[Bolton, 1976]
Bolton, 1976 (Photo courtesy of NRAO/AUI/NSF)




NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with John G. Bolton
At Parkes Observatory
March 15, 1978
Interview time: 3 hours, 40 minutes
Transcribed for Sullivan by Pamela M. Jernegan

Note: The interview listed below was either transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009) or was transcribed in the NRAO Archives by Sierra Smith in 2012-2013. The transcription may have been read and edited for clarity by Sullivan, and may have also been read and edited by the interviewee. Any notes added in the reading/editing process by Sullivan, the interviewee, or others who read the transcript have been included in brackets. If the interview was transcribed for Sullivan, the original typescript of the interview is available in the NRAO Archives. Sullivan's notes about each interview are available on the individual interviewee's Web page. During processing, full names of institutions and people were added in brackets and if especially long the interview was split into parts reflecting the sides of the original audio cassette tapes. 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.

Part 1 | Part 3 | Part 4 | Part 5

Sullivan

John Bolton on 15 March ’78.

Bolton

Well, during the long nights we spent keeping a recorder pen on scale at Dover Heights, I had been educating myself. I'd read twenty years of the Astrophysical Journal, twenty years of Monthly Notices, and that kind of thing, anything I could get hold of. And when we got these first three identifications, the people who sort of stood out in my mind were [Rudolph] Minkowski and Linblad- the elder Linblad- and [Jan Hendrik] Oort. And I wrote to all three, giving the results, you see. Now Minkowski and Oort had both written papers on the Crab, and I remember getting a very enthusiastic letter back from Oort with two or three pages on his ideas on the Crab Nebula and that kind of thing. There was just one line at the end of the paper commenting on the other two identifications. Jan said, "Of course, there are an awful lot of galaxies in the Virgo cluster." But I mean, the thing about Oort was that he still kept that in the back of his mind; and then when he went next time to Palomar, he persuaded, I think, Baade to use the 200 inch telescope to take photographs of M87 with all different filters.

Sullivan

I think you did mention in the paper- no, you didn’t. When did it come to light to you that it had this peculiar jet? Because that had been known back in 1917 or so.

Bolton

Yes, it probably had, but Oort, I think, had [Walter] Baade take a whole new series of plates from Palmar, and I remember Oort writing to me from Palomar, sending an ultraviolet picture showing the jet. I certainly wasn't aware of the earlier reference. Oort may well have been.

Sullivan

Well, he probably wasn't. I think it was pretty obscure.

Bolton

Yes. Although he dismissed the identification in his letter to me, he had enough interest to push it.

Sullivan

Did you get any reaction from the other people you wrote to?

Bolton

Oh, yes. I wrote to Minkowski and got a letter back from Baade. I wrote back to Baade and got a letter from Minkowski. In fact, for quite a number of years our correspondence went that way. If you wrote Baade you got a letter from Minkowski.

Sullivan

Anyway, what did they say?

Bolton

I forget. I mean Rudolph wrote to me. I think he was stimulated at the time to take another or try to obtain another spectrum of that central star, which of course, later became the pulsar. And he- or Baade said in this case, "Any other positions you have we'd like to have them and try to follow them up." Of course, the Puppis position which we gave him a year later enabled the identification of that supernova remnant.

Sullivan

Right, but even right from the very earliest, in 1947 or 1948, when you first contacted them, they had a high interest?

Bolton

Yes.

Sullivan

It wasn't only after the Cygnus A business...

Bolton

No, oh no.

Sullivan

That's interesting. Well, looking once again at this paper, I notice that the title of it is "Positions of Three Discrete Sources of Galactic Radio Frequency Radiation."

Bolton

Yes.

Sullivan

Which, I think, is telling also.

Bolton

Yes, of course, the radiation in those days was known as "galactic noise," or "cosmic noise."

Sullivan

But even though you had two reasonable identifications with extragalactic, you still wanted to call them galactic?

Bolton

No. There were two kinds of noise - solar noise and galactic noise...

Sullivan

I see.

Bolton

Under the general heading of cosmic noise.

Sullivan

Okay.

Bolton

These were more galactic than solar.

Sullivan

Now, going to the paper with Stanley in which you give a precise size for Cygnus A and the declination was 1° off- let’s see, I guess I didn't have any specific questions on that one. You had a follow-up paper in the Australian Journal of Scientific Research on that with much more detail. Ah yes, there was a thing where you broke the Cygnus A radiation up into two components, the variable and the steady. And you said there’s a steep slope on the variable, which makes sense. But for the steady slope, it seemed like you had a peak at 100, so that you give a sketch of a turnover at around 100 megahertz. Do you remember how this might have come about?

Bolton

Well, I think if you add those you get a steep slope. This is just the sort of average characteristic of the scintillating component of that period.

Sullivan

I see. So it was just an incorrect subtraction of the scintillating components.

Bolton

No, I don't think it was an incorrect subtraction of the scintillating component.

Sullivan

Okay, I’m...

Bolton

At 60 megahertz it's all scintillation, so there's no steady component. So it must peak. At 200 megahertz you see no scintillation, so it's all steady.

Sullivan

Yes.

Bolton

At 100 you see half scintillation, but the 100 total has gone up much more than the 200. And then at 60, it's all scintillating, so the steady component must go down, by definition.

Sullivan

In fractional amounts, yes, but not necessarily in the absolute amounts.

Bolton

Yes, the absolute amount goes to 0, or it is going to 0; it's all scintillation, let’s say, at 40 megahertz.

Sullivan

Oh, I see. I see. Now the picture is coming to me. Right. So basically, it's a confused concept - well, what were you thinking at this stage? You hadn't yet gone to New Zealand, I believe.

Bolton

No, oh no.

Sullivan

So what were your thoughts on the cause of this scintillation? I think in the paper you're very noncommittal.

Bolton

I think Bruce Slee certainly always thought it was scintillation.

Sullivan

Ionospheric?

Bolton

Yes.

Sullivan

You say, "The question of its nature is left open for the present," but what were you thinking in your own mind? Can you put yourself at that time?

Bolton

Well, I don't think we knew. By analogy with the Sun, I mean, it just looked like another Sun. The Sun does exactly the same thing.

Sullivan

Right.

Bolton

The variable component is something which is much stronger at low frequencies. So I guess it was the solar analogy which...

Sullivan

And that would also put it not that far away from the solar system, if you scaled it down.

Bolton

No. But, we didn't know. The New Zealand experiment was designed to find out.

Sullivan

Well I've just heard people say that it was quite clear to them that around 1946-47 that it must be ionospheric, and I've been a little bit puzzled by that whether they were just sort of biased in their mind or whether they really had evidence of that.

Bolton

Who would you say?

Sullivan

I'm afraid I really can't remember to name names, but I think really there wasn't that much clear evidence but they just always assumed it was ionospheric and it was a matter of someone going out and proving it.

Bolton

No. I think we had no evidence, and as I say, the varying component got larger as one went toward low frequencies, which was exactly the same thing the Sun did. And we knew that the solar variations were intrinsic; spaced aerial experiments had been done on the Sun. We naturally took the spaced-aerial experiments on the Sun as our control experiment for the scintillations.

Sullivan

That makes perfect sense. Moving on to the paper by yourself [Sullivan: Nature, 1948], that's one question I wanted to ask you. The others had been in association with Stanley and/or Slee, I was wondering how this one came to be only by yourself. Do you remember how that came about?

Bolton

Oh, I don't know. I was sticking my neck out a bit. I think that was one thing.

Sullivan

I didn't ask Bruce that, maybe I should have.

Bolton

Well Bruce wasn't working with us at that time.

Sullivan

Oh, he wasn't yet with you? He came in for the first time often with these identifications...

Bolton

Yes. Now he wasn't actually part of the identification team. It was Stanley and I who did the work, but we did use some of Bruce's Dover Heights measurements to clear up one or two points on that. I think in the case of the Cygnus experiments, Stanley and I had actually worked together. I mean, we sat up at night together; we'd gone to various places to see if we could get a high cliff together. We’d done that all together. But the major observations on those other sources and so on, essentially were all my observations. Stanley at the time had gone back to building equipment and that kind of thing. So it was essentially my own sitting up at night and so on which got those.

Sullivan

And like you say, it was a bit of a risky sort of paper.

Bolton

Yes, but others were not really involved in the observations for it. I mean the point of that paper is to say, "Well, the first one is not unique; there's a whole class of these things. We’re starting a new branch of astronomy."

Sullivan

Yeah, they're all over.

Bolton

They're all over, that's right. But then, of course, Stanley was in on the New Zealand position-finding observations and Bruce did some of the backup observations from Dover Heights. He'd been in on the scintillation part of it, very obviously. So the next paper was by the lot of us. In a way, of course, this was a departure from tradition, by including those people in papers.

Sullivan

Technicians, basically.

Bolton

Technicians in papers.

Sullivan

Yeah. Other people have mentioned that to me. Was this a policy of [E.G. "Taffy"] Bowen's or how did this come about?

Bolton

No, it was a policy of mine.

Sullivan

It's more common today.

Bolton

Yes, very common today.

Sullivan

But at that time, it was unusual.

Bolton

I'd done a lot of experimental flying and that kind of thing during the war and I always recognized the people who kept my airplane together. It seemed to me that...

Sullivan

That they should get proper credit?

Bolton

Yes. And it did become a Radiophysics model.

Sullivan

I know that. Looking at this Bolton '48 Nature article, it seems like, indeed, you have been looking at some basic astronomy texts and reading journals, because you speculate on what the nature of these "radio stars," as they came to be known, might be and I gather, couldn't make them main sequence because the Sun wasn't... Well, that's not true; the Sun was like Cygnus anyway, but the others were not scintillating so much, so you said that they’re either post-main sequence or pre-main sequence, maybe like a planetary nebula or a contracting star. Is my reasoning right there? Did you just sort of take the other two kinds of stars that were known?

Bolton

Yes.

Sullivan

Let me ask you another question. Were you unusual in this sort of methodical past-twenty years of Astrophysical Journal and Monthly Notices? Were other people doing this? I haven't heard anyone else talk about how they learned what they had to for their projects.

Bolton

I think I would be, yes. I think an effort was made to go into solar physics. This was how Steve Smerd got his start. Pawsey one day said he'd like Steve to give a lecture on the basic physics of the Sun and solar atmosphere. Thirty years later, Steve is still preparing his lecture.

Sullivan

Yes, other people boned up on the particular area that they were in, but as sort of a general education, a self-education, I haven't heard anyone mention that. Alright, now the other paper that I have a copy of here is your paper on Taurus A with Stanley, and you call it in the abstract one of the "minor" discrete sources of galactic radio frequency noise. I suppose "minor" refers to the fact that it's a small fraction of the galactic radiation, but I thought that was sort of funny. Why in Table 2 do you give times in units of minutes and fifteenths a minute?

Bolton

Because that was our measuring.

Sullivan

On the strip chart or something?

Bolton

Yes.

Sullivan

I see.

Bolton

Minutes and fifteenths of a minute is approximately four seconds.

Sullivan

One minute of arc, yeah. Somehow that one got by the referee. That's rather non-SI unit.

Bolton

That was the smallest sensible unit. Compared to the scatter, which refraction and everything else gave us. I think it was essentially the minute of arc. After all, you made your own rules in those days.

Sullivan

Yes, that's true also.

Bolton

One of the biggest difficulties I ever had was getting the concept of antenna temperature.

Sullivan

Why was that?

Bolton

Now it's a very recognized concept, but [Kevin C.] Westfold and I wrote a paper - well, I guess, Burgess was one person.

Sullivan

Right, he had a paper during the wartime [Sullivan: also 1948].

Bolton

Westfield and I shared that antenna temperature, in fact was proper physics.

Sullivan

What about the [Robert] Dicke paper in ‘46? Was that not part of your knowledge at that time?

Bolton

Oh yes, the Dicke paper was. The temperature concept had sort of been used loosely, and we showed that it was valid physics to use antenna temperature.

Sullivan

This was in an internal lab report?

Bolton

No. This was in one of the papers we produced.

Sullivan

It's in one of the Bolton-Westfold series, I see. [Sullivan: Paper I, 1950]

Bolton

One of the Bolton-Westfold series. I mean Westfold was somewhat pedantic and I sort of convinced him that this was reasonable physical concept and Kevin produced the mathematics to show that it indeed was a valid physical concept. But it wasn't in general use. The Americans actually took up the concept of antenna temperature when they came in the vast numbers [Sullivan: in the late 1950s].

Sullivan

Perhaps because they were working at higher frequencies. For instance, the NRL group and so forth. Perhaps it works there in a little more clear-cut fashion. Let's see now, that takes care of all of the papers through 1950; however, perhaps you should tell me some more about your trip to New Zealand and so forth. For instance, you mentioned to me before we started recording about how you didn't get any useful Cygnus A position in New Zealand. Why was that?

Bolton

I am not quite sure. It was probably the time of year at which we were observing and the interference from Spread-F. What we'd hoped to do with Cygnus was in fact to count the fringes to transit to get a second handle on the declination. Fringe separation in New Zealand was roughly 1/4°.

Sullivan

You were on a higher cliff there.

Bolton

Yes. But unfortunately, what happened was that near transit the fringes get very long, and after transit, even though we had a clear horizon till the second point, I think the ionosphere broke up or interfered with the pattern so badly that we didn't in fact, get very good data. And when we moved across to the west coast, we again didn't get very good data, whereas the data on the Crab, on M87, and on NGC 5128, which were at different times of the day, were really good. You see, Cygnus was transiting somewhere around midnight which is the local maximum, and transiting setting was nearly midnight and this was the maximum Spread-F, whereas the other objects were in times where spread-F scintillations were very low.

Sullivan

So this was useful for the correlation of ionospheric stuff, but not for source positions?

Bolton

Yes, you see, two problems - one is spread-F, which has sort of local maximum at midnight and sporadic-E, which had local maximums at midday. Now it so happened, I think, on the Crab and M87 that we were clear of midnight and midday and Centaurus wasn't so good. But Cygnus was rather ruined; in fact, we never published our Cygnus position. We knew it was closer to 42° than 43° but weren't able to really pin it down. In fact, I don't think we were ever able to successfully identify the second fringe.

Sullivan

I see. And you went to two different sites in New Zealand, I believe.

Bolton

Yes.

Sullivan

What was the purpose of two sites?

Bolton

Well, from the first site we had, theoretically, a view of Cygnus right through.

Sullivan

Basically north.

Bolton

Yes, north and east. And this gave us also rising patterns of the Crab, M87 and 5128. We had to move over to a west coast site to get the setting pattern for the other three objects.

Sullivan

Okay. The survey by Stanley and Slee was the first major survey to come from the Australian work, published in 1950. Once again I might ask why you were not an author on that one?

Bolton

What was the date on that?

Sullivan

’50. It was submitted in November of 1949.

Bolton

The work on that was in fact, just as much due to Kevin Westfold and me, as it was to Stanley and Slee, and this was an occasion where we simply split publication between- four names on paper seemed a little large, so Westfold and I took the general over and published it and Stanley and Slee took the source population and published it.

Sullivan

But it was really all four of you working on it?

Bolton

Yes, it was all four of us working on the same thing; in fact, I think you'll find acknowledgments backwards and forwards when it came to the sea-interferometer or phenomena Bruce Slee and I wrote that. When it came to work on scintillation, I think; Slee and I also wrote that up.

Sullivan

Okay, I see.

Bolton

When it came to try to model the galactic background, it was Westfold and I, so I mean, the splitting up was just...

Sullivan

Right. So this survey then was a complete southern sky survey?

Bolton

Not complete, because it didn't cover the southern polar regions. They don't rise and set.

Sullivan

Okay, everything you could get from the sea-cliff interferometer anyway, and you have 22 sources in here, most of which turn out in hindsight to be legitimate.

Bolton

Well, a larger fraction than was customary in those days.

Sullivan

Well, yes.

Bolton

It was customary in those times.

Sullivan

What were your impressions at this stage of the development of radio sources where, this was just about exactly the time of 1C survey which had about fifty sources, and so you probably had a pre-print of that by this time also. What were you thinking about the nature of these radio sources?

Bolton

About the nature?

Sullivan

Was this telling you anything when you could begin to do some statistics as to where they were located in the sky and the distribution and intensity, or was this too early?

Bolton

No, I think at that stage it was too early. We, I think, were more- I mean this was just, we'd done this work and here, we pass it on for general scientific...

Sullivan

Okay, so you didn't have any great conclusions to make.

Bolton

No this was toward the end of how far we, we'd gone as far as we could go without new equipment. This was done with the 9 Yagi array, which Westfold and I did the general background survey with, and our next stage was to build a rather larger sea interferometer, which had, I think, a 6 x 2 Yagi system with azimuth resolution, higher azimuth resolution in this case. And...

Sullivan

And somewhat more sensitivity?

Bolton

Somewhat more sensitivity. And then, that paper, I think, which came out in 1953 did have some statistics, I think that might have been me, Stanley and Slee.

Sullivan

1954 - 104 radio sources at 100 megahertz?

Bolton

That's right. This is the first time we mentioned log N- log S, which was anomalous.

Sullivan

Something I've never actually done or even thought about is the cross-correlation of this survey with the 1C, Ryle, [Francis Graham] Smith, and [Bruce] Elsmore, I believe it is. Did you do that in region of overlap?

Bolton

I don't think there was any correlation.

Sullivan

Even at this early stage, you mean, the pattern was set? I'll have to do that. I just don't remember any comments on that, by either side at that time. But do you remember doing it at that time? I would think it would be an obvious thing to want to do.

Bolton

I remember noting that the source in Ursa Major was exactly twelve hours different from the source in Cassiopeia and the same declination.

Sullivan

Well, from the Nature [Sullivan: 1948] article. They'd gotten rid of that by the time they published the 1C.

Bolton

That was the only thing. No, what was the date of that one?

Sullivan

1950. The other was published that same year.

Bolton

Of course, after that Westfold and I got busy doing, well, what can we find out with an interferometer, how accurate is it? That was when we started building the 80 foot dish in the ground.

Sullivan

The hole in the ground.

Bolton

Going to higher frequencies.

Sullivan

Okay, let's talk about that hole in the ground, first of all, where does that idea come from?

Bolton

Well, I suppose it was me.

Sullivan

But I mean, have you heard of the concept somewhere else or you just said well this is something that's sort of an obvious thing to do?

Bolton

No, I think it's an obvious thing to do. And it really came out of Westfold's and my work- that one has to get some primary gains into the antennas, and we built this as an experimental antenna at the same time as Stanley started to devote his attention to how to get sensitivity at high frequency.

Sullivan

I see.

Bolton

We actually worked the antenna first at 160 megahertz but it was intended to work at 400.

Sullivan

In terms of the accuracy of the surface.

Bolton

And that dish was and the receiver were really the prototypes of the Caltech dishes.

Sullivan

I see. They are similar size, that's right.

Bolton

And the receiver technology was taken directly from a Caltech dish.

Sullivan

You chose this over a 30 foot steerable thing which might have been feasible at that time, at least as a transit instrument, simply because you wanted to have that additional gain?

Bolton

Additional resolving power.

Sullivan

Oh, you wanted the resolution. Tell me, while we're recording here, what you told me before about the colloquium, the talk, that you and Westfold gave at this time.

Bolton

Well, I guess that must have been a talk somewhere around 1951. We gave a talk which we entitled "Discernibility and Detectability" and showed that in fact, one's ability to detect objects far exceeded one's ability to discern objects. And one of the things we got laughed down at was proving that with our 17° 9-Yagi ray, although we could detect many hundreds of sources, in fact, we could only sort out what corresponded to one source per four or five beam areas, and we were really laughed at for that.

Sullivan

And what was the point of disagreement?

Bolton

I don't think anybody understood. I mean an interference pattern is something which is terribly real and terribly false. That is to say, the sum of n sine waves is still a sine wave.

Sullivan

Yes.

Bolton

And that is the thing which we could not get across- that one cannot resolve that.

Sullivan

That the interpretation is laden with difficulty, yes.

Bolton

We found this out empirically, in our crossed two-aerials on the cliff interferometer and we could move one of the aerials with respect to the other. And, although, we would get interference patterns which were cross patterns for sure, and we could think we could sort out sources, as soon as we went to another spacing we’d sort out a different lot of sources.

Sullivan

Right. And this was already clear though from your first few sources where you had a couple that happened to be near each other in the sky and the two patterns were beating, and I guess it wasn't quite so clear to the solar people because they only had the one big source. So that's an interesting thing that you had there in the light of the late '50s with the controversies over confusion and so forth...

Bolton

Yes. Well, in this 1954 paper we gave 100 sources and so on and we had found the steep slope of the log N - log S and at the end of that paper, I dismissed it as the kind of thing that happens with clustering. Because if you look at the log N - log S in the Shapley-Ames catalogue, it’s dominated by the Virgo cluster and gives you a steep log N - log S.

Sullivan

So you mean we just happen to be located in a region where there was a...

Bolton

Yeah, well I had no inclination- I had to throw in some suggestion and clustering was one of the things that could clearly do it.

Sullivan

Are you talking about the concept that we have to be located somewhat in a hole so that there are fewer strong sources.

Bolton

It's the same thing.

Sullivan

The same thing that came up later - that Hoyle came up with and so on. But now by that time were you thinking of these sources as being primarily extragalactic?

Bolton

Yes. Yes, I was.

Sullivan

When did that changeover take place? Was it Cygnus?

Bolton

No. I think it was purely theoretical. In fact, I remember at one meeting we had in Sydney when Sir Harry Massie was there and wanted to hear about radio astronomy and we had a joint meeting with Stromlo and I remember having an argument with [Richard] Woolley. And I said that I felt that these things were galaxies, because I couldn't see the energy density being produced in a stellar atmosphere to give these signals- they had to be minutes of arc across. In other words, the source temperature, even though I didn't know anything about synchrotron radiation or what the limits on brightness temperature were, I felt they had to have a large emitting body, but this was just a...

Sullivan

Even though, of course, that meant their luminosity became tremendous. But when you go to a galaxy, you have more resources, also.

Bolton

Yes.

Sullivan

And that would be about when?

Bolton

I mean, at that time we recognized that all the galactic radiation wasn't going to break up into point sources, I think.

Sullivan

And what was the basis of that?

Bolton

Simply that it didn't...

Sullivan

With higher resolution it was still...

Bolton

Yes, there was an underlying smooth component. I mean, Westfold and I argued about number densities that were required to hide sources, you see. I remember one day we eventually got down to saying "Well, it's got to be individual electrons, essentially".

Sullivan

Well, but hold it now, in one of your papers with Westfold you have a model in which almost all of the radio sources are closer than 40 parsecs and you give an actual density of these radio stars- I believe that's correct. In ‘51, in one paper of that series, you talk about an isotropic component which might be extragalactic and a galactic distribution of radio stars less than 1/10,000 per parsec. The average observed radio source is much more luminous than the Sun and at distances of less than 40 parsecs.

Interruption

So this was 1951, now, so it was probably written around 1950, so at that stage you still, apparently, were thinking of the radio star idea. Can you remember when this?

Bolton

Yes, I think the changeover probably came in 1951 to 1952. And I felt that there was a smooth radiation from our Galaxy and that the majority of the discrete sources were extragalactic, and had to be galaxies. In other words to give them a moderate volume emissivity. Of course, the pulsars have proved the exception.

Sullivan

That's really irrelevant to the 1950's.

Bolton

No, I think if you can ever get hold of the proposal I wrote to Caltech by 1955, although I'd been out of the subject for a long time, that proposal came remarkably true. The thing that was obvious then, I think, was that we did have galactic sources and we did have extragalactic sources. But one of the distinguishing characteristics was that the galactic sources had the same dimensions optically as radio. The extragalactic sources had much larger dimensions in the radio, and one of the ways of getting at the identification was to look at the structure and say, "Well, we’ve got a four minute of arc source, this is either a four minute of arc galactic nebula or it's a half-minute of arc galaxy".

Sullivan

And for some reason the radio emission is outside.

Bolton

And for some reason the radio emission is outside.

Sullivan

But I'm trying to think now, what else was there besides Cygnus, in terms of a large radio galaxy?

Bolton

Ah, well, I mean Bernie [Bernard Y.] Mills' work, our crossed sea interferometer, the Formax source, Centaurus...

Sullivan

Centaurus, right. There still was only a handful, but there was a definite pattern.

Bolton

Yes, still only a handful. The known supernova remnants, the Crab and the Puppis source and so on, were the same size optically. That is to say, the radio description was the optical description.

Sullivan

But might there have been a bit of a selection effect there, because the upper limits to sizes that one give at that time were something on the order of a minute of arc? Well, there might have been some galaxies that were bigger than that. What I'm saying is that it might not have been possible to establish at that date that a radio source was smaller than the optical image it's associated with. All you could find was that it was larger.

Bolton

Oh yea. I mean there was a great spate of interferometry around 1952- [Robert] Hanbury Brown’s, Bernard Mills’ work.

Sullivan

Right, but only on a couple of the stronger sources, that was all you could do it on...

Bolton

I think they gave a very clear indication that structurally the galactic sources were similar in radio as they were optically, and the extragalactic sources were very much more extended.

Sullivan

Okay.

Bolton

Not true, of course, for things like quasars.

Sullivan

Yes, which is once again, a later story. Let's talk about the Bolton and Westfold papers in which you switched from the discrete sources to the galactic background at 100 megahertz, but from what we've been saying, this really wasn't a switch in your mind at this time, it was just a different aspect of the same problem, I guess.

Bolton

Yes. We built for the first time a steerable antenna.

Sullivan

Right, and didn’t use the sea interferometer technique.

Bolton

Well, we used the same, actually the 9-Yagi steerable antenna was on an equatorial mount, but it had a third axis in it, which when you pointed it to the sea, you could rotate. The declination axis became the horizontal axis.

Sullivan

Oh, I see.

Bolton

Yes, it had a broken declination shaft in it with a rotation.

Sullivan

But what I'm getting at is from an astronomical point of view, in your mind at that time, you would consider this just a different aspect of the same problem as the discrete sources?

Bolton

Well, we had some radiation and it was our job to study it.

Sullivan

Right. And what do you think were the primary results that came out of that 100 megahertz survey?

Bolton

The galactic radiation one?

Sullivan

Yes.

Bolton

Well, as our antenna was too small...

Sullivan

The resolution you had, I think, was 17°. Nevertheless, I think it did give you hints.

Bolton

There were technical problems, like establishing the antenna temperature scale, matching receivers to antennas and that kind of thing. That was one of the first large antennas on which spherical integration was used to determine the gains of it.

Sullivan

Spherical integration, I'm sorry, I don't know what that means.

Bolton

Spherical integration of the beam pattern.

Sullivan

By using a transmitter or something?

Bolton

Yes.

Sullivan

And actually, since you bring it up, was that transmitter far enough away as it turned out?

Bolton

It was the Sun.

Sullivan

Oh, you used the Sun. Someone else used a transmitter a mile or so away and it turned out there was a calibration error

Bolton

That was Christensen.

Sullivan

They didn't realize how far the far field was, I think. Okay, you clearly realized you didn't have enough resolution, but nevertheless, you seem to have plunged into quite a bit of analysis of this survey with Westfold, I assume, sort of churning away on models and this sort of thing. In fact, you even went so far as to jump into the argument of trailing arms versus leading arms. What was that based on?

Bolton

Well it was based on the angle of the concentration in Cygnus.

Sullivan

On the two dimensional sky, I'm just not quite sure how you can see whether it's trailing or leading?

Bolton

I can't answer that question myself.

Sullivan

I'm afraid I haven't looked at the paper, only the abstract. You can see a concentration for an arm...

Bolton

Well, it was based on the angle- we knew which way the galaxy was going around. I forget what it was based on, that, but it led to quite some arguments for a few years.

Sullivan

Yes. And what about these secondary peaks that, of course, had shown up in earlier surveys by [Grote] Reber and so forth? You had the one in Cygnus, then you have Sagittarius- was it beginning to become clear from this survey that Sagittarius might be more than just a diffuse concentration?

Bolton

No, our work on Sagittarius really didn't get going until we got the 80 foot hole in the ground.

Sullivan

Okay, so, at this stage then you weren't really thinking of it as a discrete source in any sense?

Bolton

No, although [John "Jack" Hobart] Piddington had found it as a discrete concentration at, I think, 25 cm. And then [Richard X.] McGee and I with the 80 foot dish really mapped that region. I mean, building the 80 foot hole in the ground was partly dominated by the fact that we could see the Centaurus source, and we could see the region of the center of the galaxy.

Sullivan

So what you're saying is this was another strong point for such an antenna that these things went near the zenith and...

Bolton

Yes. Well, we did have the choice of building it off axis. We could have built it on a slope.

Sullivan

Well, that's true.

Bolton

In fact, it was very difficult to build it straight up.

Sullivan

Why’s that?

Bolton

With the configuration of the land, there was an awful lot of sand to move.

Sullivan

I see. But once again, that's getting a little bit ahead. Another interesting thing is that in the 1950 paper on the 100 megahertz survey you use a type of restoration of the beam. Where did the idea for this come about?

Bolton

I can't really remember. I suppose I must have suggested it to Kevin and we simply got going on it. It’s a very similar in fact nowadays to the "Clean" Program.

Sullivan

Well, is it really?

Bolton

Yes, the integration process which we used is not allowed to produce any holes.

Sullivan

Okay. You were not bothered at all by, I don't imagine at this stage you were thinking in terms of Fourier theory, which [Ronald N.] Bracewell brought in a few years later?

Bolton

Well, in fact, we did Fourier transform.

Sullivan

So I see, in the mathematical analysis of this restoration. But you had no philosophical...

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Modified on Tuesday, 16-Dec-2014 15:52:53 EST by Ellen Bouton, Archivist (Questions or feedback)