Interview with Edward F. McClain
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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.
In preparing Sullivan interviews for Web publication, the NRAO/AUI Archives has made a concerted effort to obtain release forms from interviewees or from their heirs or next of kin. In the case of this interview, we have been unable to find anyone to sign a release. In accordance with our open access policy, we are posting the interview. If you suspect alleged copyright infringement on our site, please email archivist@nrao.edu. Upon request, we will remove material from public view while we address a rights issue. Please contact us if you are able to supply any contact information for McClain's heirs/next of kin.
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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Transcribed for Sullivan by Pamela M. Jernegan.
Sullivan
Ok. This is talking with Ed McClain at his home in Morningside, Maryland on 22 December ’73. Well, tell me a little bit about your beginnings at NRL [Naval Research Laboratory] and so forth.
McClain
I started in 1942, right after World War II. Everybody, of course, was looking for something to do in this field and I had 2.5 years of school at that time. I had a friend who was working at NRL and during Christmas vacation we applied for jobs and were hired. And stayed there all during the war and until I retired actually, 26 years in total. Originally, I worked on 3 cm radar, built the first 3 cm radar the Navy ever had, I guess. That was down on the Chesapeake Bay. I tested a lot of production radars that were coming and out, that sort of thing, all during the war. After the way, I was...
Sullivan
By the way, did you ever hear of any solar interference on these radars?
McClain
No, but like a lot of other people, we tried to radar the Moon. I remember one night we went out with this first commercial 3 cm radar which had a peak power, I guess, of about 100 kilowatts. Without any calculations or anything else, we just simply pointed it at the Moon and sat back and waited to see if we could see anything, turning it on and off, but if you put the numbers in the equation it doesn’t come out you know- they had a little 3 foot transmitter and a 3 foot receiving antenna and it was just too much loop lost that the gun detected, but a lot of people did this stuff. Now [John P.] Hagen started playing around with the sun in the late ‘40s, I guess, the middle of World War Il and, of course, it was no trick at all to take a radar receiver and point it at the Sun and see a rise in the graphs on the radarscope. But I didn't really get into radio astronomy until right after [Harold Irving 'Doc"] Ewen and [Edward Mills] Purcell did their work on the hydrogen line I'd been working on aircraft navigation systems, Doppler navigation systems, where you point a couple of 3 cm RF beams at the ground and measure the velocity vector along those beams and put it in the computer and find out what the true ground speed and...
Sullivan
Why two beams?
McClain
Well, in order to solve the vector equations, we had we had two beams that were 10° to 20° out from the axis and 20° back.
Sullivan
They were on different axes.
McClain
So you get- coupling of that with an altimeter, you get the full solution of your position. So we got, I guess, the total measurement to build these computers and take out the roll, bank, ditch and plus the two velocity components off these RF beams and you could go down to Washington, fly over the airfield, crank in your longitude and latitude and you'd be within about 1% of your position. I was also completing our Air Force project that we got involved in those days, we’d go back and forth.
[Interruption]
McClain
So then I, I guess, started working on this. Well, I'd done some work on it all during the war. It was a real low priority project during the war and then by 1951 or so it had gotten to the point where we went into pilot production and wrote up specs for the Bureau of Aeronautics, and let out a pilot production contract on it. NRL more or less backed out on it, but now I understand that [Bendix and ? Laboratory?] and probably other companies probably have these things flying on airliners and of course, the military uses them.
Sullivan
Same basic principles, yes.
McClain
It was a gratifying piece of work; it worked pretty much the way we hoped it would. You know, you come out with a 70-80- we figured if we could get under 100 pounds with it we were in business, We could put it on Navy fighter planes, maybe one that would lead a squadron, say. He could keep everybody knowing where they were. But anyway, it's been in commercial production for years and years now. But anyhow, while I was in OS, people like Fred Haddock and John Hagen were playing around with radio astronomy because after the war, it became pretty generally recognized that a lot of our success in World War II had been due to a rather solid scientific base and the swing was then to go away from application and engineering, you might say, applied science, if you will, back to some basic investigations that might in future years pay off the way such work had done prior to World War II or in World War II. So Hagen, I guess, was the one who really had a tremendous knack for being able to sit back and look ahead and figure out what was going to pay off. He was a really good poker player. And Fred Haddock was pretty much in the same boat, and the two of them got very interested in solar radio astronomy. One of them set up an eclipse expedition. Connie [Cornell H.] Mayer, for example, went on a...
Sullivan
Yes, I talked with Connie so he's told me details about those.
McClain
So these fellows were running off around the world while I was working on my automatic navigator and I didn't pay too much attention to what was going on until Harold Ewen discovered the hydrogen line and Hagen took me up to Boston to check it over- well, Hagen decided that he was going to have Ewen build him a receiver. So he took me up there to get checked out on it and as a result of that, I spent about a year or a year and a half, not exactly redoing Doc’s work, but adapting the thing to work on the 50 foot telescope at NRL and also trying to get the 50 foot telescope to work because as you probably know it was a beautiful mirror, but it was put on a 25 inch gun mount. Well, a gun mount is made to operate with very low moment of inertia at very high velocity, and of course, we had the moment of inertia at maybe 100 times over a couple of 5 inch gun barrels and the drive system just wasn't compatible with the mass that it had to move, and the darn thing wouldn’t hunt. So they put in 9 to 1 reduction gears; that didn't solve the problem. So first thing Joe Nichols and I spent must have been six or eight months building some "Dutchman", I’d guess you'd call them, a box to go between the other boxes to try to quiet this thing down, which we finally succeeded in doing it. In the meantime, we were working on Ewen's receiver trying to get it adapted to the 50 foot dish.
Sullivan
Why do you call it a "Dutchman"?
McClain
It's an old machinist's expression - a piece that goes between two other pieces to make up for somebody's mistake.
Sullivan
I see.
McClain
Anyhow, you knew Bob Myerdon...
Sullivan
No, I didn't. He retired just before I...
McClain
Bob's favorite expression. Anytime you had to fix something by that method, you had to go make a "Dutchman".
Sullivan
Let me just ask you. Do you know which ship those gun mounts came from?
McClain
I haven't the slightest idea. They came from the Navy Yard, I'm sure. See, the Navy used to make all their own guns and mounts and everything else, right down here in Washington. And they came out of stock; as a matter of fact, one thing we had to keep doing over the years was to scrounge old gun mounts that people were going to throw away to get spare parts for that one. And I guess it's still operating.
Sullivan
Yes.
McClain
Joe knew everybody in the country, I guess, who had any spare gun mount parts. As a matter of fact, a real good friend of mine right now, who I see almost every Sunday morning, was working at the Navy Yard on those gun mounts along in the late 1940's and early ‘50s. A fellow by the name of Fleming Schell. I also met a fellow by the name of Joe Dibeck. They were just a couple of guys; it's one of these things like you were talking about the measures and the fathers went away and the measures are in trouble. And the same way on gun mounts - the talent disappears and you want gun mounts to work on, you're in dire straits; there's only a couple of guys in the country who we had access to that weren't even working with the Navy anymore that we could get to come in and work on a contract basis to help us sometime. But finally Joe got well enough versed that he could keep us going. But anyhow...
Sullivan
But what about the dish itself? I mean, that was a rather large thing at that time.
McClain
It was a fantastic job.
Sullivan
Who designed that and so forth?
McClain
Well, the idea, of course, was John Hagen's. There's no question about that, as I was saying, he had a remarkable talent for knowing which direction to go.
Sullivan
And the aim was what? What was the original purpose?
McClain
Well, we in the branch during the war had been highly talented in the centimeter wave range - 10 cm and up. We worked on 10 and 3 cm, and 8 millimeters even. Nobody else in the world was working at these short wavelengths. We didn't know what was there, but Hagen figured it was something to do that nobody else was doing, and we did have the facilities and the money and the talent to pursue it and so he made this choice and had to be credited with a creditable choice. So this was also just the right time because the Navy was telling NRL to go do some basic research - keep yourself well-grounded in the basics and don't worry about building radars for us or building this or that or the other thing necessarily, put at least 50% of your effort on basics. Of course, you know, the other 50% kept on trying to advance applications and keep the fleet up to date and all this sort of thing. But we were fortunate in being one of the branches that was assigned a basic research mission and it was radio astronomy. So John Hagen's objective was to get a very high quality mirror at centimeter wavelengths- at the wavelengths we knew and find out what was there. Well, obviously there was a devil of a lot more there than we ever found out with that mirror in those early days, but we had no way of knowing. It was a thing where you didn't have the equipment available. For example, you didn't have measures that could get down below 100° at 3 cm - things like this.
So, the contract was let to Collins Radio Corporation, out in Cedar Rapids strangely enough. And what they did was- I've forgotten how many castings there are but they had a large number of aluminum castings which were edge-machined and tinned and bolted together. Then they built a huge boring mill over this assembly of castings and ground cut surface, and I think it had a f/d of something like .5, which is rather long for these days, but it also has some advantages, if you don't worry about ground radiation. It's very easy to feed. But anyhow, they machined this thing and then they disassembled it completely and shipped it here casting by casting and then reassembled it on the roof down there. It would work at 8 mm. The only trouble was with main castings- there were heavy webs in behind each, you know, periodically across the surface, there would be a heavy, deep section of aluminum, and if you had a temperature change during the day, say, these webs would soak up the heat or slow down the change in the region that they affected, And we got some very nice photographs of frost on the thing; when the frost started to melt, you could see the webbing in behind the surface. So that at 8 mm it wasn't all that good unless you had a stable temperature situation. But Hagen did make quite a few measurements and Haddock at 8 mm, I think, on the Sun- things like this.
Sullivan
I think that was all they did, wasn't it?
McClain
Just about. Up until the time when the hydrogen line thing came on, because he had all this pointing trouble, he couldn't make the damned thing point and stay there without hunting.
Sullivan
But any source other than the Sun or Moon for 8 mm; it was a long time later.
McClain
Yes. But it was used. My point was that it was actually used at 8 mm on the Sun and Moon. And if I’m not mistaken, I think, the phase effect on the Moon- Gibson was working on this; and I think that was done with the 50 foot dish, perhaps. I’m not positive.
Sullivan
Well, [John "Jack" Hobart] Piddington and [Hary C.] Minnett did a study in 1948 at K-band.
McClain
That's true.
Sullivan
I'm not sure when Gibson's was, I think...
McClain
I think their resolution was way down compared to angular...
Sullivan
Oh yes.
McClain
Angular resolution was much, much more.
Sullivan
Gibson's work was the first to show differences across the surface.
McClain
Essentially, yes. You looked at the terminator and you could see the phase changes with lags and one thing or another. Anyhow, it greatly reinforced the dust theory, I'm sure; and I don't know whether the boys when they landed found out whether it was all that dusty or not, but it worked like dust anyway.
Sullivan
Yes, right.
McClain
So that was the major thing it was used for up until, well, he started on this hydrogen line business.
Sullivan
So that was what period?
McClain
Well, it started about 1952 and I guess, in 1953 we really got the antenna so we could point it where we wanted to, and we had the receiver for we'd bought from Ewen, pretty well adapted to the dish and we'd ironed out some bugs in it and one thing and another. We made a decision to put the consoles over in one building and the dish was over in the other building, so we had the business of feeding 1400 megacycle of oscillating of power all the way across and getting the IF signal back - a lot of problems, nitpicking problems really, but it took time to solve. Anyhow we got in business some time in 1953, I guess.
Sullivan
Let me just ask you about Ewen's receivers - everyone talks about getting his receivers. What was so special about them?
McClain
Well, he'd done his original work, if I remember correctly, with a frequency-switch system, but he got this hot idea that the way to go was what he called DC comparison radiometer, which was a non-switch system and was essentially utilizing total power, and which gave it edge on sensitivity where you just compared a wide band of noise against a narrow band of noise on the line and the only trouble was that the amplifiers would amplify a wide band of noise a little bit different fashion than they would a narrow band of noise. It's very bit difficult, for example, to get a video amplifier to amplify a 2 megacycle bandwidth without the occasional peaks overloading the stays. In other words, it had to be operated at a very, very low level in order to keep it linear. So you had some linearity problems. This was part of the work we were involved in over maybe a year or eighteen months or so of trying to get these bugs out so we could believe what we saw. It was just another way of doing the thing. Not as satisfactory I’m sure at this point as frequency-switch, you know, nobody knew in those days what was going to pay off the best.
Sullivan
So he spent a lot of time thinking about...
McClain
Well, Ewen sort of went into business for himself, he sold these things to the lab and after discovering the hydrogen line strangely enough, he didn't lose interest in radio astronomy but he went into it from a businessman's point of view and built some pretty good equipment from time to time for various projects, not only radio astronomy, but that's the route he went and I think a lot of people thought less of him for it, but everybody to their own kicks. Anyhow, this was the beginnings of his company you might say. So along 1953 we got on the air with the hydrogen line receiver and proper feed and the antenna and everything, and one of the first things that we found- I guess I found it, but Hagen and I were working together and I showed it to him and he wasn't too impressed, and then we got confused over it. We looked at the center of the Milky Way and it didn't look right - instead of being a nice round profile as you might expect, this thing was depressed in the middle.
Sullivan
As you let it drift through, you mean.
McClain
Well, you drift through or stay right on the center and scan through- it had a structure in it - well, to me, I was not astrophysicist at the time, of course it's perfectly easy to figure out how hydrogen absorption works after the fact, but we weren't looking for it. And we had these profiles that were depressed in the middle and rather fine structure something like 5 kilometers per second, narrow feature, but...
Sullivan
You only had a single channel?
McClain
Only had a single channel and we could scan it either in position or scan it in frequency. We narrowed the bandwidth down to make sure, I built a special- to make sure we weren't losing anything and it was indeed narrower. The 5 kc filter apparently resolved it, but that was about what it was - about 5 kilocycles, which meant, you know, it was right down thermal - something that turned out later pretty cold. But, it was rather unfortunate for us situation. The 50 foot dish, the antenna temperature on the galactic center, was of the right magnitude such that with thermal absorption you just about come down to zero.
Sullivan
Oh, yes.
McClain
And the fact that the thing didn't go below zero, you see, didn't give us the clue that we needed. Had the thing actually backed up and gone below zero, there would have been no other explanation. So, oh there was some criticism of the way we handled it, I guess, but no excuses really, we didn't know what we had. There were some bright boys around who told it was absorption, but if it had gone below zero, you could have believed it, you know, and it went all the way from [?] to stars in the center of the Milky Way to fantastic things, you know. We really just didn't know. But anyhow, this went on for some time. I became convinced it was absorption; I talked to Ed [A. Edward] Lilley and we met quite a few hours discussing the possibilities and particularly the absorption part of it. I was pretty thoroughly convinced it was absorption - I don't know if Hagen was or not. But anyhow, the original detection of the thing was already published, so there wasn't anything we could do about that.
Sullivan
Well, did you talk with Lilley before the first paper?
McClain
No, no. Not before the first paper.
Sullivan
That probably came out quickly, as soon as you...
McClain
That's right. It was one of these things - we published it.
Sullivan
Although I think in the first paper, if I remember correctly, it says you acknowledge conversations with George Field, a young graduate student...
McClain
That's right. George was at Princeton and George visited us. There was something peculiar about this and I can't recall what it was, whether George suggested it was absorption or whether, I'm thinking about something entirely different. There was some kind of a hassle with George Field, I don't remember what it was though.
Sullivan
It doesn't say, it just acknowledges discussions with him.
McClain
Well, we had discussed it with him, but...
Sullivan
So anyway, at the time you did consider absorption but in the first paper you decided the other was...
McClain
This may have been what George suggested and what we ignored. I just can't recall for sure. But the thing just kind of hung fire for a while. We had published the fact that we had found this peculiarity, and people got interested - quite a few people came by and talked to us. The guy that's head of the department down at Indiana - I get a Christmas card from him every Christmas.
Sullivan
Edmundson?
McClain
Edmundson. Edmundson came by and he was real interested about this thing and we talked to a lot of people. But again, just the fact that there was this depression that didn't go below zero, nobody could prove anything, you see. We weren't as bright as we ought to be, I guess, but...
Sullivan
Well, you weren’t an astrophysicist.
McClain
Well, Hagen - I wasn't certainly, I don't feel guilty about it at all. Anyway, we detected the damn thing and started the ball rolling.
Sullivan
This is completely independent of what was done at Jodrell Bank a few months later when they got absorption also.
McClain
No, you're thinking of Holland.
Sullivan
No, [John G.] Davies and [David R. W.] Williams.
McClain
No - it was contemporary. It was contemporary, but I've always had a bit of a suspicion that- well, we had a visit from them between the time we made- no we're mixing apples and peaches. Galactic center absorption no one else...
Sullivan
Right but I’m just talking about absorption.
McClain
Let's get on to the real absorption story. The galactic center was hanging fire. We thought it was absorption I guess by the time Ed Lilley came down, at least Ed Lilley and I did. But Ed came down in about July of 1954, I believe, and he and I were both convinced it was absorption and so he and Hagen and I were working, and we finally got around for some- I don’t know why we waited so long. We finally got around to looking at Cassiopeia. And I'll never forget that night, because when we had the thing set up and it took about thirty minutes to run a profile, and we had the thing all set up tracking Cassiopeia and the damn pen comes along and starts dropping and it drops and it drops and it drops and I'm watching, and I said, "My God, we've got a hole!" Well below zero. Not only one, but two well below zero and a third one, if I remember right, just breaking the line. Well, this set off about a month and a half to two months of panic head stretching. We were convinced it was absorption, but it's real easy to see in hindsight, of course, that obviously the only thing that can do any absorbing is the very small column directly in front of the radio source, but at the time, it wasn't so easy to see, and it was quite some time before, well, I wanted to work on the receiver...
Sullivan
What were you thinking...
McClain
We were worried that maybe this was some kind of processing phenomena in the receiver itself, you see. Nobody had worked out the equations on how the receiver worked. So I went to work on that and that appeared as Appendix A in the paper. Ed Lilley and I scratched our heads, and he did most, practically all of the work, I guess, on the mechanism and finally between the three of us, I know at one time I'd shown Hagen how the receiver works and he told me I'd better go back, go back to my desk because I had it all wrong - he said there's a factor of two missing somewhere, I remember that real well. I said "No, John, this is the way it works - it's got to be. There's no factor two missing there's something happening." I don't know who's responsible for what, but anyhow, it finally dawned on us and we were all three convinced that what was happening was that there was a small column of absorbing gas cooler than the radio star Cassiopeia and that there was fine structure in this gas as there obviously would be on a narrow column like that and that there were three very narrow...
Sullivan
Giving very narrow features...
McClain
Giving intense absorption features across the profile associated with the spiral arms. Well, this really set off a donneybrook, because - oh who were the two guys at Caltech? The two experts- [Rudolph] Minkowski...
Sullivan
[Walter] Baade and Minkowski yes.
McClain
Baade and Minkowski had just determined that Cassiopeia was I believe at 500 parsecs.
Sullivan
That's right.
McClain
And of course, once we understood what we had on the chart paper, it was perfectly obvious if anybody believed [Jan Hendrik] Oort's model of the galaxy, that it wasn't 500 or 800 parsecs, it was about 2,500 parsecs beyond the third spiral arm. So this was something else. We were on a collision course with two of the top experts in the optical business at least. So I'll never forget, I think it was the 100th meeting of the astrophysical society, at Princeton and Baade and myself and Fred Haddock - Baade came to me and wanted me to explain to him what was going on, how this thing worked. Now I didn't feel so bad then because I figured an optical astronomer ought to understand absorption upside down and backwards and we'd already published, I believe at that point in the Astrophysical Journal, and completely laid this thing out. Showing how the fine structure and the gas could cause the deep absorption holes and how the thing actually was a tool of immense resolution because the angular dimension you were working with was a dimension of the star itself, the radio star - the radio source. So anyhow, Baade got ahold of me and wanted to know if I would go over this with him, so we went over in [Lyman] Spitzer's office and he and Fred Haddock and I were there. I got up on the blackboard and laid the whole thing out for him. I'm sure Baade at that point didn't really understand it either, because he kept saying 'yes' and you know, he gave all the appearances of understanding it, but really I don’t' think he appreciated the power of the tool.
Sullivan
The fact that it was in your beam...
McClain
He was used to classical methods and I seriously doubt he expected really to have his and Minkowski's distance proved wrong, by irrefutable evidence, let's say.
Sullivan
Yes, by such a large factor.
McClain
Or by such a large factor. So this was a really interesting session with Baade anyhow - he apparently grew to be very fond of me and I of him, because he reminded me a great deal of my grandfather - he was German, of course, and he had a limp which my grandfather had broken his leg and he had a limp - they looked alike, and I told him, "You know, you're a dead ringer for my grandfather whom I thought very much of," and every time I saw him at a AAS meeting, he'd always come and we'd have pleasant words and conversation. It was some time later, I can't tell you the exact date, but I have the letter encased in plastic upstairs. I got a letter from Baade and he says either we or I or Minkowski had just redone our work on the distance to Cassiopeia and I have come to the conclusion that we could not see the forest for the trees - that the object is actually covered by dust and we were not seeing all of it, that it is much larger than we thought it was and the distance is indeed 2500 parsecs and I thought you would like to be the first to know. And that perhaps is the thing of which in all the dealings I have had with radio astronomy, the thing of which I am most proud, because this is certainly a prince of a man and a real first class astronomer, to get a letter like that from somebody is something you cherish all your life. But anyhow, at the same time, England, the people in England did...
Sullivan
Davies and Williams.
McClain
They published a paper which I really never understood, and I think if I read it again today I wouldn't quite understand it. Have you read it?
Sullivan
Oh, yes. I think I understand it.
McClain
You understand it?
Sullivan
Let's not go into it now, maybe afterwards in detail.
McClain
No, but...
Sullivan
What didn't you understand?
McClain
Well, I just didn't understand what they were getting at. How they were proving that this was taking place. They didn't have any absorption profile.
Sullivan
They had difference profiles, between on and off.
McClain
They had, yes, some - I guess I didn't understand them like Baade didn't understand ours, it's that sort of thing. I'm not saying that what they said isn't legitimate; I'm just saying I didn't understand it.
Sullivan
Because indeed they did come up with about 3 kiloparsecs.
McClain
I know. And it was after they'd been over and had a talk with us, I might add, and I'm pretty sure we'd shown them our profiles. I can't sort it out, but I know they'd been there and I know Ed Lilley and myself and John Hagen always kind of wondered if maybe we'd let the cat out of the bag. But regardless, it's all part of the game.
Sullivan
But at that stage, though, were you aware of the possibilities for getting distances?
McClain
Oh, yes. We recognized, were sure what the mechanism was - there wasn't any question about the distances.
Sullivan
Because in the paper, the large paper, Lilley, McClain and Hagen, I don't think there's anything in there about distances.
McClain
Well, there's the distance to Cassiopeia.
Sullivan
Is there?
McClain
It's laid out- 2500 parsecs.
Sullivan
Maybe my memory's...
McClain
You're talking about the Hagen, Lilley, and McClain paper?
Sullivan
Oh, you're right - that's the, you're right.
McClain
The first paper on the galactic center, there was nothing on distances.
Sullivan
They just gave a profile, right.
McClain
You can't be very proud of that paper, but the second paper laid it all out.
Sullivan
Right, but Williams and Davies paper was published before the second paper in that case.
McClain
That's perhaps true.
Sullivan
They gave the distance estimate and then yours.
McClain
Ours was in publication, I'm pretty sure. Like I say, they had been over to see us.
Sullivan
And you say the visited?
McClain
They did visit us. This is not to imply anything underhanded or anything else. But I really never did understand what they were saying.
Sullivan
Okay, I ought to maybe tell you a little bit about it afterwards, as I remember it anyway.
McClain
But anyhow, as far as I'm concerned that original paper in the Astrophysical Journal on Cassiopeia absorption laid the whole thing out for anybody else that ever wanted to work on it.
Sullivan
Oh, sure, that's a basic paper.
McClain
It was complete. Even to the description of the receiver which showed exactly how that operated. There was no question for anybody to ask from that moment on. But it was hard and boring I mean, it wasn't easy and it took a matter of two or three months I guess, before we were sure that we covered all the bases. And then the Dutch came out right shortly after that with a similar set of profiles and they were able to get a good profile of on Taurus which they had a little bigger dish than we did. That was a paper that is most usually quoted on absorption.
Sullivan
[Christiaan Alexander "Lex"] Muller’s paper.
McClain
Muller's paper, yes. But...
Sullivan
Oh, I don't know about that.
End of Tape 27B
Sullivan Tape 29A
Sullivan
With Ed McClain on 22 December ’73.
McClain
They published on Cassiopeia and gave a little bit more refined value to [Tau?], I think in the holes. Also had Taurus and I believe another source, I'm not sure. But they had more information...
Sullivan
They had the galactic center.
McClain
Yes, possibly so. It's been quite a few years ago. But anyhow...
Sullivan
That was published in 1957 though, that was still - or 1956, maybe.
McClain
I think it was the next publication on the subject.
Sullivan
Yes, but it was at least a year later.
McClain
It probably was, but it was a more refined piece of work. They had a larger instrument and therefore could get more detail, so that when one wanted to look for detail, or the latest word on what the hole looked like, it usually got quoted, but I don't think basically the paper added anything to the method as far as that goes. So that...
Sullivan
So that was the first non-solar thing that was done on the 50 foot dish, is that right?
McClain
Well, I kind of hate to make that a definite yes because I might be short circuiting somebody, but it's the first thing I really remember - solar or linear, one or the two.
Sullivan
The 21 cm continuum and the 10 cm continuum were a little bit later.
McClain
Well, now we did some 21 cm continuum simultaneously with the galactic center part.
Sullivan
Simultaneously, I see.
McClain
There was a letter the editor I believe, published in the Proceedings of the IRE. This was simply a number of sources with the temperature of 1400.
Sullivan
That was with Nan [Nannielou H.] Dieter?
McClain
Possibly. It may well have been. Nan was in on some of the later absorption work. As a matter of fact, I remember we were still hassling over what the mechanism was, and this I believe was- couldn't have been Cassiopeia- it was the galactic center, I guess. There was a question of who was going to Ann Arbor to give a talk to AAS [American Astronomical Society]. And rather than Hagen or I either one going, he decided that Nan would go. I remember calling her up at home and saying, "Nan, for heaven sake, I don't care what John told you, it's got to be absorption, it's not anything else."
Sullivan
He remained unconvinced you mean, for a while?
McClain
John?
Sullivan
Yes.
McClain
Well, not unconvinced but not convinced. Well, like I say, it's one of those things and looking back it's real easy to say any fool should have known that, but it isn't that easy.
Sullivan
There are a lot of things like that.
McClain
So anyhow, Nan went out and gave the talk, and I guess she sided it towards absorption. I'm pretty sure she did. But there was still a big controversy at home, I guess. Cassiopeia straightened it out anyhow.
Sullivan
So what was the next thing you worked on?
McClain
I don't even remember. Well, Ed had come down by that time. We looked at some other sources, but again, we were pretty well strapped with a 50 foot diameter, we couldn't contribute much. The Dutch by this time, I guess, had probably published their thing with their, what was it- 80 footer?
Sullivan
25 meter.
McClain
25 meter dish, and so they could outdo us on that. Fred Haddock, no wait a minute, we may have short circuited Fred Haddock. I guess along about the same time that we started - after we got the dish straightened out, Fred did some 10 cm continuum work, too, and published that. Fred also did H II regions...
Sullivan
Right, that was the first detection of them.
McClain
Which was pretty good go around for him on the first detection of H II regions by radio astronomy, I guess, at 10 cm. So this was a nice piece of work. This kind of thing continued on, I'm trying to remember what Ed and I got involved with. I don't know - we had to write papers for Scientific American and stuff like this which was always, I guess you'd call them diversions but meetings to go to. But well, about that time Al [Alan H.] Barrett came down, I guess, and he and Ed got together and were going to work on OH. And also in 1956, I was named Acting Branch Head which meant I had to kind of divorce myself. So he and Ed got together and were working on OH, and by an unhappy set of circumstances didn't find it, which Al later found when he went up to MIT. And Ed and I had our famous red shift experiment, which will go down in the annals of astronomy, I guess, as one of the larger bricks that were dropped. Although it had its good aspects. It created a fantastic interest in large dishes.
Sullivan
That's right. Well, that was one of your justifications for the Sugar Grove thing.
McClain
No, I don't think- it might have been in the early stages of Sugar Grove.
Sullivan
But the idea was that it could operate at 21 cm, though, and that you could detect...
McClain
We wanted to operate at 10 cm, and it would have, if it was successful, but we were really suckered in by statistics I guess, because we, I don't know how many thousand measurements we made and we lumped our red mean and our blue mean together and then we lumped the stuff in the middle. And I don't know how many people we had go over it, I know Connie Mayer went over all the data; I think Fred Haddock went over all the data. And if you took the data and looked at it, there was a definitely a depression - it was on Virgo I guess it was.
Sullivan
Yes, Virgo.
McClain
Which later proved out to be true. I mean, the Australians I guess.
Sullivan
Well, yes. Kueller and Robinson, but that hasn't been confirmed yet.
McClain
Oh, it's still in doubt?
Sullivan
It's still in doubt, I think most people don't believe it. Because they...
McClain
That's interesting, because Jim Kenny and I on our own without any fanfare, went back and tried to repeat the thing on Virgo and we again thought we had evidence. We thought we had evidence so strongly that we went up to Harvard and actually used their 21 cm measure to do some points up there. And when we got all through we just decided it wasn't a strong enough case to publish - particularly a second time.
Sullivan
I think it still is really undecided, but this Kueller and Robinson was repeated and I can't remember who did that now.
McClain
Well, Hagen always referred to it- no, I won't say that. He referred to it as a particular kind of experiment where you know the answer before you begin - it ought to be there. Well, if it's there, it's not an easily detectable thing.
Sullivan
But you don't understand what might have gone wrong?
McClain
No, because Ed and I repeated the thing on Cygnus again very carefully, and you know, it wasn't statistically significant the second time. We had long discussions about publishing a paper and he would send me his revised version and I'd revise it and send it back to him, and this went back and forth for quite a few times and finally, I had to do an article for Scientific American and I mentioned in there that we had repeated the experiment without success, which other people had also done by that time, and that was about as far as we got as far as retraction was concerned. I think we both now wish we had actually published what we did the second time, but everybody knew what everybody else was doing. Everybody knew that we weren't able to reproduce it. We didn't publish it formally, I guess you'd say, but we should have, I think. And it's been repeated several times since unsuccessfully. So as I say, that was about the biggest brick.
One interesting anecdote - Ed was all tooled up with some kind of receiver- I’ve even forgotten the wavelength now - I believe it was the OH receiver. He and Al [Brant?] were working on it. They pointed it at the sun and got a beautiful emission line, an absolutely beautiful perfect profile. So he brought it down to me and says, "Hey, what do you think of this?" I said, I asked him what the ground rules were and he told me and I said, "Well, I don't know, Ed," - this was after the red shift, of course - I said, "I don't know, I think we better do something to make sure it's real." And I don't know, we had some tuning mechanisms in the feed horn to get the SWR down, one thing and another, and it was a Cutler feed, I guess it was called - folded back on itself type thing for the 50 foot dish. And I was just very skeptical at this point- not skeptical really, but I really wanted him to be sure before they published so the only thing we could think of to check it out was to build a horn so we got umpteen, I don't know how many, 200-300 fluorescent bulbs, built a box, lined the box with fluorescent bulbs - tucked the feed inside the box. Took it off the antenna and brought it back over in the lab, you know. Ran a profile, or ran the scan through and there was a profile. And we never did find out what it was but it was some extremely high Q, very low trebled thing in the antenna feed I presume, which you had to have an extremely high temperature - on the order of 10,000s of degrees to detect, but once you got that high background temperature in there, this little profile of 20° or whatever it was, was there. So that washed that one out - saved us more embarrassment, I guess. That taught all of us a lesson to be real, real careful, you know, to get the instrumental bugs out if possible. Because like this Cygnus occultation, I was telling you about, you can't explain these things, but there probably is an explanation, other than the one that you'd like to have happen.
Sullivan
Well, when was the Cygnus occultation?
McClain
It was one day, I don't know what the date was, it must have been around late ‘50s or early ‘60s. Jim Kenny and I were at Maryland Point doing some routine measurements on Cygnus in the continuum, I guess, I don't really remember for sure, what the experiment was basically. But everything was set up and working perfectly - the antenna was tracking, we had a switching radiometer, I guess it was continuum because it was a continuum switch against a sky horn, both the sky horn and the square wave switch out of receiver was displayed on a scope so we could watch that. The antenna dials were right there and everything, and all of a sudden we were just sitting there tracking Cygnus and the thing started to go down, the chart recorder started to drop. And the temperature of Cygnus is what? Something like 100° perhaps.
Sullivan
Yes, on that order.
McClain
And it dropped down to 20°, 30° in a very slow methodical, deliberate fashion. And slowly, in the same methodical fashion, and came back up - and we had enough time over a period of 30, 40 seconds or a minute to actually check everything in the building as far as instrumentation is concerned, and it was actually as far as we could determine a true diminution in temperature of Cygnus. But you stop and try to figure out something that can physically do this in the way of a satellite or a star or something like that, you come up against a stone wall. There's just no tidy explanation, and we talked to a lot of our buddies about, "Should we publish this?" and finally decided we wouldn't, that it was probably a fluke of some sort, but it's always bothered me because it actually did happen, we had no explanation for it. It looked like a true occultation by something, and yet there was nothing that one could come up with that looked real - there weren't any stars or anything - any obvious objects, let's put it that way. Nowadays, maybe you'd look for a black hole, but in those days...
Sullivan
Could it be a power supply for some reason? But how would a power supply do that?
McClain
We went through all these possibilities, and as I say, this was a single channel receiver with a switch on the sky horn and it was too smooth. It wasn't anything erratic. It wasn't a bad connection or anything of this sort. As far as we could determine, looking at the switching scope which were displaying switching wave forms out of the receiver, there was no question- but it wasn't a question of the sky horn increasing in temperature, it was a question of Cygnus decreasing in temperature.
Sullivan
That's right. That would be another possibility.
McClain
We had time to check this, you see. We were both firmly convinced that for whatever reason, the temperature of Cygnus actually dropped. We checked the antenna dials. The position was correct. We jotted down the time and the declination and Right Ascension and all this so we had a time with leisure to check everything and we were right on it. There was no question about that, it wasn't a hang up in the tracking system.
Sullivan
Very strange.
McClain
Because, you know, if a gear or something like that hung up, you'd expect it to some sort of erratic behavior, and this was very smooth. We analyzed the profile, if you will, and there was no explanation - it never came up. So...
Sullivan
What about the Sugar Grove dish?
McClain
This of course was a fond hope and dream following, well we built the 84 foot dish, we had that which was good for hydrogen line absorption, things of this sort. But other people did, too, this was...
Sullivan
Just about that time.
McClain
This was- we completed ours first, I think, but Haddock had one at Michigan. There were several of them. It was a common size. And quite independently, two groups at NRL got interested in big dishes. One was a group working on electronic counter measures; and ourselves, who were interested in big radio telescopes. And it turned out that both our needs could be fit by a single instrument, and we got very ambitious and decided to see what could be done about something in the 500-600 foot range. And we finally came up with essentially two studies, one done by Jim [James H.] Trexler's group which basically was a servo panel system where the surface panels would be servoed by some method or another to a parabola. It turned out that eventually they used light beams or infrared beams from a rigid member in the middle of the dish that would locate all the corners of all the panel. But we had an alternate proposal by the fellow who did the 50 foot dish. Where you'd need a controlled structure which was an entirely new idea- that is, you would put the major strength members in the structural members in the thing under strain and either increase or decrease the strain depending on what a pilot control rod down the center of the member would tell you to do.
So we had the two proposals. One I would call a controlled structure, and the other I would call a controlled surface. So we finally built these out of NRL and took them over to the Bureau of Yards and Docks. Unfortunately. Not that the Bureau of Yards and Docks isn't a fine organization, and they built very fine yards and very fine docks and all that sort of thing, but they don't build scientific instruments and that's really what we had. And it was a rather chilling experience to find that here we had two designs, both of which depended on servo mechanisms and the chief engineers at the Bureau of Yards and Docks didn't know what a servo was. He looked at me as an upstart kid, and he looked over at me and said, "Are you an engineer?" Well, it so happens I do have an engineering degree, so I said, "Yeah." And he said, "What kind?" And I said, "Electrical." And he said, "Well, I thought so." You know, this kind of thing. He was an older man and he wasn't going to be, have these young kids tell him how to build anything.
Sullivan
Anything that was that big.
McClain
Yeah - as far as he's concerned it’s like a skyscraper or a bridge or a yard or a dock. And then in actuality, it was a very precise scientific instrument that depended on several mechanisms to keep it in trim. Well, they, NRL essentially- what happened in short was that NRL lost control of it as far as being able to say what should be or shouldn't be done.
Sullivan
Because it was getting to be too large a project?
McClain
Yes. It was not so much too large a project but- well, too large perhaps, out of our province - big money and big base structure, and somebody else, NRL just didn't do things like that. So they finally awarded the contract to a consortium. It was really a sad experience because you could see all these very qualified civil engineers working their fanny off, you know, trying to design it and build it.
Sullivan
What was the final design, by the way, which method won out?
McClain
The servo panels. Baltelle Memorial Institute did a very good study on this, and they were going to set this very rigid thing made of Invire, I believe, in the middle, on the chopped axis, and from this you'd have chopped light beams or chopped infrared beams going out to panels and you would then control the whole parabolic surface from this one hard structure in the middle. But they let the contract to these people who normally, I guess, build yards and docks and they got very enthused, and we went to a lot of meetings and they were very qualified civil engineers but they never quite understood, I don't think, what they were dealing with. And not only that, the military application had a lot of priority and the fact that money would never have been allocated for the project if it hadn't been for the military side of it. So the scientific side of it was kind of taking a free ride, you might say, but Jim Trexler’s side of it, they were in as much trouble as we were because they lost control and lost the ability to have input, or to call a halt when they thought something was going wrong. So what happened, the contractors just immediately started laying foundation and were ordering steel. You see, it was a railroad type thing - it had a big pindle bearing in the middle with a circular railroad track with bogies or trucks would run around it and support this thing and then two huge ferris wheels for declination axis. And they started ordering steel for the structure just above the trucks and the pindle bearing and all this business, before anybody had ever designed anything up at the feed point. Well, you know what happened, you add a pound at the feed point and you end up with ten or twenty or a hundred pounds down on the lowest member. So the foundation got designed and the steel ordered and a lot of it delivered before anybody ever got a final figure on what it was going to have to support. And they had to back off then and redo it, and oh, I guess at the end, Jim and I were hassling with each other about how many feed capsules we needed.
We were going to have rapid changeover- roughly half the time would be available for science work and half for military. So you had to have rapid changeover; we had to keep these modules down to some reasonable weight, and even at the very end, we were still not quite sure what the weight was going to be at the feed point, and they were already on their second go-around on the foundation. Well, to make a long story short, [Robert] McNamara got ahold of it in 1963, I guess, and decided that this was one of the things the Navy could do without, or the military could do without, and called, he called PSAC [President’s Science Advisory Committee] up, Jerry [Jerome] Wiesner was head of PSAC then, Ed Purcell was set up with a committee to study the scientific potential and Purcell's committee certainly didn't condemn it, but again, in a way they kind of condemned it with faint praise - they said it might be worth $40 million, well already the price was up to $200 million and some people were saying $3-400 million, you know. Which doesn't sound like all that much today, but back then it was two or three times that.
Sullivan
It's still a hell of a lot.
McClain
Still a lot of money and still way, way more than it should have cost, by the way. At one point, I heard one of the contractors say to another one and I can't remember the names, thank goodness, but he said, "You know, I had any project this size can have its cost doubled." This sort of thing.
Sullivan
Very simply. What did it start out at?
McClain
Control was completely lost.
Sullivan
What did this actually cost?
McClain
I think today it could have been build- not counting all the military aspects of it, in other words, they were going to have a crew of people in blue suits out there to do military intelligence work. Maybe a hundred or two hundred people. So discounting this stuff, we're just talking about an antenna, I'd say $50 or $60 million, and I think $75 million at the outside, for a radio telescope of that size and design.
Sullivan
Could be built today for that, you're saying?
McClain
No.
Sullivan
That was the original?
McClain
At that time, had we not run into you might say, bad management, and unfortunate decisions that were made and discounting the overhead which the military function would entail, the antenna could have been built, I'm sure, for 50-75 million, which is what we said in the first place. And after it was all over and cancelled, I've forgotten who it was, one of the consulting engineers in New York and I had lunch together, and he agreed with me still, that if this thing hadn't gotten in trouble, he said "I still believe we could build it for $50, $60, $70 million".
Sullivan
How much did get spent?
McClain
On the order of $100 million, I'm sure, and nothing was up and a lot of steel on the ground, and surface panels were made beautiful surface panels, quite a few of those were made. And the pindle bearing was in, tracks were in...
Sullivan
I suppose the foundation is still there?
McClain
I don't know what happened to the steel for the foundation. The panels were up for grabs, I don't know...
Sullivan
No, I mean the concrete.
McClain
Oh, the concrete's still there and still the biggest single bearing in the eastern Mississippi if not the entire United States. It's a monster. But it's still there, I'm sure. There would be no way, really, to get it out of there.
Sullivan
It wouldn't be worth the effort.
McClain
But some of the steel sold off and there may still be some of it there. The surface panels were up for grabs and several of us thought about maybe putting them together as a fixed dish, you know.
Sullivan
Or a hole in the ground or something.
McClain
A hole in the ground or something of that sort, but nothing ever came of that as far as I know. So they probably ended up on the scrap heap somewhere. But it was in a way, a very funny experience, and in a way a very bitter experience because it caused us at NRL to lose three or four years of work.
Sullivan
Yes, this was your major effort during that time.
McClain
This was a major effort, and everything we did in the way, not only of our day-to-day radio astronomy, but all our plans for instrumentation, all our plans for personnel, everything, hinged around this project, and my one hope was, well there was no question whether it was available roughly 50% of the time for science, but before it collapsed, we set up a committee of outsiders to come in and try to do some scientific planning for the thing, very similar to what NRAO ended up doing with a visiting committee and what not. But I think this should have been done at an earlier stage, but then again, there was the military security aspect of it and it just seemed that nothing went quite right. Everything, you might say, went wrong. Although today I'm still convinced that that dish could have been built for $50-75 million and it would have been something else again. But look at it today from the astrophysical point of view, what really would you do with a dish that size that was good only at 10 cm because most of your chemistry you might say, is done at shorter wavelengths, so you couldn't have used that resolution and gain for that, you could have used it for hydrogen absorption studies very fine fashion.
Sullivan
You could have used it for extragalactic hydrogen, very high sensitivity...
McClain
Yes, there are things, obvious things like that, but it probably wouldn't- well, I shouldn't say this, but as far as I know, it wouldn't have caused any drastic breakthroughs in that part of the spectrum. You would have been able to do things that you already knew how to do in a much finer fashion there's no question about that.
Sullivan
So what you're saying is that it wouldn't have changed the course of radio astronomy so much, especially relative to the amount of money put into it?
McClain
Very possibly that's a correct statement. It would have been handy for some polarization studies at 10 cm on very weak objects, things of this sort. There are a lot of things you could have done. Whether it was worth the price or not, I don't know. I suppose the Very Large Array will be able to do a lot of these things with perhaps not the same collecting area, but certainly more resolution. Of course, with absorption you get the resolution. It's hard to say what might have come out of it. Every time you build a dish you find things it can do that you didn't know it could do, so at the time it seemed like a wise investment. I think the price- if it had been built as a radio astronomy as a telescope at that price, it wouldn't have been excessive. It would have been one of a kind. I don't think anybody would ever have duplicated it. Harvard, of course, and their cohorts in the Northeast up there have postulated something pretty close to it with their ray dome.
Sullivan
And Lovell also is trying to get a 450 footer or something.
McClain
So I think it was still the proper thing to do, I don't think it was wrong to try. It was just very unfortunate that it hit all the rough spots it seemed like.
Sullivan
So when that ended, then what did you do?
McClain
When that ended we were badly deflated you might say. Everything we had done for the past 3 years or so had been pointed toward that so we had several meetings in the branch and tried to decide what we could do. So we finally decided that the best thing we could do is to build- 85 feet for example was a fairly standard size, but nobody had one that would really go to short wavelengths. So we decided that probably the smartest thing would be to build an 85 foot dish that had a peak gain somewhere around 1 cm- that would be a real good dish at 1 cm.
Sullivan
Take advantage of your short wavelength expertise again.
McClain
Again, yes. What we knew best. Plus the fact that nobody else had one and the same argument on the 600 - what do you do if you cut off at 10 cm, there were beginning to be rumblings of molecular lines and things of this sort, water vapor and all this kind of business. People had already looked for them, I think, but...
Sullivan
Well, there had been a few predictions and things.
McClain
Yes. But it looked like a proper direction to go. And an 85 foot dish is something we could handle in the branch. We figured the cost would be under a million dollars which was something we could manage without undue strain in our group. So I went over to Page and laid it out for him and told him in view of the demise of the 600 foot- well there was one little thing that happened before that. There was a period when NRL was going to try to save the 600 foot and perform a holding action, I guess you'd say. They wanted to move the whole radio astronomy branch to Sugar Grove.
Sullivan
I see.
McClain
I don't know; I don't think I did myself any good, but I asked everybody what they thought and nobody was much in favor of it. And I was dead set against it; I thought it was the most stupid thing we could do to cut ourselves off. NRAO was going through the same problem right then and they finally moved back to Charlottesville. I couldn't see doing the opposite, but it sure took some strong backbone to stand up to everybody else at NRL because they wanted us out there as a kind of guarded place, I guess, to guard the bearing.
Sullivan
Hostages out there.
McClain
Yes.
Sullivan
All these people...
McClain
Make sure nobody stole the bearing, you know. So anyhow, we wrote that one over and then things kind of settled down. I went over to Page and said, "Look, we've got to do something if we're going to stay in business, and I think the smartest thing we can do is build the best 85 foot dish that American expertise can build." He said, "What's it going to cost?" I said, "I don't know, but it’s going to be a lot more than the last 85 or 84 we had." And I guess we estimated somewhere between $.5 million and $1 million. So we, and a lot of credit on this project goes to Bob Myer, who was contract officer on this one. I was contract officer, scientific officer they call it, on the 84, but I turned that over to him on the 85 and he did a lot of awfully good work in checking the mechanical design, performance on the contract. So we had, we wrote up performance specifications. We'd already seen things go down the drain with trying to tell people how to build things, so we just wrote a set of performance specifications. What we wanted- the size, of course, but what we wanted it to do when it was finally built - that is, how accurate the tracking was supposed to be, how accurate the surface was supposed to be, how it was to perform off vertical axis, everything we could think of as far as performance was concerned. And then we didn't tell anybody how to do this, we just put it out on bid.
And it shook down to Bob Hall's former place of employment, and his then present place of employment, that is, Blaw-Knox Corporation and [?] Corporation, who built the San Francisco subway. So Bob had built the Blaw-Knox dish when he was there, the one that Michigan had for example, case in point. So he bid it and Blaw-Knox bid it and the two designs were so close that it wasn't even funny. I mean, it was really a problem for us to decide between the two, because they were within a fantastically small sum on price. I don't know which one had the spies in the other one's camp but it had to be pretty close, you know. But the fact that he had been at Blaw-Knox and the fact that he was then at [?], Bob Hall, that is, accounts for it. I don't think there's any chicanery or anything of that sort. But it was a very close thing, and it got down to the point where I was firing telegrams off to one and saying, "What will you guarantee us on tracking within 45° of zenith?" Duplicate telegrams to both companies. And after four or five of these things, you know, I had some basis to make a choice. Because one of them would guarantee a little more than the other one would. Bob Hall would stick his neck out a little further than Blaw-Knox did, but other than that, it was a dead heat. As far as I know, Blaw-Knox might have done as good a job as [?] did. But anyhow, we had a performance spec and there were some shaky moments while the thing was building. At one point, somebody had under designed the bolts up at the top of the polar axis or something and they had to go in while half the structure was up and do a fix on this. But after it was all said and done, I guess it turned out to be one of the best, finest mirrors in the world.
Sullivan
I found out from entirely independent means that the military purpose of the Sugar Grove dish was to monitor Soviet radio traffic that bounced off the moon, but not to intercept individual messages, but rather just the general level of the UHF and VHF communications. The idea being that before preparations for an attack, that there'd be a great increase in the amount of radio traffic and this would show up as a brightness increase on what was reflected off the moon and those could be picked up by the 600 foot dish. So that was the guiding idea of this whole thing.
So that ends the interview with Ed McClain on 22 December ’73 at his home in Morningside, Maryland.