[Barry Clark fights punched paper tape, 1964]
Barry Clark fights punched paper tape, 1964

[Marshall Cohen, Barry Clark, Ivan Moisiev, Semeis, 1971]
Marshall Cohen, Barry Clark, Ivan Moisiev, Semeis, 1971

[Barry Clark at OVRO 40th anniversary, 1998]
Barry Clark at OVRO 40th anniversary, 1998



NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with Barry G. Clark
At the Very Large Array, Socorro, New Mexico
2 May 1978
Interview time: 45 minutes
Transcribed by Sierra Smith

Note: Note: The interview listed below was conducted as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009) and was transcribed for the NRAO Archives by Sierra E. Smith in 2016. The transcript was reviewed and edited/corrected by Kenneth I. Kellermann, Ellen N. Bouton, and by Barry G. Clark. Any notes of correction or clarification added in the 2016 reviewing/editing process have been included in brackets; places where we are uncertain about what was said are indicated with parentheses and a question mark, e.g. (?) or (possible text?). Sullivan's notes about each interview are available on Sullivan's interviewee Web page. During processing, full names of institutions and people were added in brackets when they first appear. We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of Sullivan's original cassette tapes.

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.

Click start to listen to the audio for part 1 of the 1978 interview. (Link for part 2 audio is below.)



Begin Tape 104B

Sullivan

Ok, this is talking with Barry Clark on 2nd May 1978 during a convenient power blackout at the VLA. Alright, can you tell me when you first came into contact with radio astronomy?

Clark

I was a summer student working during the summer at Owens Valley Radio Observatory summer of 1957. That was the summer after my sophomore year. I was an astronomy major and during my sophomore year I went around the astronomy department looking for a summer job. They sort of chased me over to the radio astronomy people. So I got that job.

Sullivan

What did you actually do that summer?

Clark

During the summer I lived at the Owens Valley Observatory. And at the time there was a 26 MHz interferometer there that had been built during the preceding winter, which consisted of half wave dipoles strung on 4x4s about a quarter wave above the ground. And the first thing I was put to doing that summer was enlarging this dipole covered area, which involved shoving brush and dirt out of the way with a bulldozer - figuring a telescope with a bulldozer - and digging post holes for 4x4s.

Sullivan

But this is John Boltonís style I gather?

Clark

Well to some extent, yeah. John is a very good bulldozer operator.

Sullivan

Was this all in preparation for the occultation?

Clark

No, the idea behind this instrument was to monitor ionospheric fluctuations and it was just left running all the time. And part of my job that summer was to keep it in chart paper and complain if it wasnít working. One of the reasons it wasnít working usually was that it was a power supply with a bad contact in it. So a swift kick in the power supply usually restarted it.

Sullivan

Now were you looking at radio sources to do this? Was it just a transit thing?

Clark

Right. The beam was adjustable at declination in rather coarse steps by changing lengths of feeder lines and was just a transit instrument in right ascension. And so it just saw what went by, Cas, Cygnus, Taurus, and, I guess, Virgo, were immediately obvious on the records. You could see other things if you wished but you had to look for them.

Sullivan

What was the purpose of this? Just something to doÖ?

Clark

To a fair extent. The idea behind building it, they said, was to look for the ionospheric fluctuations. So we complied a goodly amount of data about what were days of severe fluctuations, and how severe did they get, and so forth, which never saw the light of day.

Sullivan

Yeah. What Iím wondering is here is this Owens Valley Radio Observatory. It seems like a rather strange thing to do. Itís not really astronomy.

Clark

Well, that was very much in the line of radio astronomy of the day.

Sullivan

I agree. The idea was to go to higher frequencies, certainly, eventually. It was going to be directly useful.

Clark

Yes, the real motivation was to get an instrument on the site as quickly as possible.

Sullivan

So, I guess it was the following summer, now that I look at it, in June í58, when you did theÖ or was there anything else that first summer, Iím sorry.

Clark

Letís see. When was the paper?

Sullivan

The paper was í58 and the occultation was also June í58. [J.G. Bolton, G.J. Stanley, B.G. Clark. A Solar Occultation of the Crab Nebula at a Wavelength of 12 Meters. PASP 70: 594, 1958. http://adsabs.harvard.edu/abs/1958PASP...70..594B]

Clark

Right. Well, there wasnít much else going on the first summer. They were going doingÖ the east-west railroad track was already there and I did a little conduit screening. But as far as the eventual interferometer, mostly it was puttering with this 26 MHz machine, just keeping it going.

Sullivan

Was that the first interferometer with a railroad track? Now that I think about it.

Clark

I believe so.

Sullivan

Was there any uncertainty about whether that would work, or uncertainty about putting a telescope on a railroad track, do you remember? Or was that pretty straightforward?

Clark

Well, there was a bit of uncertainty in that the design which supported the 90 foot telescope on only eight wheels, and, not very big wheels at that, was loading the wheels far beyond normal railroad practice. The theory was that we would be saved by the fact that we were never moving very fast, and that the total amount of traffic over a given amount of rail was not very great. As a matter of fact, if you go back and look at the rails now, you will see that they are sort of squashed over at the top. But they still work. The rail itself was very nearly Civil War surplus. It was military surplus, mostly 60 pound rail; considered very light by present standards and put on a concrete sleeper.

Sullivan

What do we have on the VLA here for comparison?

Clark

This is 80.

Sullivan

80 pound.

Clark

Anyhow, some of the rails were dated and the earliest date I believe was 1881.

Sullivan

Oh, youíre not exaggerating!

Clark

Not much.

Sullivan

So you came back down the following summer and what did you work on then in the summer of Ď58?

End of Tape 104B

Beginning of Tape 105A

Click start to listen to the audio for part 2 of the 1978 interview. (Link for part 1 audio is above.)



Sullivan

Barry Clark on 2nd May í78.

Clark

The first instrument that Caltech radio astronomers had was a 32 foot telescope on Palomar Mountain.

Sullivan

Right, Iíve heard about that.

Clark

Which was used mostly by Tom Matthews and Dan Harris for a hydrogen line survey. Then as soon as the Owens Valley site was reasonably ready, they moved this 32 foot to Owens Valley. So Thanksgiving of '67 a group of random undergraduates bossed by Dan Harris went up and finished taking the 32 foot apart at Palomar. And the pieces were carefully labeled and carried to Owens Valley and reassembled there. And sometime in that winter or early spring, at home at Caltech campus, there were fiberglass panels made for it. The original surface had just been fairly coarse mesh and had been good enough for 21 cm. But it had been Johnís idea that when it was reassembled at Owens Valley, it would have this fiberglass surface and hopefully be good to somewhere in the neighborhood of 3 cm.

Sullivan

I see.

Clark

So that summer of í58 then, we took the fiberglass panels that had been cast at Caltech and started putting them in the dish, putting the dish together. And when we put it in and started surveying it, we discovered, of course, they didnít fit. And for reasons that are far from clear to me, the circumference of the outside of the panels was a couple of centimeters too great. So it ended up being my lot to reduce that couple of centimeters by means of a handheld sander.

Sullivan

Once again real radio astronomy.

Clark

Yes. Incidentally, the 32 foot has never worked once it was taken off Palomar.

Sullivan

Thatís what I was about to say. Iíve never seen any 3 cm publications out of Owens Valley. I have seen it over on the junk heap. Itís still there.

Clark

Yes.

Sullivan

But whatís the end of that story, just to finish that one off.

Clark

Then we tried to make the fiberglass into a reflector by rolling ordinary kitchen aluminum foil, gluing it down to the surface. And then sometime the following winter that all blew off, which is one of the reasons it never got reused.

Sullivan

And interest just had shifted after that I suppose.

Clark

Eventually the interest shifted to making that a radio link interferometer by í62 or í63, I guess. They had concluded that is what should be done with the dish. It was lifted off its permanent foundation and put onto a set of surplus airplane wheels. But again, the radio link electronics never had taken high enough priority to actually work.

Sullivan

How far away was it going to go roughly?

Clark

Well, its line of sight had to be from the design of the radio link, but the line of sight in the Valley there could be up to 30 or 40 kilometers, I guess, without any trouble.

Sullivan

Well, Iíve never heard about that dish except up on the mountain. Thatís interesting. Well, tell me about this occultation in í58.

Clark

Well, again I was just sort of keeping track of making sure the machine had chart paper, and so forth, in it, and wasnít really aware of what was happening. And John Bolton, in looking over the records, rather to his surprise, noticed that the Crab wasnít there one day.

Sullivan

Oh, you mean it was accidental?

Clark

Oh, yes.

Sullivan

Oh, I see. Well, this experiment had been done a few times before. This was accidental.

Clark

It never occurred to anybody that that was an appropriate machine for getting an additional point on the spectrum of it.

Sullivan

And you happened to be looking at Taurus A that day fortunately, I guess.

Clark

Well, as I say, the beam was pretty broad, probably parked somewhere near the zenith. Thatís enough to get the Crab. So noting when it went awayÖ well, eventually John put the paper together and put my name on it because I had been faithful about putting chart paper inÖ

Sullivan

I see. What it a matter that he looked the records and said what happened this day and then all of a sudden it occurred. Or was it a matter of someone said, ďGee, there was an occultation a couple of weeks ago?Ē

Clark

No, no, it was a matter of his looking at it and saying, ďHey, where the Crab?Ē.

Sullivan

Now thatís strange. What else did you work on that summer? Was it mainly still monitoring with that low frequency?

Clark

Well, that was a small part of my time. It was mainly working on the 32 foot and miscellaneous small chores. More pulling wires and putting in conduits for the 90 footers that were being built.

Sullivan

But you were still an undergraduate, so I guess you went back to school again.

Clark

Right.

Sullivan

And the following summer, was that the next time youÖ

Clark

Yeah, yeah. So that summer they had single dish receivers on ninety-foot one. And Dan Harris had been using, I believe, a 960 MHz receiver, measuring positions and looking at doing essentially the CTA list of sources. And Radhakrishnan was working on putting together a 21 cm receiver. The 21 cm receiver was a real beast. He was still using it to frequency switch so there wasnít a great deal of stability as a continuum instrument. That spring John Bolton had been taking observations of Centaurus A, just drift curves. One or two a day as the object drifted through at 960 MHz. And I did some of the plotting. John did some of the plotting of that. I did the reductions on it, subtracting the point source and integrating various parts of it, finding out how much flux was in each thing.

Sullivan

Modeling it more or less?

Clark

Sort of, I guess.

Sullivan

Yes, yes.

Clark

A double with a point source in the middle and a curious extension on one side. It is not clear anymore whether that belongs to Centaurus A or our Galaxy.

Sullivan

You havenít mention Gordon Stanley. What was his role in the developmentÖ?

Clark

He was the electronics guy. He did most of the electronics design and assembly. There was a technician there who had been there for many, many years, John Harrington, who did a fair amount of the assembly. Gordon, for the most part, designed those mixer receivers that were really a very nice piece of engineering at the time. They were narrow band but they have a factor of 2 lower noise temperature than anybody elseís mixers.

Sullivan

Really? I see. What were they? Do you remember the value?

Clark

About 250 [degrees] double-side band at 21 cm.

Sullivan

So now we are up to the summer of í59 actually and I guess thatís when you graduated.

Clark

Yeah.

Sullivan

And obviously by that time you were already were a radio astronomer.

Clark

Well, to a very minor extent. About that time also I did a little Schmidt print looking at Dan Harrisís positions, which didnít come to too much. Mostly pretty well known identifications, those that had identificationsÖ

Sullivan

This wasnít accurate enough to make new ones yet?

Clark

Yeah. It was a couple of minutes, maybe even 5 minutes, uncertainty. Anyhow, I took Polaroids of the positions and marked the coordinate systems on them.

Sullivan

What was the operational procedure to do that? Find reference stars nearby and then just offset them more of less?

Clark

Yes, yes, using the AGK2 [Astronomische Gesellschaft Katalog 2]. My procedure was to pick three stars and just solve for two plate scales and rotation. Other people did more elegant things later.

Sullivan

Well, this is pre-computers still, I guess, or did you have a small computer to work with?

Clark

I always worked it out with a slide rule. A couple of years, or a year later, Fritz Bartlet was doing the same thing. He used more stars and more plate constants and programmed a little computer.

Sullivan

So, as a graduate student, you mentioned this attempted identification work. What things did you get involved in otherwise?

Clark

Well, the interferometer worked for the first time, I guess, that winter. And then in the summer of í60, they essentially parked Ken Kellermann and I at the observatory to run the place for the summer. They said, ďHereís all the pieces. Go ahead and hook it up as an interferometer. Itís been done before. You wonít have any trouble.Ē Which, of course, we did. But, by golly, that was a good education. We learned so much, so fast.

Sullivan

So you got it working that summer?

Clark

Yeah. It started out on Kenís thesis measurements, the source spectra. And I canít recall when it was that we first got the line interferometer going. Sometime after that summer. But there were a couple of things that needed building ó the set of crystal filters, for the 30 MHz, or 10 MHZ IF rather. We just couldnít do anything without these filters. The previous set of filters had been about 30 kilohertz wide, just too wide.

Sullivan

You needed a swept frequency LO also, I guess?

Clark

And their out-of-band rejection wasn't good enough. We also needed the swept frequency LO, which was designed and built by Bob Wilson.

Sullivan

You say that it had been done before but, in fact, you were operating at higher frequencies than people usually did interferometry and perhaps than anyone had done interferometry. Is that correct?

Clark

Well, what I mean was, that Bolton, Stanley, and Radhakrishnan had gotten the thing working at 960 MHz early that spring. And then had disassembled the interferometer and gone by to single dish for some stuff. And then Ken and I got there and they said, ďOk, hook it up.Ē

Sullivan

But, nevertheless, this high frequency, for that time, interferometry was a good bit trickier than low frequency stuff, I would suspect. Or is that not true?

Clark

Yeah. I donít know. Iím only familiar with the one interferometer at that time.

Sullivan

Thatís right. Thatís what you were brought up on.

Clark

The baselines were considered fairly long except in comparison to the Jodrell Bank machine. But those baselines and wavelengths were still respectable except for the very longest elsewhere.

Sullivan

What was the first science that you began doing with the interferometer?

Clark

Well, it was quite a number of things. I donít really recall the order except the first thing we did was Kenís thesis observations, which I was serving as an observer assistant on. And Radhakrishnan was working on polarization at the time. I helped again as an observer assistant and what have you. And that eventually went into [George] Seielstadís thesis. Because of the fact that I had done or had been involved in this Crab occultation at low frequency, when the AFCRL [Air Force Cambridge Research Laboratory] people said that they had an anomalous broadening in the Crab at about a gigahertz, I donít recall, I was very interested in that so I did some observations with John Wyndham to check that. [J.D. Wyndham, B.G. Clark. Occupation of the Crab Nebula by the Solar Corona in June 1963. Nature 200: 766, 1963] We were able to set 20 times or 100 times smaller limit than what they said.

Sullivan

Now this was a map of the Crab or was this an occultation again?

Clark

It was an occultation comparing the same baseline when it was near and far from the Sun. Since the visibilities were the same, we were able to set a limit of the scattering that was very much smaller than the Crab itself. Despite the fact that we didnít have enough phase to map it, we made observations of polarization at the time. We were hoping to the get the Faraday rotation out and therefore the magnetic field and electron density of the Sunís corona.

Sullivan

The rotation would come from the comparison with the NRL [Naval Research Laboratory] observations at higher frequencies?

Clark

No, just watch the polarization angle as it gets near the Sun.

Sullivan

Oh, I see what you are saying. I see.

Clark

It was essentially defeated by the complexity of the Crab. You really need a polarization map or a shorter baseline than we had to figure out what was going on there.

Sullivan

One question which comes to mind, maybe not directly relevant to you though, in retrospect one can look back and say that the Owens Valley interferometer could have been aperture synthesis at a very early stage. Do you have any theories on why that didnít happen? Or was there no need?

Clark

Well, we were very aware of the possibility. With the plus or minus 4 hours in hour angle, you have to have a lot of configurations to do it. So people tended to do the one dimensional synthesis as Al Moffat did, rather than to try to do the two-dimensional thing. We did tend to think in terms of aperture synthesis. If you asked would an observation at this configuration give you need data or duplicate something you already had, then we would plot it on the UV plane. And think of it in terms of a map like that.

Sullivan

But no one quite had the interest to put in the dedication that would be required?

Clark

To do a full two-dimensional synthesis would have required just very many configurations. And even then it wouldnít have been too good because of the missing stations. That interferometer was built when much less was known about the structure of sources. So the stations were exponentially spaced rather than uniformly, or near uniformly. So while the original spacings were at 200, 400, 800, and 1600 feet, (they've added more since). It had restricted stations, restricted hour angle coverage.

Sullivan

Still it would seem like that might be of great interest, to come up with a single high resolution map. But thatís all in retrospect, of course.

Clark

It was just difficult. [2016 note added by K.I. Kellermann: Alan Moffet later claimed that Caltech, unlike Cambridge, did not have the computing power to do a full 2-D synthesis.]

Sullivan

I guess we are up to the stage where you began working on HI interferometry. Tell me about how that went.

Clark

Well, it went fairly smoothly actually. I did a little playing around with the receiver. And, as a matter of fact, cobbled up a receiver that only I used. In my life I have designed a total of two circuits. One of them was built into that, and the rest of it was miscellaneous pieces that were lying around. One of the problems with doing line interferometry with mixer receivers is rejecting the image. And the image rejection scheme we used was to offset the LOs at the two antennas. And because it was convenient, we offset them by 2 MHz. And then, we used a couple of different schemes. We had a broadband and a narrowband receiver. The narrowband one, we just mixed one IF back up. The broadband one was sort of cute in that the two IFs from the two antennas were just multiplied together to get the 2 MHz fringes. And then we did an ALC on that. It was a slightly more elegant scheme than the ALC on the IFs. And then just detected 2 MHz fringes. But that worked pretty well. The real question was how deep were the deepest absorption lines. And it was a lot of instrumental fiddling around about how much contribution was there from the wings of the filters. A number of questions like that called for a careful instrumental investigation.

Sullivan

And also I guess the confusion with how much small scale structure emission might beÖ?

Clark

Yes. For that reason, I looked at two or three regions of emission with reasonably long baselines. The sort of things towards the anti-center where things are really quite hot. And saw no fringes at the levels of interest so I was happy with that. Although with the more sensitive receivers now, you can see fringes at 200 feet anyway. On occasionÖ

Sullivan

Yeah, like you say, that is much more sensitivity. When you went into this experiment was the idea was that there might be a different component to be seen in absorption that in emission? Or was it just withÖ

Clark

It was already clear that the absorption lines were much narrower and much spiker-looking than emission lines that were general in the same direction. And this could be explained on either of two bases. That there were temperature segregated clumps and the absorptions lines see only the low temperature ones and emission lines see mostly the high temperature ones. The basic suggestion there was John Boltonís. I just took it and worked it out. Another possibility, of course, was that it was not temperature segregation but spatial segregation. So that if you looked at an emission with a 2 minute beam, you might also see this.

Sullivan

This was the original idea wasnít it of [John] Hagen and [A. Edward] Lilley andÖ

Clark

Well, they didnít phrase it quite that clearly but that wasÖ

Sullivan

It was implicit in what they had.

Clark

Right. And the thing we did that sort of convinced me was, well, just looking at enough sources to get a statistical handle on what the absorption lines look like statistically. And then you can readily add a few up and see whether you get something that looks like the emission. And the answer, of course, is that you donít, which starts making the two component model look pretty good.

Sullivan

And where did the idea of possible pressure equilibrium come, which as I mention seems like sort of a throw-in in your 1965 paper?

Clark

Well, once again, we knew at the time that these clouds are not dense enough to be stable, gravitationally stable. So they must either be made furiously or must be in some sort of pressure equilibrium. There is no very convincing argument either way even now, but throwing in the assumption of pressure equilibrium will let you do a few things that are interesting. So why not.

Sullivan

Letís see. Is there anything else that you worked on at Caltech that we havenít covered?

Clark

Well, there were the Venus measurements. This must have been the winter of í62-í63. An inferior conjunction of Venus, and I wished to resolve the planet and attempt to measure its diameter. And to do that, 10 cm and 1600 feet was just barely adequate. And it was the first time the interferometer had been used at that baseline and frequency. It had been used at each separately. And I got the calibration pretty badly loused up. And the result was that the diameter came out much too large. And this would have fit in with some of the ionospheric models at the time which called for limb brightening. But I didnít have a great deal of confidence in it at the time. And then just before I left in the summer or spring of í64, [Arkadii Dmitrievich] Kuz'min was visiting. He and I redid Venus.

Sullivan

Did that first result get published?

Clark

Yes, it was at a meeting on Venus at JPL [Jet Propulsion Laboratory] and it had extended abstracts or something. [Citation to full paper: B.G. Clark, A.D. Kuz'min. The Measurement of the Polarization and Brightness Distribution of Venus at 10.6-CM Wavelength. ApJ 142: 23, 1965, http://adsabs.harvard.edu/abs/1965ApJ...142...23C]

Sullivan

Yes, I have seen that report. Thatís where [Frank] Drake alsoÖ he was working at JPL, I guess, looking at water vapor and things like that. Well, in conclusion, as you look back at the early days of Caltech, what would you say were the main contributions that Owens Valley made to radio astronomy of that era?

Clark

Moffetís thesis observations put the double radio sources on a solid footing for the first time. Showed there were so many of them. Radhakrishnanís and [G.L.] Bergeís Jupiter observations tied that down for the first time. And Iíd rate the hydrogen line stuff below that. But it's still useful. And letís see, Matthews' positions were potentially useful. They didnít have quite the pay off one would have hoped.

Sullivan

Is there anything in the approach in the way that things were done at Owens Valley or the equipment that was there that was different or enabled one toÖ do you see what Iím trying to get at?

Clark

Yeah. There were a lot of ideas in that receiver, a few ideas for the first time. Many of them that we have copied shamelessly ever since. And then the development of mechanical analog computer for fringe stopping. Jodrell Bank did the same, more or less independently from Caltech. The fringe stopping, automatic delay tracking. I don't know who has priority on that. The interesting tricks you can do with mixer receivers to sort out side bands if you are so inclined. The fact that mixer receivers are insensitive to IF phase shifts. And as I said, those mixers were very good for the time.

Sullivan

What about the idea of the north-south, east-westÖ or was there only east-west at this time, track?

Clark

The year that the north-south was added was probably í60 or thereabouts. I remember I was out surveying in rails. Ken Kellermann was soldering together wires.

Sullivan

Do you see that as a major step in the development of radio astronomy, just the idea of having rails and putting dishes on them? Or was that sort ofÖ

Clark

Thatís a point. It hadnít occurred to me that was the first large railroad-mounted interferometer. I donít recall how that Jodrell Bank remote element moves.

Sullivan

They just carry them around in the back of the truck I believe, certainly in the early days. I was just talking to Dick Thompson. And then I think also even into the early 60s, I think, they were just carting them around the countryside. They were operating at lower frequencies, of course, and didnít have to have dishes. I donít think they had dishes, did they?

Clark

No.

Sullivan

Well, I think that covers it pretty much. That ends the interview with Barry Clark on 2nd May í78.


Modified on Tuesday, 05-Apr-2016 09:37:08 EDT by Ellen Bouton, Archivist (Questions or feedback)