Interview with Bernard Y. Mills
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The interview listed below was originally transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009). The original transcription was retyped to digitize in 2016, then reviewed, edited/corrected, and posted to the Web in 2016 by Ellen N. Bouton. Places where we are uncertain about what was said are indicated with parentheses and question mark (?).
We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of the original cassette tapes, and for a 2012 grant from American Institute of Physics, Center for the History of Physics, which funded the work of posting these interviews to the Web. Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event.
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Begin Tape 61A
Sullivan
This is talking with Professor Bernie Mills on 25th August 1976 at Grenoble, sitting in the grass, with the cars going by. Could you tell me a little bit about your educational background and what work you were doing before you got into radio astronomy?
[Indecipherable interchange, perhaps about traffic noise]
Mills
Perhaps I can start with graduating from Sydney University as an Electrical Engineer in 1942, end of 1942, when I joined CSIRO, Division of Radio Physics, to work, along with everyone else, on radar. And I became involved particularly in the receiver section, receiver development, and in electronics and display work generally. I worked on several systems and became increasingly interested in the basic design of electronic systems and developed quite a lot of some of the radar systems which were being made in Australia at that time – the receive and display sections. At the end of the War we diversified efforts quite a bit and I became interested in applying radar techniques to linear accelerators. And, in fact, at this time, some initial experimental work had been done just hooking a magnetron into a resonant cavity to see what could happen. A few hundred ke? were obtained. So I joined the Valve Lab which had been formed then in an attempt to develop this idea into something that could be more practical than was this initial experiment.
Sullivan
This was CSIRO?
Mills
Yes. The aim was, in fact, to provide a working, pulsed X-Ray tube. And I got that going, getting about a million volts. This, I think, was the first practical application of the idea, applying a magnetron to a resonant cavity, and I developed a lot of the basic ideas involving stabilization of resonant cavities in a pulsed mode.
Sullivan
Did you know about the radio astronomy group that was forming at that time?
Mills
At that time, it wasn’t. This was immediately after the War, and I became interested then in the physics of things more. I began to take an interest in high energy physics with other people in the group who were also interested in this. And this was chopped off at the end of ’46. It was obviously too expensive for Australia. So I was at a loose end. I started out with a thought of getting into digital computing which was just starting in those days. F. C. Williams had produced a digital computer in England and we were playing with the idea in Australia. So I, since physics was my background – so I played with that for a while. But then I was picked up as a TB case and put off for 6 months. I overcame it completely, but then that left me with a free field. And in this time, radio observations in radio astronomy had really got going and John Bolton had found some point sources – not identified then.
Sullivan
This was ’47 or so?
Mills
Yes, that’s right. Of course, Pawsey had observed bursts from the sun. And it all looked very interesting, so that was my introduction.
Sullivan
Let me just ask you before we go on any further; did you hear about Hey’s discovery of solar emission or any other discoveries of solar radio noise during the War?
Mills
Yes. We’d heard of the bursts of radiation from the sun which caused radar interference. And Pawsey was, in fact, quite interested in this and had done an experiment during those early days. Someone else may have told you.
Sullivan
During the War?
Mills
During the War where he stuck a simple parabolic reflector out of the window and looked at the sun and saw nothing because it was at 10cm. (laughter) But that was the stimulation, I think. Pawsey became interested and then at the end of the War he started things off.
Sullivan
I gather that there was just no time to try to do it a second time or follow it up?
Mills
Oh, no. He just looked and saw nothing.
Sullivan
Because, as you know, Southworth did detect the sun, but he probably had more sensitive receivers. Would that be true; do you think?
Mills
I don’t remember. What frequency?
Sullivan
Oh, was at 10, 3, and 1 cm. He had state-of-the-art receivers.
Mills
Well, I think they would have been comparable. We were in very close contact with the Americans in our equipment and all the microwave stuff came direct from America.
Sullivan
Do you know of any possible record of that experiment? Is it mentioned in any –
Mills
I don’t think so. Just a few of us knew about it. Frank Kerr might remember.
Sullivan
I think he did mention it to me – it’s been several years since I interviewed him. Ok, having come back in the field fresh. It wasn’t called radio astronomy then, what was it called?
Mills
Oh, I can’t really remember. I think we were a cosmic noise group and a solar noise group, if I remember correctly – the whole field. Everyone was thinking very hard what they could do. There was very little instrumentation immediately available. There were lots of questions. And I first became involved on the Sun by observing a solar eclipse. You’ve probably got the paper there.
Sullivan
That’s Christiansen, Yabsley, and Mills ’49.
Mills
Yes. You could get more details about that from Christiansen probably, because he was the main person to organize it.
Sullivan
Well, I haven’t talked with him yet, so maybe you could just tell me about what you did.
Mills
Well, I there was a partial eclipse in the southern part of Australia extending down to Tasmania, and what we did was to have – I can’t remember if there were 2 or 3 stations now – I looked after the local station and Christiansen and Yabsley went off. And whether they had 1 or 2 stations, I’m not sure. Tracking it from a different place further down south, so that we got intersecting arcs. By that mean, as the sun was occulted, we were able to get the separately placed arcs on it and locate the positions of radio emission, and we did find they corresponded to sunspots.
Sullivan
Now was this before the McCready, Pawsey, and Payne-Scott paper?
Mills
No, that was after it. But they had detected this big burst from a sunspot. But the question was what happened at high frequency, this was quite a high frequency -
Sullivan
50cm.
Mills
Yes. That’s right. Well, in those days that was a pretty high frequency. And the question was what happened there. We found that we had quite steady emission at this frequency from the sunspots. I think that was quite an advance at the time.
Sullivan
But the abstract said that you did find a few bright spots which had an effective brightness temperature of 5 million kelvins, so there must have been a little bit of activity.
Mills
There must have been, as we know it now. In those days, of course, we had no idea of what was going on.
Sullivan
Now things like this partial eclipse – was the group sophisticated enough, I’m not sure that is the right word, to be looking ahead to find out when there were eclipses? Or was it just someone reading the newspaper and saying -
Mills
I don’t know.
End Tape 61A
Sullivan
This is continuing with Christiansen on 26 August 1976.
Mills
Mills! (laughter)
Sullivan
Continuing with Mills! You were saying that you had just joined Christiansen’s group and so you weren’t part of the planning period.
Mills
Well, I hadn’t really joined his group. I joined the whole radio astronomy group and Christiansen, along with some others, was planning this eclipse work, and he asked me to join forces with him to look after the local station while he went off.
Sullivan
So I should just ask him.
Mills
Ask him, yes.
Sullivan
So what was the next step after this experiment?
Mills
Well, it rather whetted my appetite, and the question was what were the interesting problems at that time. I remember discussing this at some length with Pawsey, and I think it came down to two. One was a search for the hydrogen line. He was very interested in it at the time. And the other was trying to locate very precisely the positions of radio sources. And it was a difficult decision to make. I eventually chose the precise positioning because some of the techniques I was more familiar with, and it looked as if it was something that would lead to an immediate result, whereas the other was extremely speculative – whether anything would be seen at all.
Sullivan
You didn’t realize it was going to take you 10 or 15 years to get those precise positions.
Mills
Oh, actually we got some quite precise ones very quickly.
Sullivan
That’s true for the strong sources. But I’m interested about this looking for the hydrogen line. Of course, it had been predicted by Van de Hulst in his famous paper, and Shklovsky also had a paper about it which I suppose you knew about at that time, ’48 in the Astronomicheskii Zhurnal.
Mills
I don’t think at that time we did know about that Shklovsky paper. Only the Van de Hulst.
Sullivan
But what I’ve heard from various people is that, in fact, the Australian group was not racing with the other two groups. It was only once they’d gotten the word that it had been detected in Harvard and Holland that they then slapped together a receiver to -
Mills
Yes, that’s perfectly true. That’s what happened simply because I think no one really felt strongly enough about it to put the work into what we felt, or I felt, might be a rather speculative sort of thing. There was no certainty of the positive results. There were lots of other things to be done.
Sullivan
You knew that radio sources were there.
Mills
Yes.
Sullivan
Were the technical difficulties forbidding for the hydrogen line thing?
Mills
Well they appeared rather forbidding. One knew one had to get right down to the absolute maximum theoretical sensitivity because the thing was going to be faint probably. As I said, after some discussion it wasn’t clear that it would be above the detectable limit. So it was a speculative project from that point of view. And one knew that the Dutch were doing it too.
Sullivan
Oh I see.
Mills
I’m pretty sure we did know that.
Sullivan
Van de Hulst’s paper itself said this is only a possibility. He had not come out and said it’s a sure bet. Now the radio sources, what was the reason that you wanted better positions at that time?
Mills
Well, it was a matter of identification. I don’t think at that time there had been any clear positive identifications. I’m not even sure there’d been suggestions, but Bolton may have suggested the Crab. I’m not sure about that – it was just about that time that the first identifications were suggested. And it was clear that to make any progress we had to get onto these and get very accurate positions. I felt that the cliff technique, rising above the sea, was not a way to get accurate positions.
Sullivan
Because of the problems of the refraction?
Mills
Yes, that’s right. So at that time Little and Payne-Scott were constructing an interferometer, a very simple one with a couple of Yagis for observing the bursts in the sun and motions of bursts. In fact, they were the first, I think, to observe these bursts flying out from the sun. And this equipment was available and it appeared to me that with fairly simple modification, it could be used for determining the position of the strongest source we could see, Cygnus. And so I set about doing that as my first independent job in radio astronomy.
Sullivan
I see. Just before going any further, I’m interested in this talk with Pawsey. You listed two things that he suggested as possibilities. Was the idea that solar radio astronomy was important, but that was being taken care of?
Mills
Yes. Paul Wild was looking after that at that time. But in the other part obviously there were a lot of things which could be done and John Bolton was really the only one working on it at the time.
Sullivan
And also since I obviously can’t talk to Pawsey, I’m also interested in how he was thinking at that time. Is it possible for you to remember at all?
Mills
He was intensely interested in nature as such, and physics. He was one of the most dedicated people I think I have ever struck. As I said, he had become interested when he’d heard of the interference during the War, and this had stuck with him and he of course knew of Reber’s work by then. He was just interested in getting at the physics of all this, and finding out exactly just what was going on.
Sullivan
So he was much more on the physics side than the radio engineering side?
Mills
Yes. He was, really. He was a very competent radio engineer, particularly on antennas. But perhaps not quite so good on electronics. He was very competent indeed. But his real interest was in the physics of things.
Sullivan
So your first independent project, you said, was working on the position of Cygnus. And you and Thomas published a paper in ’51.
Mills
Yes. A long time after the results had been obtained, I’d say.
Sullivan
Any particular reason for the delay?
Mills
No, I don’t think so. It was all tied up with doing a lot of other things at the same time, and both of us were quite inexperienced in doing basic physical research – we were both engineers by training. And this took a bit of time to get used to.
Sullivan
Just how to write things up?
Mills
Yes. And we still wasted – wasted I think is the word – a lot of time on the on the ionospheric effects which were of considerable interest at that time. We did develop a sort of qualitative theory involving irregularities in the ionosphere which is, I think, quite sound as far as the qualitative theory worked, but we couldn’t get it on to a proper quantitative basis. I think that was one of the main reasons for the delay, and actually getting a paper written up.
Sullivan
Yeah, in the abstract it says that the fluctuations correlate with the F-region, the fluctuations in Cygnus correlate with the F-region.
Mills
Yes. So we did find that, but were also interested in actually constructing a theory of how it worked. And as I said, we were not eventually successful in producing anything decently quantitative.
Sullivan
So it seems like you were as willing to work on the ionosphere as on what we call radio astronomy now, anyway.
Mills
Well, both problems. Yes, I guess I was.
Sullivan
Maybe the distinction wasn’t so clear then.
Mills
It wasn’t. It wasn’t even clear that time when I started that the fluctuations were ionospheric in origin.
Sullivan
Right, well I’d like to hear more about that.
Mills
There was a big argument that they were inherent in the source. Of course this had been argued that this was a source of radiation that was fluctuating rapidly and therefore it was a point source.
Sullivan
That was in Hey’s original paper.
Mills
Yes. But people had realized, and I can’t say who or how, I know I had the (?), that the same argument applied whether the fluctuations were inherent or imposed by the ionosphere. It still must be a point source. We did some experiments. I remember we had some with John Bolton – these were spaced experiments to look for correlations at different spacings – and found none. I don’t think this was even written up anywhere – John Bolton may have, but it certainly showed that the –
Sullivan
It’s only sort of a footnote here and there, that I’ve been able to find on it.
Mills
Yes. It showed that it was clearly not in the source, but -
Sullivan
Were you aware of the work going on in England at this time on the same sort of questions?
Mills
Yes. Most of the publications were in Nature, of course, and we knew all those as soon as they came out. We’d also had some contacts with the Cavendish group.
Sullivan
Were you aware of the work going on before it got published?
Mills
Not at that particular time, although we were later. Bracewell was doing a PhD at the Cavendish at that time. He, like me, joined immediately on graduating with a Bachelor’s Degree, but he went off later to get his PhD in Cambridge. And so he kept us informed, you know, what was going on, and similarly, I think he kept them informed of what we were doing – he was one of the sources, and Pawsey himself had contacts. He was a very good friend of Ratcliffe, for instance, and there was a lot of correspondence going on there.
Sullivan
Do you know if Pawsey has any sort of archives or his widow or anything like this?
Mills
No, I don’t really know what happened to his things. Possibly in the Academy Library.
Sullivan
Well, let’s just look at the basic results of Mills and Thomas ’51. You got the position to 3 arc minutes, the flux measured to within 20%, less than 3% polarized, less than 10 arc minutes in size. Although that had also been established, I think before that time, by Bolton’s sea interferometer. Is that true, that is was less than 7 arc minutes in size or something?
Mills
Yes. I think that had been established.
Sullivan
And you say the parallax was less than 1 arc minute, therefore it must be outside the solar system, so this was apparently a distinct possibility the thing was a comet or something like that?
Mills
Yeah. Or before that it could have been anything (laughter).
Sullivan
Right. Now did these results – what sort of light did they shed?
Mills
There was one other interesting result there in that we pointed out the position of a galaxy which corresponded with this.
Sullivan
That’s right. This is the first overlay, isn’t it, in the history of radio astronomy?
Mills
Yeah. It also turned out to be the so-called colliding galaxy, but I didn’t positively identify it. I actually wrote Minkowski about this, pointing out the coincidence and in typical style Minkowski wrote back saying, “This was an ordinary 17th or 18th magnitude galaxy. It couldn’t possibly be the radio source.” And we just mentioned this in the paper.
Sullivan
I remember now. In fact, I’ve gone through this rather carefully. I don’t know if you’ve ever done this, but your position was actually off a bit more than your errors in R.A., I think. But your overlay also was a bit incorrect.
Mills
Yes, it was.
Sullivan
And this conspired such that it was still within your error box.
Mills
Well, in those days we used to use probable errors, anyway, which, of course, made things a bit easier to get coincidences. It was, in fact, I think, only some 2 or 3 probable errors off, but still a reasonable identification. It was pity, really, we hadn’t taken it more seriously and taken a proper photograph at that time.
Sullivan
That would have been a year or so before, I guess, the Smith position and so forth?
Mills
Yes.
Sullivan
So it was still a complete mystery as to what this Cygnus A thing was?
Mills
Yes. We felt that, and I think Minkowski helped here, it was probably unlikely to be a galaxy, so distant, because it was the strongest source and it was in the galactic plane. And these two made us think: “Well, maybe this was just a coincidence.” And I am sure this is what Minkowski thought about it too. It looked as if it was in fact a galactic source because of its position in the plane.
Sullivan
I get the impression from reading papers at that time that most people were thinking that these were galactic sources. For instance, in the identification paper, Bolton, Stanley, and Slee, they don’t seem to want to believe that [NGC] 5128 is a galaxy – they say it’s been called a galaxy, but so and so has said that . . . Is that right that people are fighting against the idea of these things?
Mills
The term “radio stars” was in common use. I think actually the Cambridge people were responsible for this concept. It caught on very strongly and there was a very strong school of belief that these were in fact stars. The small size – well the strongest, Cassiopeia and Cygnus, were out on the galactic plane – and things like this – made a lot of people think, in fact, they were radio stars. And I think we were also affected to some extent. I think I had a comment about it being unlikely that an object so large as a galaxy should be so unusual as to emit enormous quantities of radio emission, something like this.
Sullivan
Although, were you at the stage at this time when you were actually figuring out what the intrinsic radio luminosity of such an object would be? Were you that much into the astronomy of it, so to speak, or were you just qualitatively saying, “Well, it would be so far away that it - .”
Mills
No, I think it was more of a qualitative argument that it was a very, very large object consisting of very many stars, and it seemed in those days that everyone was thinking in terms of local physics, fairly small objects like stars, and it just seemed inconceivable that a large object could emit a great deal. We knew the Galaxy emitted radiation, but then the Cygnus one was obviously millions of times more bright than the Galaxy. It wasn’t clear - at all.
Sullivan
You were thinking of a nucleus or something.
Mills
Not at all.
Sullivan
This concept of “radio star” interests me. I was interested, for instance, to notice when I was at Cambridge looking through the theses that Edge’s 1959 thesis, which was the 3C survey basically, was titled “A Survey of Radio Stars,” even in 1959.
Mills
That was the dogma in those days.
Sullivan
But I mean then it was clearly recognized. Ryle himself, in the Bakerian lecture, had argued that these things were extragalactic, very strongly.
Mills
No, not in that. Wait a bit. I must get the dates. Oh, yes, by then, yes, that’s true.
Sullivan
And in ’55 he was talking about cosmology.
Mills
He had a complete reversal about ’55.
Sullivan
That’s right. But that’s what I’m wondering, yet the terminology, did the term linger on even though people realized they weren’t stars?
Mills
I would suspect only in Cambridge. We certainly didn’t. Right from the beginning, I think, I don’t think I called them stars. Ryle was weighted towards that belief. When I first joined the group, I sort of always had niggling suspicion that they weren’t, particularly in view of the possible identifications at that time with M87 and [NGC] 5128. Well, M87 particularly. I think that was one of the possible in the early days.
Sullivan
Oh yeah, there were three of them. Centaurus, Virgo, and Taurus. Let me ask about those three identifications, as to the Australian group’s feelings, as to their certitude, so to speak. Did it seem pretty definite that these were right?
Mills
Well, the Crab looked pretty good. Of course it was such a peculiar galactic object anyway, and it tied in with violent events. It looked as if it was probably good. The others, I think, were always a bit, we always felt in the back of our minds that they may not be exactly right. This is one of the reasons I continued on that idea and built bigger antennas with the idea of getting more accurate positions with a larger number of sources.
Sullivan
To pin it down?
Mills
Yes.
Sullivan
Now M87 wasn’t known to have, at least by you anyway, to have this peculiar jet.
Mills
No.
Sullivan
I think there actually was a photograph way back in 1918 that some optical astronomers knew of it. But I haven’t heard any radio person that knew that at that time. Continuing with Cygnus A, before we get to your first survey, there’s this famous set of three papers in Nature in 1952 by the Jodrell, Cambridge, and your group, on the positions and sizes of radio sources. Well, really more sizes.
Mills
I think that paper was on the size at that time.
Sullivan
Right, and you have Cygnus less than 1 arc minute, Taurus less than 4, Virgo less than 5, and Graham Smith has similar sort of things for Cas and Cygnus. I wanted to ask you, what did you think when these three papers came out about this asymmetry that Hanbury Brown and Jennison had with their entirely different technique? They came out strongly saying that the thing is very elongated.
Mills
Did they at that time? I don’t remember that.
Sullivan
Yes. They didn’t have the double quite yet, but they said that is was 4 by 1 or something like that.
Mills
I see. I’d forgotten that one. I can’t remember too clearly the details of that. We had just got our radio link interferometer going, and these were very much preliminary results with it. And the interesting this, as far as I was concerned, was that they all resolved. This said they were not stars. And I remember, in fact, at that time bringing this to Piddington’s attention, because he was still continuing to think in terms of stars. And I remember commenting: “Well these were quite clearly not stars.” We had to think in terms of galaxies rather than stars. That really convinced me.
Sullivan
And I guess Bracewell was back by then, or was he? Because he has this nice little paper in Observatory, “Radio Stars or Radio Nebulae.”
Mills
Oh yes, he’d come back and was discussing this with us at the time. And of course we had also measured angular sizes of some other sources, such as Centaurus A which we found to be very large.
Sullivan
And Fornax?
Mills
Fornax, too. This had been done with our other interferometer which we had constructed for the survey. Smith had built a three element interferometer for his measurement of Cygnus, but we also had this three element interferometer going, which measured these large sources. So we had a general picture, and I think this was what struck Bracewell when he came back, that our concept of the Universe was different from the Cambridge one. They thought in terms of point sources uniformly scattered over the sky, radio stars, in fact. And we had in the south, we had measured all these large angular sizes, and it was obviously much more complex than this. It has been one of the sources of the difference between us in our outlook, I think, that just by chance that the northern ones are smaller than the southern. (laughter)
Sullivan
A conspiracy of nature! But nevertheless, there were Centaurus and Cygnus, and Taurus, which Graham Smith himself measured to have finite sizes.
Mills
Cygnus and Cassiopeia. Centaurus is a southern object.
Sullivan
I said Taurus. I forget if he measured all three of those. I think he measured Taurus and Cas, it may have been.
Mills
And Cygnus.
Sullivan
Probably Cygnus.
Mills
No, he - I can’t remember, we’ll have to look that up. I know he measured Cygnus and Cas.
Sullivan
Anyway, is it your view that these were sort of looked upon as aberrations by the Cambridge group?
Mills
I don’t know. I can’t remember at that time. I think Cas and the Crab were definitely thought of as being galactic things, and I’m not sure about what they thought about Cygnus.
Sullivan
Well, I am going to try to get Graham Smith at this meeting too.
Mills
Yes, he may be able to help you on that one.
Sullivan
Well, what about this new intensity interferometer technique? Was that your first knowledge of it when you saw this one-page thing in Nature?
Mills
Oh, no. All those came out together in Nature. We’d heard about it quite some time ago when they were first getting going, when people had been traveling around and bringing back the news of what they were doing. So we knew they were doing this particular job. During the ’52 URSI meeting in Sydney, both Graham Smith and Hanbury Brown were there and we got together and talked about all these things.
Sullivan
These papers came out of that meeting?
Mills
Yes, that’s why they’re all published together. We organized a joint publication. We were not seriously concerned about the difference in sizes or shapes that we got, since it was obvious that we had only done a fairly coarse sampling of the stuff.
Sullivan
Although, I think that once again, if I had the paper here I could verify it, this is the paper where you had several spacings, didn’t you?
Mills
Yes.
Sullivan
And if you had put a couple more in, you could have gotten the double nature!
Mills
Yes, I could have. It was unfortunate.
Sullivan
Famous looking back, retroactively.
Mills
Yes, I did all my interpretations on the basis of some sort of Gaussian distribution.
Sullivan
Right. Well, if you had no other evidence, sure. But still, you haven’t said what you thought about this intensity interferometer. Did it make sense, to you, that this worked?
Mills
Oh, yes. I think we did understand roughly how it worked. I mean physically we knew it must work. I think we were a bit unhappy about sensitivities. Again, Hanbury Brown and Twiss had made were quite clear on this, it wasn’t a very sensitive instrument, they could only see the bright thing. We had discussed it during the URSI meetings, and there were no worries about it working at all. It was only the quantum physicists who were worried about it working, I think. The radio engineers were perfectly happy (laughter).
Sullivan
Right, there was quite a debate later on, as you know.
Mills
Yes.
Sullivan
Since you brought it up, let me just ask you about that ’52 URSI meeting in Sydney. Is it true to say this was held in Sydney in recognition of the Australian radio astronomy, or was it radio and ionospheric work?
Mills
Oh, I think I was too young and obscure in those days to know the basis of the meeting. You see, it would have been decided a few years before to hold it in Australia. But I think the ionospheric work would have weighed heavily, too, because Australia did have a pretty strong ionospheric school in those days. I think both together.
Sullivan
Do you have any recollections of that meeting as to the sort of atmosphere was there? It must have been an exciting time. Things were popping all over.
Mills
Yes. It was exciting. Oh, I have quite a few odd flashbacks here and there. I don’t know that they’d be of any great use. I remember discussions with Graham Smith and Hanbury Brown about some of their work. About some of the ionospheric details, ionospheric fluctuations and how they worked. I remember Hanbury visiting our angular size interferometer, radio link interferometer, having to be convinced that it would work all right, give the effects of very large separations between the antennas and the delays which had to be corrected for, and things like this. It was a very pleasant time. The main thing that I got out of it, I think, was meeting, for the first time, a lot of these people. Of course, we’d been very isolated in Australia and corresponded with a few of them, but not actually met them before.
Sullivan
Well, that’s an important point, I think. You’ve mentioned a couple of people like Pawsey and Bracewell that had mixed, but most of the Australians had not travelled. I guess Frank Kerr is another case of one who had gone to the outside world, so to speak. He went to Harvard in ’51 for a year. He was actually there when the hydrogen line was discovered.
Mills
I’d forgotten that.
Sullivan
Would you say, that this, perhaps, contributed to Australian radio astronomy developing rather independently? If it had been a European country, it might have - ?
Mills
Yes, I think it did. We did have, I think, quite an independent outlook really. We had no famous names to tell us what we should believe, and we just went ahead following our noses to some extent. And we did have quite a good technique. This was the main thing, it got things going. Of course this is a pretty obvious statement. Lots of people have made it, I think. Our radio techniques were well up to anyone else, because our radar as a result of the War was an international subject in which practically all countries, were not exactly equivalent, were at least moderately so.
Sullivan
Right, although radio astronomy did not take off in the U.S., where there was as much technique and much more money.
Mills
Oh, it could have.
Sullivan
It could have, right. Do you have any ideas as to why it didn’t?
Mills
I think you’ll have to ask the Americans (laughter). I have no idea. I know why it did in Australia; that was because we had the Radio Physics Laboratory which had now its raison d’être was finished. It was no longer required for radar, at least for war-time radar. We had to find something to do in a hurry. I think these reasons led Dr. Bowen, who was then head, to press strongly for radio astronomy once Pawsey had got it going. Because this was a field – you see, we might have got sucked into doing standards for television or something horrible like that. We had a lab which was well set up for doing radio work of all kinds, and it was just a matter of trying all the things which we could. You see, this linear accelerator I mentioned earlier was one of the things we had a go at as being a possible good line of development. Again, Bowen pushed this very hard. And it was only when it was realized that the expense would be too much for us that we dropped it.
Sullivan
Although, of course, you had the Radiation Lab at MIT. If that had kept going I suppose -
Mills
Yes.
Sullivan
I suppose it was some governmental decision that this would – it was disbanded incredible quickly. Maybe that could be blamed in some part.
Mills
I think that probably did have a large part to do with America not getting onto the astronomy so quickly.
Sullivan
Let me pick you up on that comment about you didn’t have experts to tell you what to think. Because it seems to me that there were as many experts in radio astronomy in Australia as there were in England. Is this not true?
Mills
Oh no, no. I was talking about experts in astrophysics and astronomy.
Sullivan
Oh, in astronomy. Well, who exactly are you referring to in astronomy?
Mills
I think the classical astronomers generally had rather tended to hold back radio astronomy. And this is what I had in mind.
Sullivan
Oh, I see.
Mills
Because the general idea was that they knew what was in the sky and you shouldn’t try and look for things which are not there.
Sullivan
Right, and if you can only tell me where they are within a degree, don’t bother me.
Mills
Yes. It’s not worth their doing it. Though that was a very common outlook. But we were less afflicted by it than many other groups, I think.
Sullivan
Although, obviously some groups just didn’t let this bother them.
Mills
Oh yes. Well, they were the groups that forged ahead.
Sullivan
Well, let me ask about the whole radio link technique, as I think you were the first to develop this, were you not, in -
Mills
I think so, yes.
Sullivan
Interferometry – can you tell me how this came about?
Mills
Following on our measurements on Cygnus, which were done with a couple of Yagis, or rather three Yagis now that I come to think of it, we did have three. There was one with a short spacing for resolving ambiguities. We decided we had to have larger area and so I constructed the next system. At this time Thomas, who had been working with me, left for England. Arthur Watkinson, as a technician, came in to help me on this. We got some land quite a long way away. We had to abandon the field station we had because it wasn’t large enough and there was too much interference. So we went out to Badgery’s Creek, which is a little bit further than our eventual Fleurs Station which we still have now.
Sullivan
What was the name of the first one you were at?
Mills
Potts Hill Reservoir. This was about 30 miles west of Sydney – the nearest we could find. And the reason we chose it, basically, was that it was on CSIRO property, on one of the experimental farms. So we set up there with a much larger, but still by modern standards quite small, antenna system – a system with a few arrays of dipoles – and a fairly complex calibration system, which again followed on the technique we had been using before, which had originally been developed by Payne-Scott and Little. We did our surveys there plus position measurements. But while we were doing this we found these larger angular size sources and the question arose naturally that there’s undoubtedly (?) distribution of sizes, and we knew of possible identifications and we thought things several minutes of arc would be the sort of sizes we’d be interested in. A “minute of arc up” sort of thing. There was this debate about whether they were radio stars or radio galaxies, and quite obviously the way to determine this was to build an interferometer which could distinguish whether things were smaller or greater than about a minute of arc. This was the sort of thing I was going to do. Of course, when you look at the spacing – we were still thinking in terms of low frequencies which was all we could use in those days because of the sensitivity questions – obviously you needed spacings up to about 10 km.
Sullivan
You were becoming aware of the steep spectrum, I suppose, at this time?
Mills
Yes. We had already – Bolton and Slee had done some measurements on spectra. Well, although they disagreed again with the Cambridge one, I think, they showed a steep slope and we believed those. So it was clear one had to keep at low frequencies and you had to go to a very large spacing. And it was equally clear we couldn’t afford the cable to do it. I was quite familiar with the techniques necessary to make a radio link, so I just looked into the problems and decided that the worst problem was that of getting a stable compensating delay for the long path links to the distant antenna. There my experience in digital computing was very useful because our computer, following on the original British design, had mercury delay lines for store. So I just borrowed a couple of these delay lines and used them as a variable delay for compensation. So everything sort of fitted together on that one.
Sullivan
Were you using these computers at all in your reduction of data?
Mills
Oh, no. They were purely experimental. They’re very slow by modern techniques.
Sullivan
But still faster than your hand.
Mills
Oh, very much faster, yes.
Sullivan
So, do I have it right then? It was just a matter of expense, really, that made you go to the microwave link? Not a matter of too much attenuation over such a long distance?
Mills
Well, it was also the practical convenience of it. If we wanted to try different spacings and different places, then obviously you couldn’t go coiling and uncoiling miles of cables. The radio link was the obvious way of doing it. As I said, I knew the techniques were adequate and the only real thing I was worried about was getting this compensating delay which I realized was necessary. Even that wasn’t too certain in those days, because I remember looking at how narrow the bandwidth had to be in order to get fringes over the spacing, and decided just too narrow. So it occurred to me to put in this delay.
Sullivan
These sorts of things were not written in textbooks?
Mills
No, never. Or in papers. It had to be thought out right from scratch.
Sullivan
So what was the first thing that was done with this radio link interferometer?
Mills
Our first measurement was a very short baseline just to see if it would work. On all the stronger sources we got it going on a short baseline and we stepped out, first, I think to a kilometer, then, I think, to 5 km., noting that three stronger ones, the Crab, M87, and [NGC] 5128 had all dropped off. They were dropping off at 1. They were completely invisible at 5. I’m not sure how much they’d dropped off at 1m., I’d have to look at the paper to remember that. And then at 5 km., Cygnus A dropped off quite a bit, whereas at 1 km there’d been no drop off. So, it had a constant visibility to 1 km. – these are just orders of magnitude, of course on Cygnus A – and then a big drop by the time we got to 5 km., and by then the others had been dropping off much more rapidly. So that was it.
Sullivan
What were you using to calibrate your fringe amplitude?
Mills
We injected noise, if I remember correctly. I can’t remember the exact calibration technique used in the first system.
Sullivan
It seems to me this would be a problem.
Mills
Yes. Well, yes and no. You see, we were not after an accurate measurement.
Sullivan
Well you still want to know whether it was .7 or .9 or -
Mills
No. that wasn’t necessary at all. What you want to do is take it out until it drops off to zero or drops off a long way. And that gives you the size. Now, we had the original Michelson concept on this. It was a matter of fringe visibility and if one waits until it goes right down, then it doesn’t matter very much about the calibration. I know we were not worried about it and I don’t think we had anything very elaborate.
Sullivan
Ok. And that was the paper – the first thing that was published on it was one of those three papers in Nature in ’52?
Mills
Yes.
Sullivan
And the more accurate, or the more complete, work was also in ’52 in the Australian Journal of Scientific Research, I suspect that’s right.
Mills
No, it was ’53 when the main radio link one came out, I think.
Sullivan
Six accurate positions, was that from the survey?
Mills
That was from the survey.
Sullivan
Ok, I’m confused then. Well, to keep things in chronological order here, let’s go to your first survey then.
Mills
Yes, well that, in fact, had been done before the angular size work. That was done around about 1950.
Sullivan
And published about the same time as the first?
Mills
Yes. You see, that first angular size work wouldn’t have come out at all at that time but for the stimulation of the ’52 URSI meeting and our decision to publish all our results together. And they were very preliminary. It would have been this ’53 one alone which we published otherwise.
Sullivan
Let’s go back to the survey with the Michelson interferometer, which was the first southern survey, basically. Well, is that right?
Mills
No, Bolton and Slee.
Sullivan
Bolton and Slee, that’s right. They’d done – they had how many sources roughly – 22 or something like that?
Mills
I can’t remember the number.
Sullivan
But you had many more anyway – 77.
Mills
Yes, that’s right.
Sullivan
Well, I guess the first thing to ask is what was the difference between your two surveys?
Mills
There was very little contact between Bolton’s group and mine. We each felt rather strongly our own technique was the best. Although we saw each other sometimes, Bolton lived out at Dover Heights and didn’t come into the Lab very often and I spent most of my time out at Badgery’s Creek. So we didn’t have very much contact actually. And there were quite a few arguments about interpretation of the results and things like that.
Sullivan
I see. And Pawsey was quite willing to let both these go on and see?
Mills
Oh, yes. He thought it was a very healthy situation, I think. So this was quite independent of anything Bolton had done. I can’t even remember when he did his and when he published it.
Sullivan
It was about the same time.
Mills
Yes.
Sullivan
So it was about the same frequency, 100 MHz?
Mills
Yes, I think about the same frequency.
Sullivan
And neither of you did – I can’t remember if you did in print anyway – a comparison of these surveys?
Mills
No. He may have. I know we had written ours up well before he had written his up. At least I think we did. At least before his were published. The survey results were prepared independently, basically.
Sullivan
Ok. Well maybe I’ll have to do that comparison.
Mills
The thing which we did agree on was the fact that these sources had angular sizes. We in fact found this first at Badgery’s Creek, [NGC] 5128, and then Bolton found the same sort of thing on his sea interferometer and we agreed at least on that part of it. My memory is that we did our survey and got our results out and largely written up before Bolton had got back onto his. That could be wrong, I wouldn’t stick by it.
Sullivan
Now what were the main results of this survey?
Mills
Two things: the positions, which I’ll talk about later, and the actual results of the survey itself. And there, I think, the important things were that we recognized that there must be two classes of source because of the strong sources being distributed along the plane of the galaxy and the weak ones isotropically. Now at that time we couldn’t be sure that the isotropic sources were extragalactic, but we felt pretty confident that they were. And I think if you read the paper on that survey that I wrote, you’ll find that I was being rather careful not to commit myself completely to one or the other. I was leaning towards the other because of the fact of these identifications. I think that was the important thing that came out of the survey itself. And I know it was debated quite a lot in England by people who had gone overseas – Pawsey was often touring around, for instance. At that time the extragalactic-galactic argument was building up, getting quite strong and heated at times.
Sullivan
Now these were the class 1 and class 2 sources that you refer to?
Mills
Yes.
Sullivan
These two distributions. This terminology, I think, was used throughout the ‘50’s, wasn’t it?
Mills
Yes, it was.
Sullivan
Now the 1C survey came out a bit before yours? You probably saw that.
Mills
Ah yes, that was one of the early ones -
Sullivan
It was published in 1950.
Mills
Was it? We may have had the results a bit earlier – I think we did actually. I know that about the time I was working on this, there was the Ryle survey – Ryle and Elsmore, I think it was.
Sullivan
That’s the one, right.
Mills
Yes. And there was Bolton and Slee’s initial survey – both were available.
Sullivan
But now, Ryle, Smith and Elsmore, I think it was, came out with an isotropic distribution. Why did they not come up with - ?
Mills
Well, that actually is one of the sources of our whole difference in outlook. They had missed the strong sources near the plane because they were using wide-spaced interferometers. So therefore, they divided the sky into a slowly varying component and point sources. Whereas we, by having our close spacing, were able to see these -
End Tape 61B
Begin Tape 62A
Sullivan
This is continuing with Mills on 26 August 1976. So you were saying that having the small spacings you were able to pick up -
Mills
Yes. It was obvious that along the galactic plane there was a lot of complex structure of quite small scale, but obviously not as small as would be picked up by the wide-spaced interferometer which had a resolution of a fraction of a degree. This was the basic cause of our arguments at that time with Cambridge. In fact, I understand that the Cambridge people denied the existence of extended sources along the galactic plane – you should talk to the Jodrell Bank people about this probably. They know more about it than I do. Anyhow, that’s the situation.
Sullivan
So you’re saying that even before the debate was most intense, namely the Mills Cross survey vs. the Cambridge survey – even before the Mills Cross, that there was this difference in philosophy on extended sources?
Mills
There was an argument then on completely different grounds, because the Cambridge people then believed that the majority of the isotropic sources which they’d found were radio stars at that time, and that there were no other strong sources along the plane. We were very open-minded about this and believed rather that the isotropic ones were extragalactic and that there was a very strong component of extended sources along the plane. And this was a considerable source of debate. And a completely different subject, though from the next one.
Sullivan
Well, the point I’m trying to make is, do you think this is correct, that in both debates that you were concerned with the short spacings, whereas, Cambridge was neglecting them essentially. So you got more extended -
Mills
Yes. I think that’s right. This survey was actually the basis for the Cross because – I realized that it was necessary in any survey to have an instrument which would respond to close spacings and large angular size structure. Otherwise, one would simple miss it and miss a lot of the information available in the sky. And it was as a result of this survey that I thought of the Cross as being the sort of thing on must use. One must use pencil beams for survey. That was the basic idea I had in mind.
Sullivan
We won’t have time to get into the Cross today, but let me finish up with this paper which was an extension of your survey in which you had six accurate positions in ’52. You confirmed the three I.D.’s that had been suggested a few years before, and also, had Centaurus and Fornax as being the first large angular sized objects, I think, that were detected. Were there any surprises in this? Were you surprised to find these half-degree size things?
Mills
Well, in a sense, as soon as we switched on our close spaced interferometer, we realized we had them. It was the first thing that hit you in the eye. We’d look at the same part of the sky on a close spacing and we’d see an enormous interference pattern and in the wide spacing a small one. You couldn’t help seeing it.
Sullivan
But how could Centaurus be something that’s a half-degree or degree, and you’re trying to identify it with this one arc minute Galaxy?
Mills
No, no, Centaurus A, the visible Galaxy, was about 6 or 7 minutes of arc. And we did know that things extended out beyond it. Although we didn’t know that -
Sullivan
Optical, you mean?
Mills
Optically, yes. I think at that time de Vaucouleurs was in Sydney and we had discussed with him quite a lot this question of optical extensions. What I didn’t know then was, in fact, that this 6 or 7 degrees, or even more across – these two big blobs on either side – existed. Our interferometer only gave about 20 minutes of arc equivalent size.
Sullivan
So, you were still lacking the shortest spacings that were needed?
Mills
Yes, I was.
Sullivan
So it wasn’t a matter of sensitivity. It was a matter of missing –
Mills
Missing Fourier components. Yes.
Sullivan
So then from this, there still was no conflict with the problem, which is still with us today. Namely, why is the radio emission out here when the optical object is here?
Mills
No, it wasn’t at that time.
Sullivan
That really didn’t come, I guess, until would you say, Jennison and Das Gupta’s double thing in Cygnus was really the thing?
Mills
Yes, that was probably the first time that that became obvious.
Sullivan
That was ’53 in Nature.
Mills
No, I think, before that it was clear that the emission in Cygnus A must extend quite a lot longer than the sort of 1 minute of arc which I had obtained.
Sullivan
Well, that’s true. But then, of course, you might expect at least that it would be concentrated on the optical object.
Mills
Yes. Jennison and Das Gupta, as far as I can remember, was the first indication that you could have a main concentration of emission way out from the Galaxy.
Sullivan
Although I was talking with someone, I can’t quite remember who now, who said even though that result was around from ’53 on, there were a lot of people who just did not believe it. Were you one of these?
Mills
No. Around about that time we realized that this sort of thing was observationally reasonable and it did fit my results, too. All I had done was to miss the zero point.
Sullivan
Those missing spacings you were talking about - ?
Mills
Yes, the zero point in the middle. I myself was quite happy with it and saw no reason to doubt it whatsoever.
Sullivan
But you weren’t particularly worried about radio physics of galaxies?
Mills
Oh, no. Well it was obviously exceedingly interesting as to how this happened. I can’t exactly remember what time I began really to understand what was happening. I think it was about that time. My interests were first, on the theoretical side, stimulated by Fermi’s paper on his accelerating mechanism of cosmic rays, and I had that in the back of my mind since probably before 1950 when I read that. But it wasn’t until Shklovsky’s paper came out that everything made sense and fell into place.
Sullivan
Yeah, there’s one of your later papers that talks about abnormal galaxies as radio sources, Observatory ’54. And you say that 5128 probably is not colliding galaxies, or it seems unreasonable. About this time then, you must have been thinking about the physics.
Mills
Yeah.
Sullivan
This is a good place to break, I think. We’ll have to continue. That finishes the first session with Bernie Mills on 26 August ’76.
End Tape 62A, end Part 1
Begin Part 2 of Tape 62A
Sullivan
This is continuing with Mills on 26 August 1976. So we’re looking at your first large survey in 1952, in which you define the Class 1 and Class 2 sources and you were just telling me about this paragraph on page 271.
Mills
Yes, where I had stated that for the purpose of compiling a list of sources and for subsequent analysis, a discrete source has been defined in terms of a particular kind of pattern produced on the pen recorder, etc. This indicates the sort of operational definition I’d been forced to adopt because it was clear that we had different patterns on our two interferometers. The wide-spaced interferometer in general didn’t always have the same pattern as the close spaced one. So obviously you were not dealing with a single point source all the time. So one had to make an operational definition. This, I think, is a source of a lot of the disagreement between Cambridge and us. That they were not forced to a sort of definition by working at one spacing only, they thought in terms of single discrete sources. The other thing, of course, was the repeating patterns we got, which indicated resolution as a problem. So that, again, one separated this out in terms of periodic patterns on the recorder and you defined each pattern as a discrete source. But I did want to make it quite clear, in here, and may not have used quite sufficient emphasis – that it was purely an operational definition.
Sullivan
Right, I can see that now that you point that out. Looking towards the end of this same paper you have a log-n log-s in the middle of the paper. Then at the end, you talk about whether they really are extragalactic they would be of great importance to cosmological theory. And you were telling me before about how you spent a lot of time. Could you tell me about that again?
Mills
Well, I took a quick glance through to see just what I did say. It was a long time ago. But, I think the point was we had a whole set of different observations. We had these isotropic weak sources, concentrated strong sources, and we had some possibility of identifications with extragalactic objects. We had a possibility of identifications with galactic objects. These set a series of constraints on the sort of theory which you could have. It took me a very long time to sort this all out in my own mind as to what was permitted by the logic of the whole system.
Sullivan
And you said you actually set up a logical flow diagram?
Mills
Yes, I did. I had to use this at one stage.
Sullivan
You think there’d be any hope of digging that out? That would be of great interest to see that.
Mills
I don’t think so. I have a habit of – when I was thinking about things like this – to scribble them on bits of paper. It could possibly be in one of my old notebooks. I doubt it, though. I probably just threw it away.
Sullivan
But anyway, the result of all this thinking was with what, would you say, a sort of leaning towards the idea that they’re probably - ?
Mills
I think I felt at that stage that they were pretty certainly extragalactic objects – galaxies, in fact – there was no mention or thought of quasars at that time, of course. I felt I had to check this, and this lay behind the angular size interferometer, the radio link interferometer – this was a way of checking, really, whether they were all extended like galaxies.
Sullivan
Now going on to this one, Mills ’52, in which you have six sources with very accurate positions. I’m just looking at specific questions I may have scribbled here sometime in the past. You refer to calibration and refraction effects. Now when you say refraction, do you mean scintillation also, or do you just mean what we today would call tropospheric refraction or is it tropospheric and ionospheric?
Mills
I was thinking definitely in terms of ionospheric refraction. In those days, at low frequency, you could practically ignore the tropospheric effects.
Sullivan
Ok, and scintillation, I think you used the term “random refraction” here. Well, you say ‘the random errors due to refraction effects – that’s what threw me. I normally think of that as a systematic error.
Mills
No, I was thinking in terms of the results we had obtained before, I’m sure now, looking back on it – our Cygnus results. We had obtained apparently slow variations in amplitude which we had attributed to refraction effects. It was probably this I was thinking of here, that these could give apparently systematic changes in the amplitude of the interference pattern. And it wasn’t clear to me that this may not be worse at wide spacings, and this would depend on the model one had, I think, of the ionospheric irregularities.
Sullivan
So it really was a sort of scintillation. You call it random refraction because the source was jumping around.
Mills
Yes, I’m pretty sure that’s what I meant.
Sullivan
And here I’ve noted that you have corrected the position of Cygnus from Mills and Thomas, which was 3-1/2 seconds of time off. Apparently, sometime in the meanwhile you -
Mills
Oh, this is the position of the actual galaxy.
Sullivan
No, slightly wrong, this is the radio positon.
Mills
Oh, yes. The nebula was slightly wrong.
Sullivan
For Fornax A, it turns out that the right ascension is a good bit off and I just want to ask you if you can remember why that was? It turned out to be 55 seconds off.
Mills
Oh yes, in fact, this prevented us making the identification of [NGC] 1316, although we knew the possibility of that identification for quite some time. In fact, I remember Bolton had noted a similarity between that and [NGC] 5128, which he’d read of in on of Shapley’s papers. So we knew about the possibility of an identification with [NGC] 1316. This RA was rather surprising because I was thinking that it would turn out as an identification. The reason is quite clear when you look at the distribution – this demonstrates the difficulty of dealing with interferometers when you have an extended source. Fornax A is a double source and although the centroid is nicely on the galaxy, in fact the fine structure is more distributed on one side than the other. So that the position is weighted towards the position of the fine structure. We know now what’s happened, but at that time it wasn’t clear.
Sullivan
You also mentioned in the paper that this was the weakest source investigated, which didn’t help either.
Mills
No, that’s right. The pattern was not clear. There was a bit of confusion in the pattern.
Sullivan
Well, let’s take a look at this one [1953] AJP on four extended sources. I guess we haven’t talked about this at all. Maybe you can tell me about this experiment in which you made some simple assumptions in order to get the brightness distribution.
Mills
This was an extension of the work, of course, described in that earlier Nature paper, using the same instrument, and merely trying large numbers of different orientations and baselines.
Sullivan
The entire Fourier theory I guess was pretty apparent as to how one would -
Mills
Well, that goes back to Pawsey and Payne-Scott, McCready.
Sullivan
Yeah, that’s right, that paper.
Mills
It was basically Pawsey’s idea. So we were very familiar with that.
Sullivan
Here’s your portable equipment. Was that still Army surplus, or were you buying new stuff by that time?
Mills
We were making a lot of it by then so I can’t really be sure. I don’t think it was Army surplus by then.
Sullivan
Are there reasonable prints of these things still around CSIRO?
Mills
I think it’s probably quite likely there are, yes.
Sullivan
Because I’d like to get hold of some of those. Now, you have a figure here in which you compare photographs with what we’d today call ‘radio-graphs’ of Centaurus, Virgo, and Taurus. Virgo is a little bit confusing, I guess, because its E0 or E1 and you came out with a more elliptical thing.
Mills
This elliptical thing is now known as the halo of Virgo A.
Sullivan
Right. But Centaurus and Taurus really did have basically the shape of the galaxy.
Mills
Which was actually wrong in the case of Centaurus, of course, because again, by having insufficient spacing, I’d missed the double nature of the central source. So that what I came out with was quite the wrong direction. It should have been going at right angles to my distribution.
Sullivan
To the one you had, right. But that was with only three or four spacings that you were trying to make these models?
Mills
Yes. Of course, there was no real indication then that one needed a lot of spacings to sort things out.
Sullivan
Well, here they are here. It looks like for Virgo you have about eight. Let me ask again… there are some interesting wiggles in Centaurus.
Mills
There are, certainly (laughter). Actually what had been missed there was that the visibility function crossed over into the negative region. If I had picked that, it would have made a completely different model. But it did show, grossly speaking, the sizes were equivalent and, in fact, the shapes turned out quite well for the Crab and for M87. Centaurus was missed.
Sullivan
Let me ask more explicitly about this Fourier theory because it’s of interest for me to know whether it was quite clear to you, as you went around to these various places and all, exactly what you were doing. In that if you had had enough time, did you realize that you could have done the whole thing properly?
Mills
Oh, yes. That was known, but it wasn’t considered to be technically feasible.
Sullivan
And why not?
Mills
Because of the phase problem. Simply, the amplitude information was only useful if one had a completely symmetrical system, and we didn’t think it likely that we had that. We felt we had to have phase information if we were really going to plot things. I didn’t think it was technically feasible to carry the phase information at the large spacings and get high resolution.
Sullivan
Did you ever try it, actually?
Mills
Not really, no.
Sullivan
The next thing I come to here is the whole Mills Cross and you mentioned briefly yesterday how the idea came to you. That you really felt that you needed a pencil beam. Could you explicitly say that the Cross and the phase in-phase technique was not obvious at that time, I don’t think.
Mills
No, it came out of our three-element survey. That’s the one we were talking about earlier. There were two things which came out there, I think. One was this difference in the responses of the closed-spaced interferometer compared to the wide-spaced interferometer, which showed we had to have all Fourier components, as we now say. The other thing was we thought that there were resolution problems because we had lots of beating patterns which showed that sources were not being observed individually. I think these were two of the important things. Then the technical thing which suggested to me the use of the Cross was our attempts to sort things out- sources out – when they were beating together by pointing our antennae in different directions so that we were getting the product of the two diagrams. I was familiar with that at the time and it’s mentioned somewhere in the paper that we did this. We used to put out, for instance, a zero on one of the sources and we’d try and -
Sullivan
This was radar technique also, or was it - ?
Mills
No.
Sullivan
Didn’t they used to put an object on the side of beam in order to -
Mills
Yes, that could have been. I don’t remember how it came about that I was using this, but I think I had simply noted that you could cut down the response of a source by putting one of the antennas near a minimum.
Sullivan
Well, that must be this remark which indeed I was going to ask you about. “A similar method of using a common portion of overlapping beams to produce a narrower effective beam has already been used.”
Mills
Yes, that’s right. That was what we were doing. And this really suggested it to me. I think Cambridge were also thinking of this sort of thing at the same time. Obviously, because it came out later in a paper by Ryle.
Sullivan
Well, that was ’52 also. Yours came out in ’53, actually.
Mills
Well, this paper came out in ’52, You see, we had been playing with this sort of technique. And, in fact, the whole idea of the Cross was worked out in ’52.
Sullivan
Did you know about Ryle’s whole phase switching thing while this was going on?
Mills
Yes, we knew of the phase switching technique. This was used in the survey which we had done, in fact, it was mentioned there that I used a phase switch similar to Ryle’s. This was a survey done in 1950 and 1951. So that was a great advance at the time, and we recognized that and we went over entirely to the switch system. And so I felt familiar with how it worked. The things which appeared were that to do surveys you need a pencil beam and I spent a lot of time thinking first of large arrays because it was also obvious that one needed high resolution. And then it did occur to me that you didn’t need all the area of the arrays because you were confusion limited. All those ideas were sort of worked out in ’51, ’52 and I can’t remember exactly when. I can’t remember precisely when the idea of cutting out most of the dipoles in an array and just using a cross came to me. But that’s how, in fact, I thought of it, in terms of an array.
Sullivan
Then it didn’t need to be a filled thing -
Mills
Then I realized that just having a Cross and taking total power was useless, because of the wide responses. And it suddenly came to me, well, I can use the same sort of technique I’ve already been using and just multiply one against the other. That was sometime in ’52 that the idea came to me. So it didn’t really come as a basis of an interferometer, thinking in terms of interferometers, but thinking in terms of pencil beams. I was quite convinced this was what was needed for a survey.
Sullivan
Going back to this comment about using a common portion of overlapping beams. You say that it was used in the survey, Mills ’52, yet I don’t think there’s any mention in that paper.
Mills
Yes, there is. I’m sure you’ll find a mention there somewhere for sorting out beating patterns by pointing antennae in different directions.
Sullivan
Ok, I’ll take a look at that. Well, then you built a model of this. I guess it wasn’t clear that it really was going to work.
Mills
No, there was considerable internal criticism, I’d say. (laughter)
Sullivan
What was the nature of this?
Mills
No, I had to convince the head of our laboratory, Bowen, it was worth putting any money into it.
Sullivan
But why wouldn’t it be?
Mills
I don’t know. He didn’t believe it would work. I can’t, for the life of me, now remember the argument used, but it was something to the effect that if you had two separate antennas, they must give a null result over the sky, so you’d have no – it wouldn’t give any response at all, or something like that. I can’t really understand it. I couldn’t at the time, but I had to get the word to build one.
Sullivan
And so you did and I guess it worked famously.
Mills
Oh, yes, very nicely.
Sullivan
And then came the business of trying to get the full thing built. And I guess you were able to convince Bowen then?
Mills
Oh, there was no doubt about it. After all, we observed the Magellanic Clouds for the first time with that thing – the small one.
Sullivan
Well, since you brought that up, was that a publication?
Mills
No, I think it’s just mentioned as a by-product in this ’53 paper. [Turning pages]
Sullivan
You have a thing of the sun here. Here is one figure showing the observations of the Magellanic Cloud.
End Tape 62A
Begin Tape 62B
Sullivan
Continuing with Mills on 26 August 1972. How much did this large Cross cost, if it’s of interest?
Mills
Oh, very small, around about – perhaps 10,000 pounds or less. But it was constructed almost entirely in the laboratory so it was very difficult to cost – I mean labor, the work force there just worked on it. But the external cost was less than 10,000 pounds. I should have said there was a little more to it than this in building the first experimental one. I had to convince people it would work, but there were also a number of basic problems I wasn’t quite clear about myself which I wanted to experiment with, principally on this matter of the intersection of the two arms, and what was the best way of dealing with it.
Sullivan
How they interact with each other -
Mills
Yes. The experimental model was very useful from that point of view.
Sullivan
And what about the grading? Was that obvious as to how to do that best?
Mills
No. There were several possible ways of doing that. Well, it was just a matter of thinking them out and deciding which in the end would be the easiest to make and to maintain.
Sullivan
Perhaps this is a good place to ask you about how you went about building something like this instrument. Was it a matter that you sat down and designed it, and, more or less, supervised the entire construction of it? Or was it a matter that you were out there with people having fabricated things but you had to be pouring cement and all this sort of thing?
Mills
No, actually I got out of most of the hard physical labor involved, because I went overseas for a while, while it was being constructed. The original design I did almost entirely – the electrical design I did almost entirely alone. The mechanical design, just the physical construction of the thing, was done by McCallister who was our mechanical engineer in the place. I got all the major things sorted out at that time and then Alec Little joined the group. Oh, no, he’d been working before. He did a lot of the test equipment development for it. We were sort of working together on this. He was developing equipment to set it up while I was doing some of the basic design work. Then when we had it all figured out and I had this opportunity to go overseas for about six months, and at this stage most of the bits had been made and the assembly had been starting by our workshop at Fleurs Observatory – it wasn’t an observatory then, just an open field. The actual setting up of it and the adjustment of it was Alec Little’s responsibility and he did a very good job on this. He, as I said, designed all the test equipment which we used for setting up and getting the currents right and the phases right and things like that. We had discussed it all beforehand and we had a pretty good idea of how it was going to work but he was responsible, actually, for getting that all set up. When I came back the thing was almost complete, as far as I remember. Still some adjustments to be made and things like that.
Sullivan
It must have been a pleasant sight?
Mills
Oh, it was a very pleasant sight.
Sullivan
What determined the overall size of the thing? Was it just constraints on funds or was it the size of the field or was it something astrophysical?
Mills
Well, the astrophysical requirements set the minimum size of the thing. It had to be beyond about 1,000 feet, something of that order. The frequency was chosen on the basis of where we thought the maximum sensitivity for point sources would be. In fact, there’s a bit of discussion of that in a later paper in the astronomical issue of the Institute of Electrical [i.e. Radio] Engineers ’58 -
Sullivan
Ah yes, that special issue.
Mills
Yes, there’s a long discussion of the basic design -
Sullivan
The American Proc IRE?
Mills
Yes. And, there’s a long discussion of all the basic design factors which were considered and things like that in that paper. So we had a minimum size from the astrophysical requirements. The maximum size wasn’t so much money, but the amount of effort we were prepared to put into it. And I had originally hoped to have it a bit larger, something around about half a degree, but it became pared down a bit with time until we finished up with a beam width of about 49 arc minutes.
Sullivan
You say the frequency was chosen for maximum sensitivity and you were at 3-1/2 meters, but why not go to a lower frequency?
Mills
In that paper I mentioned earlier, I have a discussion of this – at the moment I can’t quite remember what all the various factors were. It was connected with resolution, too, of course. If you go to a lower frequency the antenna has to be larger. You get a balance between resolution and sensitivity, particular frequency, particular receivers. There was a very wide maximum but roundabout 80 MHz seemed to be about the right frequency to do it.
Sullivan
Were you thinking at all in these years that by observing at only very low frequencies that you might be biasing the nature of the objects you were finding – that there might be lots of other types of sources -
Mills
No. It was purely (?)
Sullivan
Which you would pick up at 800 MHz or something?
Mills
Yes, well it was known that the sky would undoubtedly look different. We didn’t at that time, I think, seriously consider that there would be sources stronger at a high frequency than they were at a low frequency.
Sullivan
Although Taurus was sort of puzzling.
Mills
It was more or less flattish. Everything we knew, except for HII regions, was falling off at high frequencies. The reason we chose a low frequency, of course, was simply that the receivers in those days – the high frequency receivers – were very poor.
Sullivan
Can you tell me about your six-month trip away from Australia? Where did you go?
Mills
This was spent in two places – at Caltech, which was the basic reason for me going there, to learn some basic astronomy, which, I felt, I was sadly in need of in those days. Caltech was the center, of course, and this was the obvious place to go to talk to Minkowski and Baade. I’d been corresponding with both of these. And to look at the sky survey plates.
Sullivan
Which was in progress then?
Mills
Yes.
Sullivan
When was this?
Mills
This was in ’53, about mid-53. It was shortly after the publication of our first Cross paper. From my point of view this was a very fruitful part of my life because I could stop worrying about instrumental developments and sit down and start thinking about astronomy, astrophysics, and physics generally. I spent about three or four months there and another three months or so at the Department of Terrestrial Magnetism, which I looked on more as being in the nature of paying for my visit since they were just becoming interested in Radio Astronomy. And I, in fact, did help them out on a lot of technical matters. It was also very valuable too, of course, to work in different groups.
Sullivan
This was with Tuve.
Mills
Yes, Merle Tuve.
Sullivan
And was anyone else with this?
Mills
Well, Graham Smith was overlapping me there – he spent some time there.
Sullivan
I guess you were the one that probably influenced them to build the Cross, and I think Graham Smith actually finished it up.
Mills
Well, no. Actually they had seen my paper before I came over, and had decided to go ahead and make a Cross. So that when I came over, in fact, they were already starting on this. I didn’t really contribute very much on that Cross part at all. The decision had already been made.
Sullivan
I’m interested about your visit to Pasadena. Can you tell me what Baade and Minkowski were thinking at that time? What they were teaching you, essentially?
Mills
It was all colliding galaxies in those days, of course. There were a lot of discussions about probability of collisions, and how many there should be. There were some worries because there didn’t seem to be enough collisions for the radio sources which were observed. There was very little physics in that part of it. It was purely looking for abnormalities in the galaxies which might be identified with radio sources. But how the physical connection, no one had a clue about. The mechanism wasn’t known then. I think I mentioned before I’d had some suspicions that it was connected with synchrotron emission after reading Fermi’s paper on acceleration of cosmic rays. And there were one or two papers, I think Herlofson produced one -
Sullivan
Alfvén and Herlofson and Kiepenheuer.
Mills
Yes, and Kiepenheuer too. Although somehow I missed his. I don’t know how I missed Kiepenheuer’s paper.
Sullivan
This was back in 1950. They were both in Phys. Rev. within a couple of months of each other.
Mills
Yes. Well, it was odd but I was aware of the Alfvén-Herlofson one. I had a sort of feeling this was probably the reason, but I didn’t know how enough astrophysics then.
Sullivan
Did you know about the Russian work that was going on in ’52 and in ’53?
Mills
No, I knew nothing. I didn’t strike that until later. I can’t remember where I first struck that, but as soon as I saw it, everything clicked. That was it.
Sullivan
You were a believer then?
Mills
Yeah, I was a believer. (laughter)
Sullivan
So it sounds like from what you’re saying that Baade and Minkowski were really the only two at Pasadena that were interested in radio sources. Is that fair to say?
Mills
Yes, I think so. Greenstein had a bit of an interest, too. But they were the two fundamental people. As I said, we were all searching. Then, as to just -
Sullivan
Was Hubble still alive when you were there? He died in ’53, I think.
Mills
Yes, I met Hubble actually. I had a short conversation with him and he died while I was at DTM, I think.
Sullivan
Was he going along with this or was it outside of his realm?
Mills
He was interested in an off-hand sort of way. He wasn’t professionally interested. Whereas Baade and Minkowski were really working hard on it, and devoting all their efforts at that time to these problems. We just didn’t know – it was a question then of, for instance, whether Population II objects were connected with it because of M87, an elliptical galaxy. Or whether Population I -
Sullivan
Because of all the dust and Centaurus?
Mills
Yes. This was one of the things which worried me because I had this tendency towards thinking in terms of synchrotron mechanism – just in the back of my mind – but it seemed to me M87 was the thing which really killed that. And also 5128.
Sullivan
Because it looked like such a tranquil galaxy?
Mills
Yes. 5128 also appeared to be an elliptical. So it looked as if these were elliptical galaxies and in fact, as if these might be associated with the radiation in old stars. These were a lot of the things we discussed at the time. One of the first things I did with the Cross when it was going (if I can anticipate a little bit) was to look for emission in globular clusters to see if that particular problem could be resolved.
Sullivan
What about Hanbury Brown and Hazard’s work on showing that spirals had strong continuum radiation?
Mills
Well, we knew the Galaxy did.
Sullivan
Well, M31, then they began to pick up more and more spirals.
Mills
I can’t remember the exact dates of these. This picture was fairly clear that we had normal galaxies like the Milky Way which we knew produced emission. M31 produced it. Others you’d expect to produce it. But these were thought of as normal galaxies. What we were worried about was this very abnormal emission.
Sullivan
Right. And were they beginning to be called “radio galaxies” then?
Mills
I think they probably were. I can’t remember exactly when the term arose. You’d have to look through the literature to find that.
Sullivan
Well, this probably explains the source of your short article in Observatory in ’54 entitled “Abnormal Galaxies as Radio Sources.” You talk about Fornax and you say, “5128 may not really be colliding galaxies.” I guess that was just what you’ve been saying now that you had this feeling.
Mills
Yes. This was written after I came back from my American visit, and after the Cross had started working and these were some of the first results. I was working through some of the ideas which I had picked up in Pasadena. I can’t really remember that particular paper. I know that it was written about that time, but I can’t remember what was in it.
Sullivan
Do you think it’s fair to say that the collision hypothesis was sort of grasped on because there didn’t seem to be anywhere else to get such an amount of energy – kinetic energy?
Mills
Yes, that was it, certainly. And, in fact, there was no doubt in ’53 between Baade and Minkowski that they were dealing with colliding galaxies. In fact, Minkowski paid off a bet of a bottle of Scotch to Baade as soon as they got the spectrum of Cygnus A.
Sullivan
Right, I’ve heard that. When did they become converted on that score?
Mills
I can’t remember the exact sequence, but more information was coming in all the time and I think, in fact, 1316 and 5128 did go a long way towards checking them on this. Because it was very difficult to really get a consistent colliding picture. And there was also another northern multiple source, I can’t remember which one it was now, which Minkowski originally thought of a colliding but then he found several different velocity groups in it, and this just looked very peculiar. I mean, you can have two velocities with things colliding, if you’ve got more, it suggests something expanding.
Sullivan
Was this Perseus A?
Mills
Yes, it was.
Sullivan
Of course we still don’t understand what’s going on.
Mills
That’s true (laughter). But, I think that was, perhaps, the first thing which shook Minkowski.
Sullivan
But why do you say Centaurus didn’t fit in? Because when you took spectra you didn’t find two sets of lines? I don’t remember in Centaurus whether you do or not.
Mills
The disposition of the lines, I think, didn’t really suggest two separate colliding systems. I can’t remember the details of it.
Sullivan
Ok, moving on - a short article in Observatory in ’56 on the radio source near the galactic center, which you apparently were interested in. What was the feeling about this radio source?
Mills
Well, there’d been a number of papers produced by the high frequency people – NRL had done a lot of work on the galactic center source. So obviously this was something that one should -
Sullivan
Also, I think Bolton and somebody had a paper, didn’t they, on the whole galactic center region where they mapped it?
Mills
Low resolution map, I think.
Sullivan
Right, I guess you really wouldn’t see the source.
Mills
No, it didn’t show up at all. So we looked at it with the Cross. And as soon as you look at it, you saw that you didn’t have one source flat at the center, but you had two sources straddling the high frequency source. This was obviously an intensely interesting result. It was one of the few occasions I think where I sat down and write a paper very rapidly as soon as we had the contours worked out. It was clear that this non-thermal emission was arising on either side of the thermal emission. I didn’t believe some of the distance analysis which had been made at the time because it was obvious to me that this thermal source which the high frequency people found, you couldn’t really estimate the emission from it with any degree of certainty. Because the baselines – base levels – were so uncertain there. I felt this must be at the center of the Galaxy because everything was symmetrical. I went ahead on that basis.
Sullivan
There was a paper by, I think, Bolton and McGee on Sagittarius A and I think it was about ’53 or so. It was definitely earlier than ’56.
Mills
Yes, that’s right.
Sullivan
But I can’t quite remember what it said.
Mills
I can’t remember what it said either. I remember, though, it was observations using a hole-in-the-ground type of antenna with a moveable feed, but also Piddington had done measurements at high frequency too. So the galactic center was an interesting thing. I mentioned Haddock and the NRL people because at that time they had the best observations.
Sullivan
Of the high frequencies?
Mills
Yes. In the Australian group there had been observations of it before then.
Sullivan
So it sounds to me like in the mid-50’s there was quite a bit of controversy then as to what the nature of this thing was. There were arguments that it was only 2 or 3 kiloparsecs away and other arguments like yours.
Mills
Yes. As far as I remember the short distance arguments did originate in the NRL group.
Sullivan
From HI
Mills
And from HI.
Sullivan
I guess that may have been a little bit later. But I think they had a distance even before they got the HI absorption.
Mills
Yes. Based on, I think, the ratio of the emission to the background, or something like this. I can’t remember the argument.
Sullivan
Ok, let’s clean out a couple of these others before we get into the whole MHS survey. Absorption of 3.5-meter radiation in 6357 which was an HII region. According to the abstract, said that it was the first ever seen absorption, but hadn’t Ryle and Scheuer shown that the whole galactic plane absorbed, in ’52 or ’53?
Mills
No, that was an inference rather than a direct observation. And the inference was actually wrong because there were a large number of emission regions. In fact, this was the only absorption at 80 MHz, that is the only obvious absorption.
Sullivan
Anywhere in the Southern - ?
Mills
Along the galactic plane. Eventually we found others, but widespread absorption sets in at very much lower frequencies, around 30 MHz. So that, in fact, was one of the other sources of argument between Cambridge and Sydney.
Sullivan
Well, I’m going to talk with Scheuer sometime this week, so I’ll see what he says about that.
Mills
You’d have to look at the original papers to sort that one out because I can’t remember the details.
Sullivan
And you, in fact, Mills, Little, and Sheridan in ’56, you published looking at 14 bright nebulae – I guess they were all HII regions and detected 6in. emissions and got one in absorption. And you were deriving electron temperatures of 10,000 K, so it seems like you’re really getting into astrophysics here.
Mills
Yes, we were.
Sullivan
How did that come about? You didn’t learn about HII regions at Caltech, did you?
Mills
Oh, I learned about everything at Caltech. I had also, of course, been reading astronomical literature. I think the Caltech, Pasadena visit was mainly a gelling of a lot of things and gave me more of a feeling for the astrophysics of it.
Sullivan
Ok, here’s another paper in ’56 Mills, Little and Sheridan, in which you look at two supernovae and 10 novae and only detected one supernova, namely, Kepler’s. And yet you make a suggestion the galactic background may just be the (?) integral of many supernova remnants which is a rather interesting comment. Were you thinking of these remnants as actually being the source of energetic particles perhaps?
Mills
Yes. At that time the picture was simply that the supernovae were going off and what you were seeing was the integrated result of old remnants which had expanded.
Sullivan
Well, you say the picture. I don’t think everyone accepted that. This is more of the Russian line, tying in cosmic rays and background radiation -
Mills
Yes. I think around about that time I became – my outlook was quite close to the Russian. As I said, I was very impressed with the Shklovsky work and the interpretation of the Crab Nebula and things like this. I think I was just generally thinking along similar lines.
Sullivan
Another one here on looking at bright galaxies in which you detected 10 and the Magellanic Clouds – more than the detected, but mapped them out – and you made a model with a Type I distribution, I guess that meant Population I, and a spherical halo. I guess this was under the influence of the astronomy that you’d learned also, I mean about Populations and such.
Mills
Oh, yes. I was by then quite familiar with all the latest ideas in astronomy and astrophysics. There’d been a lot of work on Populations and formation of the stars in the early days of the galaxy. I remember Martin Schwarzschild was at Pasadena at that same time and he was thinking of these things, too. And there were numerous colloquia which I attended. As I said, it was terribly fruitful. I learned all my astronomy in one year.
Sullivan
That was your graduate education in astronomy, essentially.
Mills
Exactly, yes. It put me right in the forefront of thought at that time.
Sullivan
Were you thinking of the two Populations being analogous to your Class I and Class II?
Mills
No, in this model -
Sullivan
No, that’s not right, I withdraw that question.
Mills
Obviously for the Population I emission, there were supernovae and HII regions and things like this. There was also a halo then, which came directly from the observations. In the paper you’ll see some sections through the Galaxy which did suggest this. And this had been suggested by Shklovksky, of course, and I rather looked on these observations as confirming this picture of a more or less spherical halo. But the Population I component he had attributed entirely to thermal emission in his model and I think I pointed out that this was non-thermal as well as thermal. That was the difference.
Sullivan
I see. So are you saying that in the maps of these nearby bright galaxies you could actually see a hint of what we now call a disk and a halo?
Mills
No. I could see that in the Galaxy, in the Milky Way.
Sullivan
And maybe the Magellanic Clouds?
Mills
In the Magellanic Clouds there was no evidence of a halo. When you plotted the emission, you found that the spirals, the Sb spirals in particular, seem to have more emission than some of the earlier type galaxies. There seemed to be a maximum. You had irregulars like the Magellanic Clouds with low emission – the radio to optical ratio was low. Things like the Milky Way and M31 and other normal spiral galaxies seemed to be high. When you got to the ellipticals, it dropped right off again except for the odd radio galaxies. So it appeared that, this again, I thought, was dividing it into Population I and Population II objects and it did appear that you had to have a combination of them to produce the emission in normal galaxies. I discussed this to some extent in my paper on the “Observation of Normal Galaxies.”
Sullivan
And then your observations of these other galaxies was consistent with this picture but didn’t really establish the picture.
Mills
Yes. [It appeared, in fact, that strong emission requires large mass (Population II component) and a Population I component. I think that was a suggestion I made to get radio emission in significant quantities. Tap garbled and difficult to understand]
Sullivan
In mass?
Mills
Yeah, well, that seemed to be the thing that correlated with it. I think you’ll find that discussed in one of those papers.
Sullivan
We come along to Mills and Slee ’57 which is the first installment of the MSH survey which I guess was the main thing you always had in mind to do with the Cross. Or is that true? Was it your main idea to really do a super survey?
Mills
Yes. A survey -
Sullivan
But you had all these other projects on the side?
Mills
Yes. We were interested in galactic structure and the survey in the beginning and the survey, of course, takes more time. That’s why the other ones came out first.
Sullivan
Can you tell me how this survey proceeded? Was it pretty straightforward?
Mills
Yes, each night we did one strip of the sky and moved on the next night, interspersing this with observations of individual things which came out in the earlier papers. But the basic bread-and-butter work all the time – when it wasn’t doing anything else – it was on the survey.
Sullivan
Were there any ambiguities as to how this data should be reduced – not interpreted, I mean, but reduced?
Mills
There were lots of discussions as to what would be the best way of reducing it. This was determined in the end, I think, by our facilities. But I don’t think there was any real – we didn’t consider two possible ways and decide between them. It was just a matter of gradually working out what was the best thing to do.
Sullivan
Could you just describe the procedure?
Mills
I’ll try. We first set up – calibrated everything before the observation started with noise diodes and things like this. We got an observational run through. If you look at the paper, you’ll find that the actual records consisted of a series of interleaved observations at different declinations. We then employed girls to trace these out and give us a series of scans at neighboring declinations. We had five beams at a time. Yes, here’s a typical record.
Sullivan
Figure 1.
Mills
Actually some of the other earlier papers showed it better, with sections through the Galaxy and things like this which showed larger sources and so forth. But each of these scans on here, vertical lines, represents an integration and the peak value of a scan represents a value of the integrated emission over that time. And we simply switched from one declination to the other at the five declinations and back again. So that every fifth point there represents the emission at that declination. So we had to separate these out and we did this with the aid of girls, being very cheap to employ in those days.
Sullivan
And this is what you got in Figure 2, then?
Mills
Yes, that’s right. Then we looked through these looking for bumps. This is basically what we did. Most time it was obvious, sometimes it wasn’t. The ones which were not obvious, of course, caused all the difficulties and problems.
Sullivan
How did you handle them if it was not obvious? What were your criteria?
Mills
Dealing with the obvious ones first, we’d draw a baseline under it. We’d get a deflection on different declinations and a time of maximum (time of maximum would determine the RA) and by fitting these to an exponential beam – we had a little formula for doing that – the ratio between them would give the declination. So we got our positions that way and we got the flux density by drawing a baseline underneath and measuring the deflection above that baseline. That was the basic technique. And when we had troubles, when things were apparently extended, we know now that a lot of these extended ones were background irregularities, we thought at the time they might be, but we included all these in the catalogue because again we didn’t know what we had to look for at that time. And any concentration of emission of any sort appeared to be worth cataloguing. And we were not necessarily thinking of cataloguing only point sources, but all concentrations of radio emission.
Sullivan
Once again it was sort of an operational definition – “a bump on the record.”
Mills
Yeah, that’s right. If a bump were extended, we would proceed in the same way and obtain an integrated flux density rather than just a deflection and again give a position and we’d give the ratio between the point and the integrated flux density.
Sullivan
Did you think about trying to make a little contour map if they were extended?
Mills
We did think of it and we, of course, did so for specific sources, but we didn’t in our catalogues. It was just too much labor involved. All this was manual reduction. No computers available at all.
Sullivan
No computers even up to the late ‘50’s?
Mills
We didn’t have any available to us. The one which had originally been developed at CSIRO in radio physics, which I mentioned earlier, was abandoned. It was decided that that wasn’t an appropriate field for radio physics to deal with and so that was sent away and we had none. Computers didn’t become available in Sydney probably until about ’54, but they weren’t available to us then.
Sullivan
Now, these extended sources, of course, are critical to the whole argument. Were you thinking that a goodly number of extragalactic sources might well be extended also, because you were thinking of Population II and this sort of thing?
Mills
Well, it was obvious these could be either galactic or extragalactic. We tended to think that most of the extended ones were probably extragalactic, and now this is probably wrong – they were probably galactic, I would say. And either way, of course, they must be included in any statistical analysis. Account must be taken of them because they are there.
Sullivan
Well, something must be done about them.
Mills
Yes, that’s right. One could eliminate them entirely or else – but since one didn’t know whether they were galactic or extragalactic, the reasonable thing to do is to do statistics on them and see how these statistics turned out. And see if that would indicate anything, which is what we did.
Sullivan
Were you worried at all about blending of two point sources, essentially?
Mills
Yes. Well, we knew that some of these would be blended sources and we worked out the statistics of that.
Sullivan
As to how many could be?
Mills
Yes, and we found that there were many more, so we thought there may be extra blending. You see, at that time, we knew about radio galaxies, we knew there were clusters of galaxies, so we thought there was a distinct possibility that there would be physical associations between several radio sources. So we felt that a lot of these extended things – you see, our basic resolution was ¾ of a degree, and we couldn’t resolve very much. So we thought it was highly probable that some of these extended sources did, in fact, represent blends of radio galaxies.
Sullivan
This survey began about when?
Mills
It would be ’54 that it actually began.
Sullivan
Well, you talk about Pawsey going to the Jodrell Bank Symposium and appearing to contradict the Cambridge observations. Was that the first inkling that you had that things were looking rather different in the two surveys?
Mills
Yes, that was the first we heard of the Cambridge 2C Survey – at the time of this report of Pawsey. I don’t know when that was, ‘55, yes.
Sullivan
And was this a very small section of the total survey which came out – submitted in late ’56. How did this come about? I can’t quite remember – did you see the published 2C catalogue?
Mills
No, not at that time. I just remembered something. Actually, before we heard from Pawsey, I got a letter from Fred Hoyle asking or mentioning a very great excess of faint sources which they had obtained at Cambridge, giving a slope of 3, minus 3, and asking what did we get. We had rough results then. It was obvious we couldn’t possibly have a slope of minus 3, so I wrote back saying this. And then this was formalized more in the Jodrell Bank Symposium that Pawsey attended. But, the first indication, I think, was a letter from Fred Hoyle.
Sullivan
I see. Do you think that letter could be dug up?
Mills
No, it doesn’t exist, I’m sure. Well, it might be possible. I’ve got some old files there; I’ll dig through them.
Sullivan
I’d appreciate it. But anyway, so that was your first indication that you were getting rather different results. And how were you able to make the detailed comparison? Did you have that before publication?
Mills
Yes, I think Pawsey organized this while he was over there. They sent over a section of the catalogue and we reduced – I’m not sure how this was organized, whether I had actually told Pawsey, “Well, we’ve nearly reduced this area, see if you can get something on that.” I think that’s actually what happened. Anyway, we had this result and -
Sullivan
It wasn’t published yet?
Mills
Ryle may have sent it, I can’t remember. We certainly had it before – it had been submitted at that time. All the work was completed. It was just a matter of sending a sort of preprint of the thing.
Sullivan
And what did you think when you had a few hours to look at this?
Mills
It was obvious that Cambridge were not listing radio sources as such. Because one could take an area where there were some quite strong sources in the Cambridge catalogue, look at our records, and see that there was absolutely nothing there. And so it had to be explained. Now I was already quite familiar with the effects of resolution then. So the explanation was obvious. We did a bit of simple figuring on the beam areas available and it was clear that they were listing about one source per beam area.
Sullivan
Right, which you point out in this paper. Of course, you explicitly show the 2° by 15° Cambridge beam and your 1° beam approximately. You say it was obvious, but how did the Cambridge group go wrong?
Mills
I really cannot understand that. They knew about the resolution problem, I’m quite sure, at that stage. In fact, I had discussed this with Graham Smith way back in ’52. How they could possibly have interpreted their records as being isolated discrete sources, I simply cannot understand. You ask them. (laughter)
Sullivan
Well, I have, but I’m trying to get opinions all around. I don’t know, maybe it was something to do with a sort of hunger for a great number of sources to do cosmology or something.
Mills
Yes, that could be.
Sullivan
There’s more in science than science sometimes.
Mills
Yes, if one has some preconceived idea, then an observation which apparently confirms it will often be accepted without very great criticism.
Sullivan
I don’t think that’s fair to say that there was a preconceived idea because it’s my understanding that when Cygnus came along… Well, I should ask you before I say what I was going to say. When you started off on this survey, were you interested in cosmology? Did you see this as something useful that could be done with the survey?
Mills
Not when I started on the first one. That’s the ’51 survey.
Sullivan
Not that one. Right. But, I meant with the Cross.
Mills
By then I knew that one had a chance of doing cosmology. I certainly knew that when we started with the Cross survey.
Sullivan
But what was the reasoning of it?
Mills
Well, you were dealing with distant sources and they were at cosmological distances. The identifications showed that they were very, very faint things. Strong sources were very faint optical objects. Obviously it was relevant to cosmology. We all knew that.
Sullivan
The Cygnus identification really is the key to it, being the weakest optically.
Mills
But on the other hand, we knew that 5128 and M87 probably wouldn’t have very much cosmological interest. And it was just this appreciation that there was undoubtedly a very wide luminosity function.
Sullivan
So when you started the MSH survey then, you were thinking that it would have a cosmological use?
Mills
That it could. I didn’t have any ideas that it would turn out to have such an enormous cosmological impact from the Cambridge viewpoint.
Sullivan
Did you actually sit down beforehand in terms of the design of the survey at all and say, “Now, in order to test this cosmology or that, I need so many sources per - ”?
Mills
No, not at all. The design of the instrument and the planning of the observations was based on what we had observed before in the radio sky entirely.
Sullivan
I guess you felt compelled to come out with this small part of the survey.
Mills
Yes, we did. We had some correspondence about this and Ryle was quite unshakeable and I know Pawsey had argued with him about this. His opinion was that Ryle, again, was quite unshakeable. (laughter) So we felt the only thing to do was to do a thorough analysis, as far as we could, of comparing the two catalogues, as being the only way of sort of bringing things to a head and establishing -
Sullivan
Any of that correspondence would be extremely interesting, if that could be located.
Mills
I’ll see if I can dig anything out. But as far as I remember it, my efforts were met with no reply. I remember sending one letter to Ryle, for instance, saying, “Well now, here are our records of this area where you have these sources shown. There are no sources there. Could it be resolution effects?”, or something like this. And I never got an answer. We were a bit fed up with the Cambridge attitude at this time, I might say.
Sullivan
Well, it comes through in this paper.
Mills
They just ignored us. (laughter) So we went ahead and did what we felt had to be done.
Sullivan
Well, related to an earlier question I had, do you think it possible that if you’d been working at Oxford that this could have gotten straightened out?
Mills
I think it probably could. Although, I think Jodrell Bank had difficulties straightening things out with Cambridge, too. (laughter)
Sullivan
Well, they never conflicted so much on direct observational things.
Mills
No. I’m sure if we’d been able to take our records along and put them down on the table and look at their records -
Sullivan
But now someone had told me, I think David Edge told me, that you actually came up and did this in connection with the 1958 Symposium. This is getting a bit later.
Mills
Oh yes, that was later. We did that then. At this earlier stage, our results were, as far as we could determine, just completely dismissed by the Cambridge group. So we had no alternative but to come out with this sort of paper.
Sullivan
What was the result when you finally did get up and were able to lay your records out?
Mills
I think by then the Cambridge people were fighting a rearguard action on the 2C survey, and were starting off on the 3C. I think they appreciated the detailed criticisms. I’m sure they had all abandoned it by then.
Sullivan
So this was mainly to look at 3C.
Mills
Yes, that’s right.
Sullivan
Well, as you’ve already hinted at, this paper doesn’t mince words – “It is found that the survey is almost completely discordant,” a couple of other good quotes in here. But the paper largely seems, rather than talking about your survey, I mean you do analysis of it and so forth, but rather it’s much more emphasized towards showing that 2C is wrong.
Mills
Yes. This was necessary at the time, I think. Because there was enormous publicity attendant on the 2C.
End Tape 62B
Sullivan
Continuing with Mills on 26 August 1976. You were mentioning that there was quite a bit of publicity about the 2C survey.
Mills
The cosmologists appeared to jump on it, of course. The general picture that came back from the ’55 Jodrell Bank Symposium, as relayed by Pawsey – and I think Bolton was there too, if I remember correctly, I’m not sure about that, but I know Pawsey was there – was that it was creating enormous interest and if it were taken seriously, of course, the implications were really very strong. We knew that it was quite strong. I had perfect confident in this, although one didn’t have any great confidence in some of these weak sources that we had. It was quite obvious that the Cambridge 2C survey was strong at a very much higher level than ours could possibly be. So it obviously had to be straightened out. We couldn’t get through to the Cambridge people and this came strongly, as reported by Pawsey. And of course I discussed this with him again as to what was the best thing to do, and we decided on this critical paper.
Sullivan
Did Pawsey bring along strip charts and try to - ?
Mills
I can’t remember whether he brought any back. He didn’t have any of mine.
Sullivan
That’s what I meant.
Mills
No.
Sullivan
But no you also mentioned that Ryle had a popular article in Scientific American about this thing and you felt compelled to write a Letter to the Editor.
Mills
Well again, I think it was Bart Bok who suggested I do that. I’m actually not too happy about getting into public controversy, but this was one thing which was obviously terribly important. I felt, and other people felt, it should be straightened out. Hence, both the reply to Ryle in my letter to Scientific American and bringing out this paper. Though this paper was well advanced, of course, at the time of that letter and exchange in Scientific American.
Sullivan
Right. This was submitted in the same year as the other one was published. Now did this survey get a lot of publicity in Australia or elsewhere?
Mills
Oh, yes. People were using it quite a lot, I think, at that time. But it’s known publicity came as contradicting the 2C, I think.
Sullivan
Right, but that’s what I mean. Did it get fair press?
Mills
Yes.
Sullivan
Ok, let me just got through the paper and ask you a couple of specific questions. You say, “Reliability of better than 90 percent of greater than 20 Janskys (as we would now say) – these should all be included and completely reliable.” Your catalogue actually has a greater number of sources between 7 and 20 Janskys.
Mills
Yes, that’s right. Again, the purpose of the catalogue was to produce a catalogue of all concentrations of radio emission and this was the thing which we were aiming at. We were not aiming at making sure we were cataloguing only radio sources as such, point sources.
Sullivan
So you were just giving the reader the whole bit of data and then just saying -
Mills
Yes, “I would choose to cut at about here.”
Sullivan
You do a lot of analysis here on the effects of confusion in extended sources. And you end up saying that some 90-odd percent of yours are reliable and 98 percent of the Cambridge ones are not reliable, which, not quibbling about a few percent, basically turned out to be right.
Mills
Yes.
Sullivan
And here’s the great difference in log-n/log-s. Although I think that even your 20 Janskys in retrospect turns out to be a bit optimistic.
Mills
As far as the point sources were concerned, it was ok. But there were a lot of extended sources which, in fact, turned out to be background irregularities and some, I think, though, I don’t know how many, were, in fact, background irregularities.
Sullivan
You talk here about clustering of sources and you think that you have a .02 result here, a 2 percent chance of it being random. Was that just a statistical fluke?
Mills
I think so.
Sullivan
Did you try to do this later with the entire survey and it didn’t pan out?
Mills
Yes, I didn’t get any significant results. It was just a fluke.
Sullivan
And here you talk about the problem of the background variability, and you’re trying to estimate how much noise this would add.
Mills
No, the basic idea was in effect a crude “p of d” analysis. All I was doing was measuring the rms background fluctuation and interpreting it in terms of the integrated effect of many sources. And this led to this sort of cosmological model.
Sullivan
And you come up with a density that turns out to be about a thousand times that of Ryle and Scheuer. I figured that out.
Mills
I think I suggested one possible cosmology which would fit, but I realized all this must be terribly preliminary and it was just a matter of seeing if it was reasonable or not, which it was.
Sullivan
Yes, here’s: “We therefore conclude that the discrepancies in the main reflect errors in the Cambridge catalogue and accordingly deductions of cosmological intent derived from its analysis are without foundation.”
Mills
Yeah.
Sullivan
But, now here’s something philosophical – “An analysis of our results showed that there is no clear evidence for any effect of cosmological importance in the source counts.” But that’s not true because -
Mills
No, it is true.
Sullivan
If you say that they’re extragalactic and you have -
Mills
Of cosmological importance. There were cosmological effects. But I don’t think – we couldn’t demonstrate anything of cosmological importance in our counts.
Sullivan
One could argue that this established that these were a Euclidean Universe.
Mills
No, we merely say that we didn’t go far enough to distinguish between different universes. Every universe you start off counting looks Euclidean. And it’s only when you go to weak sources that you get cosmological effects coming in. This was my thought.
Sullivan
I see. So through your analysis you had determined that you really didn’t think that you were going deep enough, as we would say?
Mills
Yeah. Or, there may have been a population of more local objects, too.
Sullivan
Still extragalactic but - ?
Mills
Possibly. Anyway, local shall we say. One couldn’t distinguish in those days. You could say everything was distant extragalactic objects, but then we knew that some were not. We knew that M87 and 5128 were quite close extragalactic objects. We had no clues about the luminosity function. There were not sufficient identifications to give a hint, except for Cygnus A which suggested there were some very strong ones. These may have been a very small proportion. And if things were all like M87 and 5128, one wouldn’t have gotten any cosmological information. We got nothing statistically different from a Euclidean slope and the fluctuations in the background were such as could be explained by a fairly local population of extragalactic sources. So that we didn’t have anything really significant.
Sullivan
Did I hear you right though, when you said that there was still, in the back of your mind, some thought that they just might all be local within our Galaxy?
Mills
Not all. There could be a dilution.
Sullivan
Or a goodly fraction?
Mills
Yeah, I still kept that as a possibility at that time. I didn’t have any strong feelings about the necessity for them being cosmological or local at any time, I don’t think. Not until we started doing lots of identifications and it became clear you could account for practically everything by extragalactic sources.
Sullivan
Another technical question. Were you aware at this time that the steady state universe, in fact, would not predict 1.5 for the slope?
Mills
If you take them sufficiently local it does!
Sullivan
I mean if you put in the redshift effect, for instance?
Mills
If you’re looking at distant sources, again, no cosmology with redshift would predict 1.5 and I knew that.
Sullivan
Ok.
Mills
But it was a question of where this turnover occurred. That was the crucial thing.
Sullivan
I guess the thing that I’m learning from you that I didn’t realize is that you were really thinking that they were probably extragalactic, but that you weren’t going deep enough to do cosmology.
Mills
Yes, that was my thought.
Sullivan
I hadn’t quite seen that in the paper before. And, therefore, you only say a couple things about cosmology. You don’t try to put forth a whole cosmology of your own.
Mills
Yes.
Sullivan
You brought up “p of d.” Now, the Cambridge people, in general, will say after two or three years they began to have to admit 2C was pretty bad. But they hung on to “p of d.” It seemed to be right and it seems to me, and I haven’t gone into every bit of analysis, that indeed history has shown that it was basically right. Would you agree with that, first of all?
Mills
Well, the measurements were obviously right, but they were open to several interpretations in those days.
Sullivan
But I mean that the whole concept of a “p of d” analysis was a valid procedure.
Mills
Oh, yes. I think Minkowski put that very well when he said that “The ‘p of d’ carried information, but very low-grade information.” And that, in fact, was our feeling.
Sullivan
Was that at the Paris Symposium?
Mills
No, that was a private comment to me. He visited Sydney actually about this time we were finishing off this paper, and some of my conclusions there probably reflect my discussions with Minkowski about these things, particularly on the confusion problem. I know we had a number of discussions about that. Anyway, that was a bit of diversion from your actual question.
Sullivan
It seems to me that the outside world looked at “p of d” as sort of a rearguard action, trying to save what had been years of work, etc. Although it also seems to me that, because it was so complex, the outside world didn’t really give it the chance it deserved, to find out that it could establish some things.
Mills
I think we had a correct view of the importance of it. That it carried a lot of information, but I think Minkowski was right. It was low-grade information because you had to put a lot of assumptions into it. If you assumed that sources were discrete point sources randomly distributed through the universe, it carried an enormous amount of information. But at that time there was just no reason for assuming this.
Sullivan
Ah, yes, you were still trying to establish that?
Mills
Yes. Only Cambridge knew this, no one else did. (laughter) It was an assumption that went into “p of d” and as far as that assumption went, its conclusions were valid. And I quite accepted that. There may be a bit of discussion in the Paris Symposium about that on my part because if the sources were not randomly distributed, if there were groupings -
Sullivan
And you had some indication of clustering.
Mills
Yes, I did. Then the “p of d”, all it can do is set an upper limit on the population, because clustering increases the probability of deflection. I was aware of this, so that this excess, which was also indicated by the “p of d”, could also simply indicate clustering. Again, I discussed this with several people and I think Minkowski and I were in fairly good agreement on this. I had spent a lot of time with him on it. I didn’t at all reject the importance of “p of d” analysis then, in fact, to some extent, still feel that one has to be very cautious interpreting what comes out. Then, and I think I would still say, “p of d” sets an upper limit on the distribution of point sources.
Sullivan
Above a given flux level?
Mills
Yes. But I don’t think it can really set a lower limit because of the possibility of new populations coming in, and the possibility of clustering on some scale, which one hasn’t been able to investigate. It’s all an extrapolation from what one knows. And you should accept it with a grain of salt. That was my very strong view in those days and, as I said, to some extent I still believe this.
Sullivan
You’ve mentioned the Paris Symposium. I’ve heard from many people that that was a rather interesting meeting, to say the least.
Mills
Yes, it was (laughter).
Sullivan
One thing that interests me is that it was really the last time that virtually all the radio astronomers in the world could get together at one meeting. So you had a complete overview of the field. What struck you, as one of the main participants, as to the nature of that meeting?
Mills
Of course, I was particularly interested in the galactic and extragalactic work – not the solar, so we won’t talk about that. I think it settled a sort of way of outlook. I found that most people were now in agreement on what should be done and the problems of observation. Although Cambridge was fighting rear-guard action, it was recognized a rear-guard action. Their new survey was obviously going to be a lot better. And really the ideas of synchrotron emission had become clear. I felt it was really, sort of represented the peak of achievement up to that time. All these ideas were now making sense. A lot of the earlier uncertainties had been completely resolved. There was this little bit of argument with Cambridge but it wasn’t, in my view, a serious argument at that time. The really serious one was on the reliability of 2C, and that they had quietly dropped by then. And I was quite prepared to go along with arguing mildly about some of the others which were not so clear. So I felt it was a very effective and very worthwhile meeting which sort of summarized the present state very well.
Sullivan
But what about optical identifications though? That was an unsettled question still.
Mills
That was an unsettled question, yes. Dewhirst had started doing some at that time, Bok reported some, but there was nothing really systematic. In fact, immediately following that meeting, I went to Pasadena again and spent two months doing identification work on a combination of our survey plus the 3C results which were then available. I used 3C for right ascensions and my positions for declinations. And that is written up in the later paper you mentioned, about ’61, on the identification of radio sources. It took rather a long time to write up. I sometimes was rather tardy about writing up results when I knew what the answers were.
Sullivan
I don’t think I have that one because my bibliography ends in ’60. So this is one by you alone?
Mills
Yes. You did find it somewhere on your list yesterday, I think. The identification of radio sources. It was in the Australian Journal of Physics.
Sullivan
I’ll check it out.
Mills
And I think it was quite important because it was really the first time that a proper luminosity function was produced. It wasn’t a terribly accurate one then, of course. No, you had some data giving the number of identifications, the identifications which clusters and things like this, because I looked for all of them, the Abell cluster identifications as well as the radio sources. It was a systematic attempt and Bolton and Minkowski had also been working. They’d started a bit later, but they published about the same time giving essentially similar results. They all came out about 1960, or early ’61, I’ve forgotten when exactly.
Sullivan
But wasn’t it discouraging to people like Minkowski that such a low percentage of these things were being identified?
Mills
I think it was mainly challenging rather than discouraging. (laughter)
Sullivan
Dewhirst has told me that he went over and spent 6-9 months looking at all these 3C fields and coming up with such a small percentage of -
Mills
Of course, one of the troubles with 3C, even then, was that there were large numbers of lobe shifts and a lot of the positions were quite unrelated to the actual.
Sullivan
Well he looked ± one lobe shift.
Mills
Yes. I think we may have overlapped at the time. He may have been there a little before me. I think we overlapped for a time. But, certainly I was quite happy with the results I got out of it and I think, I as I said, both Bolton and Minkowski continued on, later one, with even more and better positions and got basically similar results. And a lot of the later identifications were summarized in that first paper. There was a horrible mistake, incidentally, in the Appendix, but never mind, you’re not interested in things like that.
Sullivan
What?
Mills
A mistake, a mathematical slip.
Sullivan
In the Appendix of which?
Mills
Calculating source counts in various cosmologies – in my identification paper. It didn’t alter any of the basic conclusions. It was just a source of embarrassment later on.
Sullivan
Back to the Paris Symposium. You also talked about the corona of our Galaxy, thermal – non-thermal emission and so forth, Class II radio sources being non-thermal. I would gather that you would say this was beginning to become settled in your mind.
Mills
Yes. I think by then I had an overall picture of radio galaxies being the Type II sources that I had, and supernova remnants and the HII regions being the Type I basically. The galactic corona I had sort of grew out of Shklovsky’s suggestion, but the model which I produced was quite different and quite different from the one which Baldwin produced earlier. It was much smaller and flatter.
Sullivan
How did that come about?
Mills
I was using southern hemisphere data around the plane rather than northern hemisphere data around the pole. It was just different data. But they were not all that different. Everyone seemed to agree at that time there was a lot of emission at high distances above the plane at high-z distances. And this was questioned later on and it’s clear my separation probably gave too much to the high-z radiation.
Sullivan
But, like you say, there was general agreement that there was a halo in some sense, and you were arguing about detailed shape -
Mills
Yes.
Sullivan
About the whole controversy over 2C, would you agree with this, that there was a tremendous mixture of arguments going on? There was the validity of ‘p of d’ as a check which was independent of confusion effects. There was the level of confusion in the two surveys. Percentage of extended sources and how this would influence N(S). Different Fourier frequencies, for example, and how this influenced N(S). Then steady state vs. evolving cosmologies got dragged in, and what should be counted as a source. Often the arguments got cross circuited – short circuited.
Mills
Yes, it was a very confusing period, really. I think we all had our ideas one way or another, and it’s pretty hard to work without some working hypothesis. But the arguments were only heated when it was a question whether 2C was basically right or wrong. Now that was one which was settled. But it was a very heated argument about that at one time. And there was also a heated argument on a question of whether the steady state could be shot down by the radio data. And there I tended to feel for a very long time that it couldn’t be. That there wasn’t sufficient precision in the radio, or the populations involved in the radio information were not sufficiently defined to shoot down the steady state. Well, you know what’s happened since then. But at that time we were really very ignorant about what class of source we were dealing with.
Sullivan
So you would say that those were the two most heated arguments?
Mills
They were the heated ones, yes. Because of personalities, basically.
Sullivan
You also made a survey with the Cross of the galactic plane which was Hill, Slee, and Mills in ’58. And what did this show?
Mills
The interesting things that came out of that, of course, were the steps in the distribution suggesting tangential arms.
Sullivan
That’s right, I forgot that was the paper that… yes. That was in the Paris Symposium also?
Mills
Yes, that was mentioned there. I produced a spiral arm model there which actually hung around for quite a long time afterwards. Because, in fact, the radio steps did correspond to regions where one was looking along the major spiral arms.
Sullivan
Were you being influenced by all the HI work going on?
Mills
No, not at all then. That particular interpretation was purely based on looking at my results and seeing, “We’ve got steps there. They must be spiral arms.”
Sullivan
I see. It wasn’t talking with Frank Kerr at tea or something like that?
Mills
No, not that one. So I sat down and read up about spiral arms and saw that the one possible interpretation was a logarithmic spiral, a good thing to fit, we tried to fit it, and it did.
Sullivan
But did you know about the HI work?
Mills
Oh, yes. I knew, of course, that spiral arms -
Sullivan
And this whole method of getting distances - ?
Mills
In those days the HI spiral arm map was just an unholy mess. It was not circular -
Sullivan
It still is.
Mills
It still is. Well, if you’ve looked at some of the old maps, you would realize that in those days it was even worse. I think many people felt that the interpretation of the distance was very difficult and this occurred to me as being a possible way of getting a model without that. If the spiral arms were basically logarithmic spirals, then all you had to do was to fit the tangential directions and you had it.
Sullivan
But, of course, that was a big if.
Mills
Yes. Of course it was.
Sullivan
At least in one case of our own Galaxy. You mentioned that the Paris Symposium was a rather nice synthesis of many things that had been going on over the previous years. At that time, what did you see as the main thrust that needed to be done for galactic and extragalactic radiation? What direction needed to be traveled?
Mills
For the extragalactic work, clearly the identification work was the essential thing. I think everyone believed that at the time. It was, in fact, why I went back and did it, or did some of it. That, I felt, was the most important on the extragalactic side. Obviously higher resolution was a real basic necessity, too, and I was thinking about a larger instrument which would help to overcome – to give more precise positions and better resolution.
Sullivan
This is what eventually became the Molongolo Cross?
Mills
Yes, that’s right. There was a vague suggestion by then that high frequency work was going to become important and again we were thinking of what is now the Parkes dish, a big steerable high frequency dish. These seemed to be important, mainly for the extragalactic work. But also for the galactic work, it was clear that high frequency was extremely important and again one wanted high resolution. These were both reflected by the Parkes reflector and our Molongolo Cross. These represented continuing along our present path and doing what we felt had to be done. And on the galactic side, I wasn’t so terribly interested in the Galaxy then. This became a thing that interested me more later on.
Sullivan
About this time, I believe, that something had to give in terms of big proposals for CSIRO, is that right?
Mills
Yes.
Sullivan
Can you tell me about that?
Mills
We had the three proposals around ’59-’60. Paul Wild’s ring-type instrument, for a big Cross, and for the large steerable reflector. That had been on the books ever since ’52-’53 and the chief protagonist for a large steerable reflector was Taffy Bowen, our Director. It was obvious this was necessary. I didn’t argue about that. This was number one priority and we simply had to have that. But when funds were solved for that – our available funds – very little was left for anything else. Again, solar astronomy obviously needed more instrumentation so there was a very strong case for Paul Wild’s instrument. Again, I couldn’t really argue with it. That really should be second priority.
Sullivan
You’re saying you didn’t try to argue?
Mills
I think, perhaps, there were some basic arguments whether it was worth pursuing solar astronomy at all. Give that it was worth pursuing, obviously you had to make that decision and, in fact, he made it, that solar astronomy had to continue. I would have been quite happy to see it not, but of course, that was just my own personal view. So, therefore, there was very little chance with our money available, and our facilities, to also continue with the Cross. Since I felt it was most important to go to a very high resolution and sensitivity, and my analysis then had shown that the steerable dish wouldn’t do this – at that time we had no thought that it would go much beyond 10cm and receivers in those days were pretty poor at high frequencies. Although the maser had come along and this looked as if it would make a big boost for high frequency work. It still looked as if that really wasn’t what was necessary for surveying the radio sources and precise position measurements. I felt that the Cross-type instrument was the best for that particular type of operation. I felt we had to do it, so I just looked around to where I could find some support. And I did find it in Sydney University.
Sullivan
And so when did you move there?
Mills
Towards the end of 1960.
Sullivan
Was this after all the MSH survey?
Mills
It had all been published then, yes.
Sullivan
Well, it hadn’t been published yet.
Mills
I think the last paper came out in 1960. It had all be done. It may have been ’61.
Sullivan
Right, and then you began working on the design and the funding, and so forth, for what became the Molongolo Cross?
Mills
Yes, that’s right.
Sullivan
Just very briefly, when was that built?
Mills
It started in ’62 when we got our first infusion of funds from America. And it was fully completed in ’67. The east-west arm was operating some years before that, but the north-south arm was a very difficult engineering problem – a one-mile-long phased-feed system – and that was ’67 before that was actually fully operational.
Sullivan
You bring up a point which I’d like to ask about. Namely, I didn’t realize some American money had gone into the Molongolo Cross. Was this Ford Foundation?
Mills
No, NSF.
Sullivan
NSF?! Those were the halcyon days when they were giving money to the Australians. (laughter)
Mills
In total, they gave about a million dollars.
Sullivan
But also, it’s my understanding that American money basically made the Parkes dish possible. This was the Ford Foundation.
Mills
Yes.
Sullivan
And Culgoora, was this Ford Foundation also?
Mills
No, it would be Carnegie Institute, I think, gave the Parkes money and the Ford Foundation the Culgoora instrument, if I remember correctly.
Sullivan
But why is there this recalcitrance apparently on the part of the Australian government to come forth with the goodies when obviously if you had to name, I think, one science in which Australia has excelled, which puts it on the scientific map – this is it?
Mills
Well, a prophet in his own country has little honor, I think. (laughter) And this applied exactly in this field. If some overseas people were prepared to acknowledge Australian science by putting some money into it, they were prepared to put some, too. Although, they didn’t put any into mine. This was the attitude – rather funny, but here we are. I think basically the politicians couldn’t believe that anything worthwhile could come out of Australian science. (laughter)
Sullivan
So, you are saying this is not confined to radio astronomy?
Mills
It tends to operate over all science. Although they had provided a lot of money before that for optical astronomy – in building the 74’’ telescope. They had provided funds for a high energy accelerator, Oliphant’s enormous homopolar generator which consumed a large amount of money. It could be they thought there had to be some limits in the amount. Anyway, we couldn’t interest the government in providing direct support without some stimulation from outside.
Sullivan
Well, early ‘60s is about where I’m ending things. I thank you very much for your time. That ends the interview with Mills on 26 August 1976.
End Tape 63A