomy in India. I'm going to talk about Govind Saru and his contributions primarily to the development of radio astronomy in India which are you know enormous but at the same time if one were to talk about radio astronomy in India in general there would be other names and other you know developments that one should take note of but I won't be doing that today I'm clearly going to focus on the work that Govind did and secondly because of this the talk is going to be partly biographical and you know particularly in the early part of the talk but I will try and draw you know connections between whatever biographical things I say and you know the work which came later and of course you know given the theme you know and this biographical nature of the theme we are not really going to be doing you know an introduction to radio astronomy I think you'll have probably had that anyway already but we will of course you know of course because of the topic we will touch upon radio astronomy at least the techniques of radio astronomy a little bit as we go along I won't be saying very much at all about the science that is done with radio astronomy and finally you know this talk is I'm being given to the NIUS students but I understand also that it was meant to be more of an evening talk and at more at a kind of popular level so I have tried to keep you know as much as possible at a relatively popular level but I've also you know got some technical things in here not not a great deal and not very very technical but nonetheless there are some you know technical details that I can do now you know which should be relatively straightforward for NIUS students but you know for non people from a non-technical background there may be some things which you you know just have to skip over and finally the story that I'm going to tell us of course interesting in itself but I think it's also a very useful reminder for us you know I think we tend you know to take the existence of facilities and institutions a little bit for granted you know we assume that yes of course you know there are opportunities to do science there are institutions to do science etc and you know when we do that we tend to forget you know the effort and you know just the you know the huge chain of things which needed to be in place for these facilities and institutions to come into being and you know we tend particularly also I think you know at the undergraduate level when we're reading science and so on to to you know to think of it as a kind of knowledge you know knowledge which somebody sitting yeah you know in a room and thinking deeply about things that's come up with ideas and things and so on and so forth and put it all down and we lose a track a little bit I think of the complex interactions between science and society which is the thing which allows both science to grow and of course society to flourish and so I think this story that we're going through today is a little bit of a correction to all of that it's sort of you know both the story of the development of radio astronomy in India but also more broadly about the development of facilities and about you know all the thing ways in which science and society interact to create you know the science ecosystem that we have around us today so with that let me start so I'll start with something biographical some biographical simple biographical details of Govind Govind was born in 1929 I haven't given the date over here but in 1929 in Thakurwada which is in Uttar Pradesh and he was born actually to quite a well of rich landowning family and you know his father and his grandfather had you know they had both land and they had business enterprises of various sorts and so the family was quite well off Govind's mother was particularly keen that he should become an engineer but Govind himself wanted to do study science and his father sort of felt it important that he be allowed to follow his inclinations and so he did in fact go and study science so he did a BSE at Ewing's Christian College in Allahabad and that's the photograph I'm showing you over here and then an MSc at Allahabad University and his time over here culminated you know it overlapped and culminated with the freedom struggle for you know the freedom struggle culminated in independence around the time that Govind was doing his BSE and his MSc and all of these had a very strong influence on his character and his thinking as you know it did for many of the scientists of his generation and so that I think is something that we you know I think marked his career as he you know through through the years the ideas and the ideals that he acquired at that time. The other thing I'd like to note was that he had actually very good teachers he had excellent teachers including K S Krishnan who a name I'm sure you're all familiar with who had worked with Raman on discovering the scattering which is now called Raman scattering and for which Raman got the Nobel Prize and you know these teachers made a very deep impact on Govind and I think that impact had to you know was twofold one of them was that it motivated him very strongly to take up a career in science and secondly and I think equally importantly it gave him a very lasting conviction about the critical importance of very high quality undergraduate education so that he was convinced that if you know science was to flourish in in the country it was very important to have high quality undergraduate education and I'll come back to to these we'll come back to all these teams as we go through the talk. All right so as I said independence happened around the time that Govind was finishing his MSc and K S Krishnan who had taught him in his MSc years was appointed as the first director of the national physical laboratory which was established very shortly after independence and Govind joined NPL after finishing his MSc and started working with K S Krishnan and so he was assigned the first job he was tasked he was assigned by K S Krishnan was to develop instrumentation for for measuring paramagnetic resonance at at wavelength of three centimeters and this was a high frequency compared to the kind of things that people had done at least in this country before and so you know Govind was in a way starting from scratch starting cold but nonetheless he managed to make this equipment in three months and he did this using a surplus world war two radar equipment which happened which NPL happened to have access to and also you know with help for understanding how to go about the design etc from a very important series of volumes called the MIT radiation lab series so this had been this was the fruit of you know research done at MIT again during world war two for radar research and they had come out in a series of volumes I think maybe even 28 volumes altogether very detailed very you know informative and very influential volumes as far as microwave engineering was concerned and you know I recall you know even decades later as an undergraduate you know going through these radiation lab series but again this our teams were going to come back to this this fact that there was a lot of development of technology in world war two for radar and also that after the war there was surplus equipment leftover which then could be used for pursuing science right so this is the first project that Govind had done at NPL and in his biography he notes that Krishnan actually was very impressed and pleased with him for having been able to do this project very quickly and successfully and now here is where we start getting into these you know these felicities and these odd ways in which science develops so in 1952 Ursi which is a large body of of radio scientists an international body of the the premium international body of radio scientists they met they had their general assembly in Sydney in Australia and KS Krishnan happened to go for that general assembly and several of the presentation in this general assembly were in this new field of radio astronomy and in a way that was not surprising because Australia was one of the very few countries in the world at that time doing radio astronomy and you know it had it had a very clearly a leadership position and it had built a large number of very innovative radio telescopes so it was natural that if there was an assembly of radio scientists over there there would be a number of talks on radio astronomy and so KS Krishnan attended these talks perhaps he even went to the visit some of the telescopes I'm not sure whether he did or not this photograph is taken during the general assembly but it doesn't show KS Krishnan it's just other astronomers at one of the telescope sites in fact a site called Pottson which we'll run into again so but you know KS Krishnan was very struck by these new developments and this new field of radio astronomy and so maybe I should you know just for people who haven't heard of radio astronomy before let me just say a little bit about radio astronomy I think I have a one-slide introduction to radio astronomy so radio astronomy started with the sedendipitous discovery of radio waves by Karl Jansky in 1931 and so this is a photograph of Karl Jansky with his telescope and what he was doing was he was working for the Bell telephone laboratories which was wanting to set up a transatlantic radio link and to set up that link they wanted to understand what all are the sources of radio noise which would affect this link and so they had set Karl Jansky the task of identifying and characterizing sources of radio noise he was a very careful engineer and so he did this observation very meticulously and he realized that there's a faint but very clearly detectable source of noise which arises from the center of the Milky Way galaxy and that was the first source of radio frequency energy detected outside of the earth right so it was a discovery which caught the popular imagination it received considerable coverage in the press but interestingly enough it was not followed up by professional astronomers and the reason for this I I think is twofold one of them which I've put down over here is that the techniques used by radio astronomers are very different from those used in optical astronomy radio astronomy required a kind of engineering basically an electrical and electronics engineering which was very different from what the optical astronomers were familiar with they did not have these kind of expertise in the department of astronomy that existed at that time and so I think for that would be a strong reason why it never really got picked up a whole lot by people and the other is which we'll come to and look at in great detail is that these early radio telescopes had a very very poor resolution so Karl Jansky was able to say that the radiation came in the general direction of the center of the galaxy but you know it's not that he could actually pinpoint it to the center of the galaxy but it was a reasonable guess to say it came from the center of the galaxy but in general the resolution of radio telescopes was so so poor it was very difficult to associate any given radio emission with any particular source that the optical astronomers could see with their telescopes and so because of that it was impossible for them to to to progress this field a whole lot because you know it was like it was apples and oranges they it wasn't tying together to to give a more coherent picture of the cosmos and we'll come to that in a bit so you know after this 1931 discovery there was a little bit of work by an amateur but more or less it's fair to say that progressive radio astronomy stalled until the end of World War II and at that time there were two factors which sort of caused a big spurt one was that there was this huge group of electrical engineers who had been involved in the war effort particularly in the development of of radar and you know as part of this many of them had realized as they were setting up and testing their equipment that there were other sources of radio waves in the sky which they were not able to follow during the war time but which immediately after the war finished and they returned to their universities you know there was things that they began following up on what are these radio sources where are they and so on and the second was at the end of the war there was a lot of surplus radar equipment and so it wasn't you know it was relatively straightforward for these people to to sort of rejig all of this to make equipment to detect radio waves from outer space. All right so now let me just look at this thing which I mentioned briefly you know that the radio telescopes resolution is very poor and you know and this is something I'm sure all of the NIU students are familiar with that although you know we used to talk about a parabolic antennae having a focus or a parabolic telescope having a focus the images which you make with the telescope with a finite size you know even if I have a point like a point source like a star or something if I make an image of the star I don't see a dot in the image in fact what I will see is is a pattern like the one which I've shown you on this thing where you see a diffraction pattern like what is shown on the screen so you know the central dot gets smeared out blurred out and you also have these diffraction side lobes or rings or whatever you'd like to call around them and the size of the central blur that you know a point like source is smoothed out to depends on the wavelength and that's a formula I've given you over there it's more you know roughly 1.22 times the wavelength that you're observing at divided by the diameter of the telescope now radio waves that we're talking about have extremely long wavelengths compared to optical waves it can be millions or even more times longer than optical waves so you know the angular resolution of of early radio telescopes were all very poor and I'll show you an example in a bit as we go along and so early radio telescopes certainly could not make out anything about the internal structure of radio sources and further they could not locate them accurately enough for people to be able to identify which is the optical source which corresponds to this radio source and in the absence of that identification as I said it became they became like two distinct fields there was a bunch of radio sources but nobody knew anything more about them other than the fact that they emitted radio waves and of course optical astronomy continued along whatever research path that they were doing they you know so in that sense almost radio astronomy was almost not some you know almost not astronomy beyond being able to say that it's coming from the sky it's not coming from the earth there was very very little I mean there were certainly certain things you could say more about it and the sun for example was studied in detail because that was one source where it was clear which is the source that we are observing but apart from that you know there was very little progress that could be made in radio astronomy and understanding the nature of the sources so you know supposing I do want to make a radio telescope to match an optical telescope in resolution right so as I said the resolution goes like lambda over d the optical wavelengths are given by things of that of that nature if I have an optical telescope it's angular resolutions typically of the order of an arc second which is set by turbulence in the atmosphere for a one arc so let's say I want to make a resolution of one arc second at a typical radio wavelength of say 21 centimeters and so then I would need a radio telescope which is about 200 kilometers in size and so that you know just to to put that kind of pictorially I've shown a 200 kilometer circle roughly in a in a in a super post on a map of Maharashtra and you can see that it's you know the proposition that we have over here is obviously crazy there's no way that anybody can build a telescope at large so if you do want to get very high resolution it's instead obtained by a different technique a technique called interferometry and again I just motivated very slightly you know that supposing instead of observing a source with just one antenna I have two antennas and I hope you can see my cursor you know you have one antenna here and you have another antenna there and I have a source which is not vertically overhead but you know at some direction so the rays the rays from that source are coming along this direction so you can see that the ray or the wave front will hit this antenna first and it'll hit the second antenna after some more time and so because of that the signal at the second antenna is delayed or has a phase difference compared to the signal of the first antenna and if that phase difference is exactly an integer multiple of two pi or exactly you know this distance is an exact integer multiple of the wavelength the two will add up in phase on the other hand if it is you know half a wavelength difference between the the two path lengths or you know pi radians apart they're in phase they will cancel right so you can see therefore if I had a source which was vertically overhead the the the signal at the two antennas would add up in phase but if it was at a small angle theta off from the vertical then the phase difference between the two would go roughly like l theta divided by lambda of course and so the resolution then would go as l over lambda where l is the separation between the two antennas and not the diameter of the two antennas so I get some information about the location of the source with very high angular resolution by with a resolution corresponding to the separation between the two antennas so I could for example conceive putting these antennas many kilometers apart and that could then give me you know an ability to to locate the source with high accuracy it doesn't give me imaging capability but it at least allows me to locate compact sources I will come back to these ideas as we go along so basically if you one uses interferometry more than one antenna to add up the signals you can begin to locate source quite sources quite accurately and so you know and in Australia they had come up with a very clever idea where you do all of this with only one antenna you don't need to use two antennas so what they did was they put an antenna on on the on the edge of a cliff right next to the sea and now you look at some source and so here is the antenna over here on the cliff and that antenna is going to receive two signals it will receive a direct ray from the source it'll also receive the ray which has hit the sea and the sea turns out to be quite reflective at these low radio frequencies so it acts somewhat like a mirror so it hits the sea over here and is reflected and it gets collected by the same antenna so this antenna will now see both these rays and depending on the phase difference between these two rays you'll you'll get constructive or destructive interference and that phase difference of course will depend on geometric factors like the height of this cliff and this angle that the source makes with the horizon that is which will of course change as the source rises and sets so you'll see if you just sit and observe with that antenna you will see an intensity which varies with time because this path length will vary with time and so you'll get interference like you know increases and decreases in your intensity and that allows you you know because the distances we are talking about over here are large they could be many hundreds to kilometers kind of distance it allows you to accurately locate the source assuming that it is compact if the source is not compact you know if there's one part of the source where the path lengths are such that things add up in phase there would be another part of the source where things are such that they add up out of phase and on the average you would expect all of this would come out in the wash and so there won't be any strong interference patterns so but if provided the source is compact you would see these interference patterns and that would allow you to locate the source and that's how some of the you know first identifications of radio sources were made so for example the crab supernova eminence was identified as one of the bright radio sources in the sky and that immediately told you then that supernova eminence are capable of generating radio emission so Anvesh had asked me to stop from time to time to check if there are any questions so maybe I'll do that now. No I don't see any question from the chat so I think you can go ahead. All right all right so let me continue I'll press on so you know so this was a bit of a digression on radio astronomy and you know things happening in radio astronomy in Australia and the reason we digressed into it was we were you know at that point in our story where Sir Krishnan had gone to Australia for attending that Ursi meeting and where he heard all of these exciting new developments in radio astronomy and the field itself was new you know he heard all about this exciting new field of radio astronomy so when he returned to India you know KS Krishnan decided that he should trans set up a radio astronomy group at the National Physical Laboratory and so he set up a group with a bunch of young MSCs who he you know sort of talked to and inspired to take up this new field and that was actually a you know a turning point because several members of this group that KS Krishnan set up went on to become quite distinguished radio astronomers so you know so KS Krishnan had decided that he'd like to set up a radio astronomy group in India which was you know a bold step but if one were to proceed with it now step by step and ask the question okay now what it turns out that the first thing one would do is to try and understand how to say how to do radio astronomy how to build a radio telescope and so on and since no radio astronomy had been done in India it was difficult for people to you know to to figure out how to do all of these things and so it was decided that you know probably some people should go to Australia to CSIRO in Australia which is where all the radio astronomy was being done and you know get some exposure to this field understand the techniques of the field and so on and so Govind left for a two-year assignment with CSIRO in Australia in February of 1953 and he was funded under something called the Colombo Plan and you know again you know these are all strange things which fall together at the right time the Colombo Plan itself came from an initiative by an Indian diplomat called K. M. Panikar who was felt it was important to set up you know schemes by which human resource in India and other South Asian countries could be developed rapidly and so he had set up an international group which provided you know not monstrous amounts of funding but sufficient you know and which made a significant impact to allow people to go and get trained in various things and you know it it did this whole scheme which I think is still running as the Colombo Plan as over the years provided you know quite made a quite a significant impact in human resource development so anyway so Govind left on this plan to go to CSIRO and over there he undertook a number of projects so the first thing that he had been assigned to do was that you know there was an array telescope at CSIRO which you know had done strip scans of the sky and he had been assigned the task of the sun particularly that was the source which you know that was basically the radio source which was studied in the early days and his job had been to convert these strip scan images into a two-dimensional map which you know I won't get into the details I will admit I'm not you know there are hairy details in here which I'm not completely on top of but you know he did it and it was a lengthy manual calculation involving having to do Fourier transforms numerically but you know in those days numerically meant literally with one of those mechanical calculators scanning things in reading of values with a with a with a scale and so on and so forth and so it was painstaking work but he did end up being able to make an image of the sun and this is the first 2D two-dimensional radio image of the solar radio emission and what you will notice straight away is that the emission is not spherically symmetric there are these two regions over here on the left and the right where the emission is much brighter than elsewhere it's much brighter along the longitude along you know the equator of the of the of the sun then along the poles and it's also much brighter at the edges and this was you know as had been predicted by theories of limb brightening and it was you know very important that that observationally one was able to to confirm that in addition he this was this map which I'm showing you was done at a frequency of 21 centimeters and this had been with data that had already been taken and Govind basically redid observations of the sun but at a much lower frequency at a frequency of 500 megahertz using observations with the antennas in the pothill array this is the array over here of of antennas and what he did was he modified all of those antennas to work at a frequency of 500 megahertz and he and a colleague of his Parthas Arthi who was also there I think on the colombo plan equipped the array to work at 500 megahertz now basically what you know happened in an array like this was that the signal from all of the telescopes had to be added up together in phase and it produced a kind of fan beam on the sky and so you know at the end of each and small antenna like this we have an amplifier and that amplified signal is taken in an electrical cable and then it eventually has to be combined together and so it's very important that you know since you have to add the antennas voltages up in phase there when I transport the signal in a cable there will be some phase that the cable itself will introduce to the signal and it's very important that we make sure that the phase introduced by the cable for the first antenna is the same as that in the second and third and so on and so forth basically all of those cables have to give you the same phase and so the signals had to be transported in phase and there was a very very cumbersome procedure to do that you know you compared the phase that you got from the cable in this antenna with that you got in the next antenna you equalized it by doing whatever adjustments you need to do then you compared the second with the third antenna and so on and so forth and it took a very very long time so it was a cumbersome phase adjustment procedure which took days but you know they did do it and they did find that even at a frequency of 500 megahertz the solar limb does show edge bright the sun does show radio emission from the sun does show you know limb brightening and we'll come back to all of these issues in a bit about transmission lines and and you know having things in phase etc because these were issues that you know are going to crop up you know as we keep going through our talk all right so you know after that stint of two years in Australia government returned to the national physical laboratory and you know just before he left the CSIRO where he was working had agreed that the pot sales antennas that he had been working on could be transferred to India and he could take them to India and he could set them up over there to do a solar radio astronomy in India but unfortunately because of various bureaucratic issues you know these that the plan actually never got actualized and for this and other reasons radio astronomy never properly took up at end you know never really took off at NPL and the group that KS Krishnan put together gradually dispersed but as I said you know many members of that group actually went on to become quite well-known radio astronomers and they've all played you know an important role and so that effort that KS Krishnan put in was very important in the development of radio astronomy in India so Govind himself decided that he would continue radio astronomy in the US and so he went to Texas where Harvard University had set up a radio astronomy station which at that time was the most sensitive system that you had for spectroscopy so radio spectroscopy of the Sun and that's the system being shown over here in the upper photograph there's Govind in the middle over here and two of his colleagues on either side of him and there's the antenna behind them so you know the work done during this period included Govind's discovery of a new type of radio burst from the Sun which was called a U-type burst it's actually an inverted U which is what's shown in the lower picture so it's showing a picture of the frequency at which the burst emission is coming against time so you can see that the frequency increases with time and then decreases again and that gives you some idea as to what is going on over here where is this emission coming from it's understood as coming from electrons which are spiraling along magnetic fields that rise out into solar corona and loop back again towards the Sun so after his stint at Harvard College Observatory Govind took up a PhD at Stanford University with a very well-known radio astronomer Ron Bracewell and he worked here he had you know he was supported by Stanford University and as part of that support he was expected to do work on the Stanford cross antenna array and the work that he had been that he was assigned to start with was to phase the array and that he had to do very early in the morning before going for graduate classes and as I explained the short while ago it's very painful and mind numbing work where you just go on comparing phases of adjacent antennas and keep adjusting things to get it to be equal and so you know needless to say you know if Govind found that repetitious job you know somewhat irksome and he you know wanted to find he was keen to find a way to you know make it less irksome and do it more efficiently and he did in fact come up with a way of doing that which is basically that he modulated the signal at the far end and sent it back and that allowed him to measure the phase of the cable very efficiently and and you know you didn't have to do this pay-by-pay comparison kind of thing and it cut down the quite time very dramatically this called this round trip way of measuring the phase is now called the Swaroop and Yang system and it is a technique which is widely used in many applications which require synchronization of separated equipment including atomic blocks and so on and so forth so it was you know very important contribution he made on the way to his PhD his PhD work itself was on solar radio emission and after his PhD he continued at Stanford as an assistant professor he had you know offers from other leading universities in the US but by that time he was quite clear that he wanted to go back to India and for that and other reasons it made sense to just continue at Stanford where he was already based and where things were already set up and where work could continue while he sort of sorted out his return to India so you know around this time when he was finishing his PhD Govind along with other young radio astronomers including people who had been with him in the NPL group had developed quite a detailed proposal for starting a radio astronomy group in India and five of them sent this proposal to major five major scientific organizations in India and the proposal was also sent to prominent international radio astronomers and they had been asked to these astronomers were asked to send assessments directly to the funding and other scientific organizations in India and the most positive response this group got was from Homi Baba at the Tata Institute of Fundamental Research and he said yes we will set up a radio astronomy group at TIFR and you know he did go further than just saying they'll set up a group he said that if and here's the quote if your group fulfills the expectation we have of it this could lead to some very big equipment and work in radio astronomy in India than we foresee at the present so he's basically saying that you know if provided you fulfill our expectations we will sort of you know ensure that you're able to to do big things so Govind you know took up this offer and sort of moved back to India to the Tata Institute of Fundamental Research and on the way back he stopped at the Netherlands which at that time was a powerhouse in radio astronomy and it continues to be one of the leading countries where you know for radio astronomy research and he met with Jan Woot who was you know probably the most prominent astronomer in the Netherlands at that time and Jan Woot at that time was setting up a 25 meter which for that time was a pretty big telescope at Dwingelow in the Netherlands and that time yeah sorry to interrupt your audio has gone down quite a bit so yeah okay I'm not sure why I'll just go a little closer to the mic is that better yeah so yeah so he stopped by the Netherlands I hope I'll just start from there I hope that earlier parts were audible and he met with this you know very famous astronomer Jan Woot who at that time was busy setting up this telescope to observe the radio emission from the hydrogen atom and so you know during the war you know I think it was Jan Woot's student himself in the Netherlands who had done the calculation which sort of showed that you would expect to be able to detect the radio emission from hydrogen in interstellar space and that emission had in fact been detected after the war just shortly before all of this and Woot were setting up a large telescope to study the distribution of hydrogen in the interstellar space it was a very hot topic at that time cutting-edge science and what Woot suggested was that the telescope he was setting up in the Netherlands would only be able to see the northern sky and he suggested to Govind that he would help Govind set up you know an identical telescope in India which would then be able to see the southern sky and between the two they would be able to do complementary science and get a full understanding of the distribution of hydrogen in you know in the galaxy. Govind and this was partly because of you know his own inclinations and also partly because of all of the advice he had got from his mentors in Australia he felt it was much more important to carve out a niche for Indian radio astronomy than to follow fashions and you know each one was certainly you know the hot topic at that time and so he decided that he wouldn't follow this path he did do a lot of work in each one later and we'll talk about it in a bit but at that time he felt that he'd much rather strike on on his own and that of course had major implications for the development of Indian radio astronomy and you know in my own view it had quite positive implications for the way in which radio astronomy developed in India. So at TIFR you know Govind's first project was to set up the Kalyan radio telescope and he for to do that he used the old Pots Hills antennas which had finally reached India and of course you have to transport signals from one antenna to the other and for that he used a novel and much simpler transmission line system than the original had used and so that of course was an important innovation which allowed things to move fast and cheaply but more importantly it also provided a training ground you know setting up this telescope coming up with their own ideas about how to you know those were just dishes how to take just dishes and make them into a radio telescope you know with the with all of the amplifier systems and so on and then connecting all of them together. So all of that engineering was actually a good training ground for young man power and so that's what's being shown in this photograph over here. It includes Govind's first PhD student Vijay Kapahi who himself was a very prominent radio astronomer working on setting up the Kalyan radio telescope. So this telescope was set up quickly and it yielded again you know quite interesting results and observations of the Sun but while all of this was going on Govind was already thinking of a much bigger telescope. You'll remember that you know before he had joined itself Baba had indicated that TIFR would be open to big developments in radio astronomy. So you know let me now you know motivate the idea that Govind came up with and to motivate that idea I need to step back and go back to something we were talking about you know many slides ago about resolution and about how radio telescopes of that time typically had very poor angular resolution and because of that you couldn't identify which exact source was the one emitting the radio waves right and as I said that happens primarily because radio wavelengths are much much longer than the optical ones and so you can't you know you find a radio source in the sky but the region from in which that source could be is so large it's very very difficult to identify which of the optical you know stars or galaxies you have in that region is the radio source. So I'm showing you know just to illustrate the magnitude of the problem this image that I have shown you over here is of a field of view roughly comparable to that of the park's radio telescope which was a 64 meter parabolic reflector antenna and one of the biggest antennas fully steerable reflecting antennas available at that time. So this is the field of view of that radio telescope and it contains a very very bright radio source called 3C273. Now if you were going to ask the question which of these optical sources that I see in this image is the one which is emitting the radio waves you will realize it's a hopeless question. I mean there's just so many sources over here you know it's meaningless to ask you know to even try and do some correlation and try and understand which one of these is the one which is emitting the radio source. If you want radio waves if you want to find that source you need much much much better pinpointing of the source of radio waves and that in fact is what the park's telescope managed to do and it managed to do it by using a technique called lunar occultation which I'll come to in the next slide. But you know they did this lunar occultation which allowed them to locate the source much more accurately and the moment they had an accurate position for the source then it became identified with a unique optical object and the spectrum of the object immediately showed that it was at what at that time was an extremely high redshift it was a redshift of 0.158 which meant that the source had to be extremely powerful it was probably you know the most powerful source known at that time. We know now that the that source is powered by a supermassive black hole. At that time it was unclear what was powering the source all they knew was that this is an extremely powerful source in the sky more powerful than things that they've ever seen before but at the same time in the optical it looked like a dot like like a star and so it was called a quasi stellar object radio source or a quasar. So these things are called quasars so now they're called quasi stellar objects. Right so but the way this source had been identified was by looking at at lunar interferometry. I should pause and I also realize that my the clock is ticking. Anvesh I need to finish at seven right? No yeah but you can go a few minutes over there. I might run a little bit of time it's okay fine and I'll pause also in case anybody wants to ask something. Yeah I don't see any raised hand or any questions yet. All right so then I'll plow ahead. So so lunar occultation so basically the moon can be approximated as a semi-infinite screen for distant radio sources. So you know as the sharp edge of the moon occults the radio source you will get diffraction patterns because of diffraction at a sharp edge and that's what's being shown to you over here that there's a distant radio source s over here there's the sharp edge of the moon which is near you know approaching the line of sight to this radio source and as it approaches that line of sight you begin to see diffraction patterns. The moon of course is moving in the sky and as the lunar edge moves this diffraction pattern will drift over the earth. So if I have a telescope at a fixed location on the earth as the moon drifts over the the radio source I will see the intensity vary according to this diffraction pattern and that's what's being shown in the lower panel over here that you have the moon drifting across the radio source line of sight to the radio source. You see the intensity sort of go up and down because of diffraction just before the moon completely occults the source then you see no emission while the moon totally occults the source and then as the source emerges from the other edge of the moon you see diffraction patterns again and you know looking at this pattern allows you to very precisely locate the source and so you know you get very high angular resolution even in your though your telescope's native angular resolution it's not high and again it gives you a measure of its angular size. So this is what you know go the 3C2173 that paper identifying it as an extra galactic radio source at this very high redshift had just been published in nature and Govind was realized that this is a very important technique going forward and he he decided that what would be worth doing is to build a very large telescope to to to measure you know the the angular sizes of a number of radio sources. So you know triggered by this possibility he decided that what's worth doing is to build a telescope to characterize the angular size of a large number of radio sources and the scientific driver behind that was to measure the angular size flux relation of radio sources and that would allow one to distinguish between two competing models of cosmology at that time the steady state and the big bang cosmological models so there's a big science question and certainly worth doing but you know technologically there are two major challenges which needed to be overcome the first is that the telescope would need to be steered fully it couldn't be a telescope which was just parked and looking vertically upwards or something like that because you do need to observe a large number of radio sources radio sources which you know the moon might occult at some time or the other so you need the ability to point at many many locations in the sky so you need a fully suitable telescope otherwise the number of sources you that you could observe would be very limited the second is that you know you need a very sensitive telescope so it would have to be very big it would it actually had to be four times bigger than the largest fully steerable telescope ever built anywhere in the world at that time so how do you build in India you know with the budgets that were available then a telescope four times bigger than you know the largest fully steerable telescope available anywhere in the world so Govind came up with this very innovative idea that building a large cylindrical telescope cylinders are much cheaper to build than parabola parabolic or paraboloid antenna so you know you build a large cylindrical telescope but you keep its long axis parallel to the earth's rotation axis yeah and if you do that then just by rotating around one axis you can track a source right because your rotation axis is parallel to the earth's rotation axis so you get this terrible antenna with just one axis motion to keep it parallel keep it parallel to the earth's rotation axis you need to keep mount the antenna on a north-south hill slope and the gradient of that slope has to be equal to the latitude of the of the telescope and so that exactly what he proposed to do homibaba was extremely supportive of the project he obtained funding for the telescope as well as an associated inter-university center for training of students but you know one should appreciate that the engineering challenges were enormous I mean it was a huge mechanical structure and that structure had to be built on a slope right and so you know there had to be very specialized designs which were done at that by TCE what is now TCE when at that time was called Tata Ivesco and also it was a time when foreign exchange was very very scarce and so all kinds of things you couldn't import things you had to find ways of making them in-house and so and that included very basic things like coaxial cable and n-type connectors and so they weren't available in India and so they had to be developed for the first time and Govind sort of got people to develop these things based on conceptual designs that he provided again going back to you know the MIT radiation lab series notes that I had mentioned earlier and so you know basically one had to work with industries work with vendors you know get them to start doing things you know which were more challenging than they had taken up earlier and so it's a herculean task but nonetheless you know it was all done and the telescope was made operational by 1970 and what I'm showing you over here is basically you know the first sort of the first sort of digging in the ground being ceremonially done by MGK Menon who was the director of TIFR at that time because Homi Baba had you know tragically passed away in an accident all right and so the UTI radio telescope came into being on a north-south hill slope in UTI with a slope of 11 degrees it's a very very large telescope it's a parabolic cylinder 530 meters long that's more than half a kilometer long 30 meters wide it's an offset parabola and it's fed by a set of 1056 dipoles located along this line that I'm running my mouse across and it's an offset parabola which has two advantages one of them is that this feed line when the telescope is brought to the western limit that feed line comes very close to the ground and it's easy you get easy access for maintenance and the second is that there's no blockage you know if it was a standard parabola where it was fed from the center then the shadow of the feed would fall on the reflector whereas if you offset it in this way the shadow of the feed doesn't fall on the reflector and so there's no blockage the reflecting surface which you can't see over here but there is a reflecting surface which reflects the radio waves it consists of thin stainless steel wires and as I mentioned earlier this telescope can be rotated about its long axis to to to track sources in the sky and the dipole signals are combined together to form the telescope beam and those signals can be combined with different phases and depending on how you face the antenna you can actually steer the beam in the north south direction so that allows you know two axes pointing off this telescope and so you get a fully steerable telescope one axis being steered mechanically the other axis being steered electronically the UTI radio telescope had a transformative effect on Indian radio astronomy and did work on all kinds of things including of course the angular size flux relation of radio sources which had been one of the main science drivers it also enabled you know quite detailed studies of the interstellar medium of the galaxy via radio recombination lines studies of the heliosphere via scintillational sources studies of pulsars studies of redshift and hydrogen which I'll tell you about in a little bit and it also you know and I think this is equally if not more important it trained a whole generation of radio astronomers and engineers the telescope is still functional it continues to be in regular use today and in fact it's being currently upgraded and is expected to continue to do important science all right but you know at the end of the day it was a single dish radio telescope and so it had limited resolution and what one would really like is to have a telescope with better resolution and as I said that kind of resolution is achievable if you use an array and ideally you'd like to have an array which allows you to make images of the source you'd like to make radio images with the images having very high angular resolution and that is possible if you combine the signals in a more complicated way than just adding them together the very simple things we looked at together just earlier you just take the voltages of the two antennas and add them together but instead of doing that simple operation if instead you correlate the voltages which is basically you multiply the voltages together and then you integrate for a little while you get something called a visibility and that visibility actually has very interesting mathematical properties it basically is you know one component of the Fourier transform of the image and that corresponds to the spatial frequency of b over lambda where b is the separation between the antennas under a set of assumptions which I want to go through so basically you know since you are measuring a component of the Fourier transform you could imagine if I lay enough interferometers on the ground I can measure you know a large number of Fourier components of the image if I know the Fourier components of the image I can do an inverse Fourier transform and I will get the image itself right so that is basically the way in which images are done in radio astronomy you synthesize a telescope of size equal to the array size and certain tricks that you could use when you're doing that and so one of the main ones is that you know if you're looking at an astronomical source it its statistical properties really don't vary a whole lot with time typically some sources do but the bulk of them don't so if I want to synthesize a large aperture I could use repeated observations but it's very small number of antennas whose spacing can be varied that is I measure all the small spacings then after a while I measure the long spacings and you know next month I measure still longer and next year I measure still longer and so on it just doesn't matter I can still put them all together and you know invert them to get a Fourier transform another way of doing this is to just track the source as it rises and sets because as the source rises and sets the projected separation between two antennas will change you know if they assume the antennas are located on an east-west line you know the projected separation would be maximum when the sources are are rising and they would be minimum when the I'm sorry there would be maximum when the sources are vertically overhead and there would be minimum when the source is rising or setting right so you know just by tracking the source I will measure a large number of Fourier components without actually having to move any of the antennas and I think I don't know if you can see that animation I hope you can it's a very simplistic animation showing you the basic idea that the projected separation between the antennas changes and so I end up measuring many more components in the Fourier plane than you might have otherwise imagined all right so you know with that in mind a synthesis telescope was built in UTI to enable imaging an array of small telescopes were set around the main telescope and that's what this schematic is showing you over here you know showing you the main UTI telescope and then all the small telescopes set up around it they were called baby cylinders which were 22 meters into 9 meters long they were connected to the main telescope via radio links and it that you know this UTI synthesis radio telescope was operational for relatively short period of time and was made used to make several images at 327 megahertz excuse me I'll just check if this is unbeach no no it's not me it's not okay yeah so then let me continue but what has happened I've lost the screen share right yes you have lost the screen share so just give me a moment yeah all right here we go yeah we are back so it was used to make several images at 327 megahertz but I think more important you know than the science that it did was that it was a very very useful training ground for building up expertise in the country for building a synthesis radio telescope and that gave the confidence to to to build the next big telescope which is the GMRT the giant meter wave radio telescope but the mid 1980s government was planning the next big telescope you know given the high importance of high angular resolution it was clear that this telescope should be an array the original concept was to have an array of 34 parabolic cylinders spread over a large area and this already was quite challenging because the only existing interferometer of this size was a very large array located in new mexico in in in the US which is an area which is very thinly populated very flat and what they did to you know in in an interferometer or in an array of any sort you have to transport the radio signals to a central location right and you have to transport it with very low loss now if you have short distances you could transport it along a coaxial cable or something like that but if you're trying to transport a signal over many kilometers it doesn't work to try to transport it in a coaxial signal coaxial cable the loss is too much so the very large array used wave guides you know which are these rectangular hollow aluminum you know tubes which have to be of you know of an exact dimension to transport the wave without loss and you know they also have to be they can't have bends or kinks in them they you know you'll have reflections and losses and so on and so forth so all of this was laid out in the desert in new mexico to connect the antennas together that's not practical in india where you know you don't have such large flat thinly populated areas you know you know there's people everywhere there's agricultural activities and all kinds of things going on it's not practical to think about connecting things with this sort of you know waveguide so govind decided that what he would do was to use optical fibers to connect this the antennas which is you know was a very very radical innovation at that time optical fibers were just beginning to appear in the scene and you know when the jmrt finally was built and put up the order that was placed for optical fibers for the jmrt was by far the largest order for optical fibers ever to go out of india after that time now of course optical fibers are you know a completely standard thing but we should remember we are talking about a time which is about 40 years ago there was another radical innovation that was needed so originally the idea was to have parabolic cylinders but these cylinders actually have limited frequency and steering ranges compared to parabolic dishes but as i had mentioned they are much cheaper to build than dishes you know but ideally you would have preferred to have dishes and so there's a lot of this thing that why are we doing cylinders why can't we do dishes etc and so govind finally came up with a radical design for a lightweight cheap radio antenna and you know this exploited the fact that this telescope is to work only at low radio frequencies it had to be located in areas with you know in puna basically where it never snows so you don't need that much of a backup structure so the kind of traditional designs for parabolic radio dishes which were built for cold countries to work at high frequencies and so on and so forth really wasn't needed for a telescope of this sort so govind came up with a design where the parabolic shape is achieved not by having you know heavy steel trusses which are you know shaped parabolic shapes and onto which you put panels that's not how it's done at the gmrt instead of these you know you have very few of these structural steel parabolic frames but instead you have a large number of rope trusses instead of you know steel trusses and these rope trusses with appropriate tension are given a parabolic shape and the mesh is then just attached to this truss and that allowed the fabrication of the antenna as a fraction of the traditional cost and that's really what allowed the gmrt to be built of course you need you know you want to build a big array etc you should have a major science goal that you're trying to achieve and the science goal that govind had was to detect hydrogen you know so going back to things that people have been talking about when he was first moving back to india govind of course had been very interested in this problem of you know of atomic hydrogen and very distant objects i'm going to skip over this slide given the time so you know given the time at which the at the time when the gmrt was proposed there were two models for the formation of galaxies one of them was a top-down model where you have very very large objects which form first and then fragment to form galaxies and the other was a bottom-up model where you have small objects which collapse first and then merge to form the galaxies and you know which of these two happened depend on the kind of dark matter that you have in the universe the top-down model was favored if you had hot dark matter and the bottom-up model was favored if you had cold dark matter and you know at the time when the gmrt was proposed the top-down model was a favored model and so you would expect to see collapsed objects you know in the very early universe and these objects were also predicted to have a flat geometry there was supposed to be flat like a pancake or a dosa and so they were called zeldovich pancakes and so a major goal for the gmrt was to try and detect the hydrogen emission from these early structures in the universe and in fact it was something that he had already started doing with the uti radio telescope and then formed the phd thesis of ravi supramanyam another you know person who went on to become a very prominent radio astronomer to search for each one from these protoclusters of pancakes using the uti radio telescope and the design of the gmrt also folded in the science case it has a kind of hybrid design where you have a number of antennas 12 antennas in a compact area at the center and the angular resolution of that configuration is well matched to that expected from protoclusters there were also a number of antennas which are spread out on a y-shaped arm and that gave you very very high angular resolution also and the combination allows makes the telescope extremely flexible and allows it to do a huge range of tackle a huge range of science problems so i think i have probably gone for another five minutes you know the gmrt was built and commissioned in 2000 it consists of an array of 30 antennas each 45 meters in diameter using this novel cost effective antenna design and it is the most sensitive radio telescope in the world and most of its frequencies of operation i'll just skip over this which i've already talked to you about and you know i'll just say that this hybrid configuration gives it you know the very very good coverage of the furior device so this is on this left panel i'll show you the coverage that you get from just with the y-arm antennas the antennas which are in those y-shaped arms and you can see you get good coverage going out to about 25 kilometers i've also zoomed in at the center to see what these antennas in the central square give you and this is what inside the central one kilometer you would get using just the antennas within that and you can see that again gives you a very good coverage over there so you get you know good coverage of both the central parts of the furior plane as well as you know going out to large distances and that gives you the ability to image all kinds of sources you know large diffuse sources as well as compact complex sources with a lot of fine structure. The GMRT as it was built had you know feeds on a rotating turret over here and depending on which frequency you wanted to observe you could rotate that given feed to face the focus and it allowed operation in five different frequency bands there were four phases of the turret but one of the feeds worked at two frequencies the electronic chain and digital back end supported a maximum of 32 megahertz bandwidth and the back end also simultaneously supported two major modes of operation one was correlation which allowed you to make images of the sky and the other was a high time resolution you know beam formed mode which allowed you to study pulsars and other kind of compact objects since then the telescope has been upgraded the upgrade has just recently completed you know and it's could be regarded as a completely new telescope everything apart from the steel and concrete has been changed and you know now the telescope offers nearly seamless coverage from 30 megahertz to 1500 megahertz there are small gaps in the coverage and this is typically chosen to be in regions where there's very very strong radio frequency interference from television or something else which means that in any case you're not likely to get useful data there the instantaneous maximum bandwidth has changed to 400 megahertz which increases the raw sensitivity by a factor of three and also gives you large improvements in image fidelity the GMRT has an open sky policy which means that astronomers from all over the world could submit proposals they invited twice a year and time allocation is done by an independent time allocation committee which just allocates time based on international peer review and it turns out about half the time is used by Indian astronomers and half the time is used by astronomers from around the world. All right there are many things which I have not covered I'm wrapping up here I haven't talked about Govind's contributions to building you know antennas for satellite communication which again was a very major and important contribution all of which got you know enabled by the fact that radio astronomy and antenna design and antenna engineering you know that expertise became available in the country and played a major role in in early development of ground stations in India and also not talked about Govind's efforts on the international regime where you know he had again a very major role in working with international radio astronomers to try and set up large collaborative international facilities you know including the facility which now is you know the big facility of the future the square kilometer array Govind was one of the early people pioneers who sort of pushed and set up that collaboration to build this telescope but I won't you know given the time I'm not going to talk about that I'm going to end by talking about Govind's role in student training and you know as I mentioned in the very first slide he was fortunate you know to have very excellent teachers and that you know really impressed on him the importance of having you know good training in undergraduate years and the importance of training scientists and engineers and so you know throughout his career training of people was very important as I mentioned the whole generation got trained at the Kalyan radio telescope the UTI radio telescopes and so on and and including at the GMRT so when the GMRT was being designed again a whole fresh young team of engineers was recruited and you know got sort of trained in these new techniques using you know at that time all of these new technologies like fiber optics and so on and so forth and in addition of course he had a very large number of PhD students many of whom went on to themselves have more students and so on and so forth and which led to the growth of the radio astronomy community in India. He also played a very important role in setting up various training programs including the JAP program which is a multi-institutional program for training of astronomers but you know he was always sort of of the view that undergraduate science education in India is very very important and he felt there particularly by you know 2001 he was winding down from the GMRT that he should now take up this course and he felt that you know that the universities in India were not imparting adequate you know training and science to students and you know this this idea that students and large number of students should be trained was always important to him if you recall the UTI telescope was originally supposed to be part of the university center that unfortunately did not happen and that was again related to the fact that Homi Baba you know passed away unfortunately through in an accident and after Govind retired from TIFR he got back into this effort of trying to set up new institutions for undergraduate education in science and engineering and you know it was I would say more than a decade of effort of working with the government and their developing proposals taking it up with the ministry and so on and so forth and till they finally took shape with the help of other people of course and that finally took the shape of the establishment of the ICERS you know which got founded about a decade ago and which you know themselves have had now a major role in the training of young people in science in India so I think that's all I had to say today thank you thank you thank you for my excellent talk I invite the students to raise your hands and ask questions or to write your questions in the chat there are a couple of questions from the youtube chat so I will just relate that so somebody has asked regarding this lunar occultation method why is it why is the moon called a semi-infinite screen oh okay meaning that it it has an edge an infinite screen would have no edge at all you know it would be infinite in all both directions right so it wouldn't have a boundary so it's semi-infinite because it has one edge and you assume that the other edge is infinitely far away yeah okay then somebody has asked why do we use radio waves for astronomy purposes that's a good question and the answer is you know the two-fold one of them is that if you you know look at the atmosphere and the you know the way we are located we are located on a planet a planet which has an atmosphere and above the atmosphere there is this ionized gas which is called the ionosphere and so you know if you ask yourself the question what kind of electromagnetic radiation from body electromagnetic radiation from outer space can reach you it turns out that actually most of the radiation from outer space will not reach the surface of the earth and this is a good thing because much of this radiation is harmful to life it turns out that there are very specific wavelengths at which a radiation generated in outer space can reach the surface of the earth one of them is the optical wavelengths you know the wavelengths at which the sun is the brightest and the wavelengths at which we see and which you know all of that light is what sustains life on earth that is one wavelength set of wavelengths which reaches the earth and the other important window is radio waves radio waves are the other place where the atmosphere and ionosphere are transparent and these waves can actually reach the earth so you know you can build telescopes on the earth and observe extraterrestrial sources if you want to observe at other wavelengths such as gamma rays or x-rays or something like that it's not possible to do that from the surface of the earth you have to launch a satellite to do that and you know that's a very expensive business and it's very difficult to launch big telescopes into space and so on and so that is you know one of the reasons why radio astronomy is important it's one of the things you can do very sensitively from the surface of the earth and the second thing is that you know if you look at the interstellar space it has stars and things in it but between the stars it's not a vacuum you know as I indicated there's definitely hydrogen gas between the stars but there are also dust particles very fine particles of dust and these fine dust particles eventually begin to obscure the starlight so you know beyond a certain distance it's difficult to see stars or in certain directions where there's a lot of dust you can't see in the optical because the dust completely blocks the light however these are directions in which radio waves have no problem propagating because the dust does not obscure the radio waves so you can see in directions where you cannot see with optical telescopes thank you thank you so I don't see any raised hand yet or any question one thing I mean this is just a comment I think you brought out very nicely what you said at the very beginning that I mean sometimes we take for granted about the facilities and the laboratories and I think your talk brought out very nicely how years of hard work by scientists backed up by very solid science goals are necessary to build such things I mean these things are not given and are not readily available I mean just out of nothing I mean so I mean I think your talk brought that out very nicely and I encourage the students to take note of that also because I mean I think it's very important to realize that it is scientists who have to bring up I mean really nice proposals and to convince people the funding agencies and to put in years of hard work to build such facilities like GMRT and everything else that we have so thank you for highlighting this in this talk it came out very nicely I thought yeah okay so if there are no further questions I request everybody to unmute and applaud Professor Chengaloo for his talk and thank you Jerome for thank you for taking out time from your busy schedule to do this well there is one question now that has again come up somebody has asked that can you tell us why the 21 centimeter hydrogen line is so widely studied okay that it's widely studied because hydrogen is the most abundant element in the universe and so it allows one you know to trace you know if I have big structures and so on and so forth they will definitely contain a lot of hydrogen and that can be traced with the 21 centimeter line so you know you are tracing the dominant component of the universe it's you know an excellent way to begin to understand how things formed even you know in the very early universe and so on because the best tracer in a sense is this 21 centimeter line it's tracing this bulk component of what you have out there thank you thank you I think with that we will close and thank you Professor Chengaloo for doing this for the IUS thank you all right yeah thank you all yeah