Robert W Wilson Two astronomical discoveries

Bob Wilson shared the 1978 Nobel Prize in Physics with Arno Penzias for their discovery of the CMBR. Wilson is a Senior Scientist at the Harvard-Smithsonian Center for Astrophysics and Technical and Computing Leader of the Sub-Millimeter Array Project.

As a child I acquired an interest in electronics from my father. I also learned from him that I could take apart almost anything around the house, probably fix it, and then reassemble it successfully. In my high school years I fixed radios and later television sets for spending money and built my own hi-fi set. Thus when I enrolled at Rice University, I declared a major in electrical engineering. During my freshman year I switched to physics after realizing that much of the EE course work would be in power engineering. Having read my father's copies of Review of Scientific Instruments I realized the physicists had the interesting instruments (good toys). At Rice and later at Caltech I had two formal courses in electronics for physicists. My earlier interest had prepared me to enjoy and absorb this material thoroughly. My senior thesis at Rice was centered on designing and building a current regulator for a high-field magnet in the low-temperature physics group. These early experiences, especially the trouble-shooting skills I learned in my high school days, have served me well while fixing many problems with radio telescopes.

I entered graduate school in the physics department at Caltech in 1957 after receiving my B.A. in physics at Rice earlier that year. I had no clear idea of what I wanted for a thesis topic. During my first year I became friendly with David Dewhirst who was visiting from Cambridge University and was using the original Palomar Sky Survey plates in the basement of the astronomy building for identifying 3C radio sources (the Third Cambridge Catalog, Edge et al. 1959). After David learned of my interest in instrumentation as well as physics, he suggested that I consider working with the new radio astronomy group which John Bolton had formed. There was the added enticement that they wanted to make maser amplifiers for the telescopes. The original Owens Valley Radio Observatory 90-ft antennas were nearing completion and it was an ideal time to join such a group.

My thesis was intended to be interferometric observations with these antennas at the 21-cm hydrogen line. I built the local oscillator and other parts of the receiver system for those observations. That project stretched out and my actual thesis was based on an intervening project John Bolton had started me on - making and interpreting a map of the plane of the Milky Way at 960 MHz (Wilson and Bolton 1960). We used one of the two Owens Valley 90-ft antennas before interferometric observations started. I used load switching against a liquid nitrogen-cooled load and scanning or drifting from the west to the east across the Milky Way. I covered up to about 20° either side of the plane of the Milky Way - enough that the radiation was falling off very slowly at the edges of my map. Having no better reference, I took the edges of my map to be zero. Since we are inside the Milky Way, it was clear to me that this technique only worked because the Milky Way is very thin compared to its diameter. I knew I did not have a true zero reference for my map. It is interesting in retrospect that I added 2.8K to my observations to improve the comparison to a lower frequency survey in analyzing the radiation from the galactic plane into thermal and nonthermal components (Wilson 1963).

My only cosmology course at Caltech was taught by Fred Hoyle. While I had not had a course in general relativity, Hoyle's lectures did not require an understanding of the tensor math which general relativity is based on. Philosophically, I liked his steady state theory of the universe except for the fact that it relied on untestable new physics.

After a one-year postdoc at Caltech doing 21-cm line and polarization interferometry, I took a job at Bell Labs' Crawford Hill Lab. A major attraction there was the 20-ft horn-reflector antenna, and the promise that Arno Penzias and I could use it for radio astronomy. A second reason I was favorably inclined toward Bell Labs resulted from the help they had given the radio astronomy group. They had offered Caltech the opportunity to send someone to work in the group which had designed TWMs and make a pair for the observatory. TWMs were the lowest noise receivers at that time. I had hoped to be the person to go, but because I needed to finish my thesis, Venkataraman Radhakrishnan was chosen to go to Bell Labs. I worked closely with him to put the masers to use and developed a very positive opinion of the people and the working atmosphere at Bell Labs.

In the late 1950s, plans were made to start working on communication satellites at the Bell Labs' Crawford Hill site. John Pierce (1955) had had a long-time interest in communication satellites resulting from his science fiction writings. The first satellite tests were planned with NASA's Echo balloon. It was known that the return signal from Echo would be very weak because a sphere scatters the incoming radiation in all directions. While reading a paper by John Pierce describing the parameters required for a satellite system, Rudi Kompfner had the idea of using a traveling wave maser. Derek Scovil and his group at Murray Hill (De Grasse, Shulz-Du-Bois and Scovil 1959) had developed them for a high-sensitivity military radar. They worked at liquid helium temperatures and had a noise temperature of a few kelvin. Even after making a room-temperature connection to it, one could have a receiver with a noise temperature of 10 K or less.

It was natural to combine a TWM with a horn-reflector antenna. The horn-reflector was invented at Holmdel by Al Beck and Harald Friis for use in a microwave relay system. In addition to turning the corner between the waveguide going up a tower and the horizontal communication path, the horn-reflector has the distinct advantage that when two of them are put back-to-back on a tower and have a very weak signal coming in on one side, a strong regenerated signal can be transmitted from the other side without interference. Its front-to-back ratio is very high. The corollary of this is that a horn-reflector put on its back will not pick up much radiation from Earth and will be a very low-noise antenna. Therefore, Art Crawford

Fig. 4.12. The 20-ft horn-reflector with its parabolic reflector on the left and cab on the right. Since the cab does not tilt, almost any kind of receiver can be conveniently put at the focus of this antenna (apex of the horn). It is clear that the horn shields the receiver from the ground, especially when it is looking up.

Fig. 4.12. The 20-ft horn-reflector with its parabolic reflector on the left and cab on the right. Since the cab does not tilt, almost any kind of receiver can be conveniently put at the focus of this antenna (apex of the horn). It is clear that the horn shields the receiver from the ground, especially when it is looking up.

built the large (20-ft aperture) horn-reflector pictured in Figure 4.12, to be used with a TWM to receive the weak signals from Echo (Crawford, Hogg and Hunt 1961).

Figure 4.13 shows a polar diagram of the gain of a smaller horn-reflector antenna compared with the gain of a theoretical isotropic (uniform response) antenna. If we put an isotropic antenna on a field with the 300-K ground down below and zero degree sky up above, we expect it to pick up 150 K; half of its response comes from the ground. The response of the horn-reflector is more than 35 dB (a factor of about 3000) less responsive to the ground than the isotropic antenna. So one would expect less than a tenth of a kelvin for the ground pickup from the horn-reflector.

In December of 1962 I went on a recruiting trip to Bell Labs. Of the groups I was interviewed by, I was most interested in the Radio Research Lab at Crawford Hill. I met Arno Penzias there and he showed me his OH experiment and the 20-ft horn-reflector. At that time, he had been there a year and a half. We had much more time to talk a week later at the winter American Astronomical Society meeting, where I gave a talk. He was clearly trying to get me to join him at Crawford Hill. Setting up and carrying out an observing program with the horn-reflector was certainly a job better done by two people than by one.

We were very different people and, as it turned out, had complementary skills. We made a good team for that job. Arno was as garrulous as I was

140 ° 150 ° 160 ° 170 ° 180 °170 ° 160 ° 150 ° 140

Fig. 4.13. Polar gain pattern of a horn-reflector microwave antenna. The radial units are dB and the gain is normalized to 0 dB at its peak.

140 ° 150 ° 160 ° 170 ° 180 °170 ° 160 ° 150 ° 140

Fig. 4.13. Polar gain pattern of a horn-reflector microwave antenna. The radial units are dB and the gain is normalized to 0 dB at its peak.

reserved. He was interested in the big picture and tended to think of ways to most effectively use the resources at hand. I tended to be shy, persistent at getting all of the details correct, and liked to do things myself and with my own hands. As a graduate student Arno had built a maser amplifier and made observations with it. While I had had some experience with the maser from Bell Labs, I had worked much more on the back-end signal-processing electronics and antenna control at the Owens Valley Observatory. We were both intent on making accurate measurements.

Crawford Hill in 1963 offered a remarkable environment for us to work in. Several of the people there had been at the original Bell Labs building in Holmdel in the 1930s when Karl Jansky discovered extraterrestrial radio waves. They included our first department head, Arthur Crawford (unrelated to the family for which Crawford Hill was named). Crawford Hill, although part of the research arm of Bell Labs, was somewhat more mission-oriented than the rest of research. Long-haul communications was their primary focus. George Southworth had developed the waveguide at Holmdel in the 1930s. They had been a strong force in designing the first microwave relay system. Many of the Members of Technical Staff (MTS) had started there before the technology of microwaves was developed and had contributed to its development. There was a strong curiosity about new things and a feeling that new fields should be understood rather than exploited for the easy solutions which might be found. Bell Labs had "written the book" on many new fields and writing comprehensive books was still an ongoing endeavor. The Bell Labs merit review system rewarded good research and recognized the value of cooperative and interdisciplinary work, something which seemed to be missing at many universities. I could find experts on many subjects at Crawford Hill or other parts of Bell Labs who were happy to help.

The Crawford Hill building was built to house the original Holmdel group when the land they had occupied since the 1930s was taken over for the big lab at Holmdel. They moved in 1962. The front part of the building has a long hall with MTS offices on the front side and laboratories on the backside. Nearly every experimental MTS had a lab and often a technician to help him build things. The back part of the building had an extensive machine shop, a three-person carpenter shop, and a well-supplied stockroom. The machinists had a lot of experience building microwave components and were used to working from hand sketches rather than formal drawings. The head of the carpenter shop, Carl Clausen, had built Jansky's original antenna in the 1930s. When I arrived they were building a replica from the original drawings for the NRAO in their spare time. These resources were available to us with little evidence of limitations from accounting.

At that time, there was no computer at Crawford Hill. Mrs. Curtis Beatty, a mathematician who had come from Murray Hill, would either write and run Fortran programs for us or take care of the complexities of running our programs on the Holmdel or Murray Hill computers. She would often fix small errors by changing the assembly code to avoid the cost of running the Fortran compiler again.

It is reported that Karl Jansky, in common with many others of his era, had built a measuring set as his first job. There were "standard Holmdel measuring sets" in many of the labs whose design probably dated from the 1940s, but were logically derived from Friis' design which Jansky had used. They were very simple but effective and were capable of measuring with 0.01 dB accuracy over tens of decibels in the microwave bands which were used for communication. I was to use these extensively in building and measuring components for our receiver for the 20-ft horn-reflector.

One might ask why two young astronomers wanted to work with such a small antenna as the 20-ft horn-reflector with its collecting area of only 25 m2. While other radio observatories all had much larger antennas, we knew it had very special properties. First, it is a small enough antenna that one could measure its gain very accurately. It was necessary to be only about a kilometer away to be in the far field for making an accurate gain measurement. And that, in fact, had already been started by David C. Hogg (Hogg and Wilson 1965).

The TWM amplifiers, which were available at several frequencies, would make this small antenna sensitive enough for work even with small diameter sources. For sources which were large enough to fill its beam, it would have been the most sensitive radio telescope in existence at the time. The other important thing is that we expected to be able to account for all of the sources of noise and make absolute brightness measurements. Radio astronomers don't often understand the background temperature when they do the usual on-off experiment (subtracting a measurement pointing away from the source from the measurement on the source), but the 20-ft horn-reflector offered the possibility of absolute temperature measurements. My interest in that possibility, of course, came directly from my thesis work at Caltech with John Bolton.

Soon after I went to Bell Laboratories, the 20-ft horn-reflector was released from the various satellite jobs it was doing. It had been designed for the Echo experiment which required operation at 13-cm wavelength, but it had later been used to receive a beacon from the Telstar(R) satellite. Thus when Arno and I inherited it, there was a 7.3-cm maser receiver on it (Tabor and Sibilia 1963). At that time it had a communications receiver with three low-noise amplifiers connected in series which a radio astronomer would find hard to believe. The maser was followed by a low-noise nitrogen-cooled parametric amplifier which was followed by a low-noise traveling wave tube amplifier. The gain stability was unbelievably bad. Our jobs were to turn all of this into a radio telescope by making a radiometer, finish up the gain measurement, and then proceed to do some astronomy projects.

We thought about what astronomy we ought to do and laid out a plan that would take a few years. The first project was an absolute flux measurement of Cas A, the brightest discrete source at that wavelength, as well as several other bright sources. We were planning our radiometer so that we could know its sensitivity to 1 or 2% accuracy based on physical temperatures we could measure.

Shortly after arriving I had joined Dave Hogg to make an accurate gain measurement of the 20-ft horn-reflector. Putting these together would let us measure the standard astronomical calibration sources more accurately than had been done before. This would be a service to both radio astronomers and the Bell System (and anyone else buying satellite Earth stations). The sensitivity of an Earth station could be accurately and easily checked by measuring its signal-to-noise on one of our calibrated radio sources.

I planned to follow up on my thesis by taking a few selected cuts across the Milky Way Galaxy and then confirm the spectrum of some of the sources that I had looked at. Next we wanted to check our ability to measure absolute temperatures so we could look for a spherical or halo component of the radiation from the Galaxy. Extrapolating from a lower frequency, we did not expect to see any galactic halo at 7-cm wavelength. We wanted to prove that when we did try to make such a measurement, we got a null result. After doing these projects, our plan was to build a 21-cm receiver similar to our 7-cm receiver. We already had the maser in hand. We would then make the halo measurement and do a number of 21-cm line projects including reworking Arno's thesis of looking for hydrogen in clusters of galaxies.

At one point during that time John Bolton came for a visit so we laid out this plan of attack and asked his opinion. He said that the most important thing to do in that list is the 21-cm background measurement. He thought that it was an unexplored area and something that we really ought to do.

By the time I joined Bell Laboratories, Arno had started making a liquid helium-cooled noise source (cold load) (Penzias 1965). Figure 4.14 is a drawing of it with an odd perspective. There is a piece of standard Bell System 90% copper 4-GHz waveguide, which runs from the room-temperature output flange down inside the 6-inch diameter Dewar to the absorber in liquid helium. About halfway down, the waveguide is thinned to reduce its heat conductivity, and finally there is a carefully designed absorber in the bottom. There is a sheet of Mylar in the angled flange near the bottom which keeps the liquid helium out of the upper part of the waveguide and makes a smooth transition from gas to liquid. Some holes in the bottom section allow the liquid helium to surround the absorber and there was no question of the physical temperature of the absorber itself. The heat flow down the waveguide which otherwise would have boiled the liquid helium rapidly has been

Fig. 4.14. The cold load for our radiometer.

taken care of by the baffles. They exchange heat between the cold helium gas leaving the Dewar and the waveguide. We realized that we had to know the radiation from the walls of the waveguide, so there is a series of diode thermometers on the waveguide for measuring its physical temperature distribution. We calculated the radiation of the walls using these temperatures and the measured loss in the waveguide.

When we first transferred the contents of a 25-l Dewar of liquid helium into the cold load, it would fill up to a high level. We calculated the radiation temperature at the top to be ~5K - just eight-tenths of a kelvin above the temperature of the liquid helium. After 15 hours or so (we usually ran down before the helium did), the liquid helium level would be down near the absorber and we would calculate the flange temperature to be about 6 K. Comparing it to the horn-reflector, the change agreed within something like a tenth of a degree over that period, so we felt we had a reasonably good calibration of what was going on in our cold load.

While Arno was doing that, I set up the radiometer shown in Figure 4.15 (Penzias and Wilson 1965b). As with most of our astronomical equipment

Fig. 4.15. The switch and secondary noise standard of the radiometer used for our measurements of the flux density from the radio source Cas A and the CMBR. The noise injected by the noise tube and its coupler was calibrated in three ways against thermal sources.

Fig. 4.15. The switch and secondary noise standard of the radiometer used for our measurements of the flux density from the radio source Cas A and the CMBR. The noise injected by the noise tube and its coupler was calibrated in three ways against thermal sources.

(To cold load)

(To cold load)

at Bell Laboratories, this is somewhat unusual. The 20-ft horn-reflector was fitted with an electroformed throat section which made a smooth transition from the square tapering horn to the circular waveguide which had been used in the Echo receiver. After a waveguide rotary joint, a second electroformed waveguide made the transition to circular 4-GHz waveguide. We decided to use this in a switching scheme which Doug Ring and others at Crawford Hill had used in the past. It takes advantage of the fact that two orthogonal polarizations will pass through circular waveguide. The polarization coupler near the antenna couples the signal from the reference noise source into the horizontal polarization mode traveling toward the maser and allows vertical polarization from the antenna to go straight through. The polarization rotator is the equivalent of a half-wave plate. It is a squeezed piece of waveguide with two rotary joints; another polarization coupler at the back picks one polarization off and sends it over to the maser.

By rotating the squeezed waveguide, we could switch between the reference noise and the antenna. An important aspect of this radiometer design is that except for the unused port, all ports of the waveguide were terminated at approximately the same low radiation temperature. Thus small reflections would not have a large effect. We adjusted all parts of the system to be well matched, however, and the unused port could be opened to room temperature with no effect on measurements. In addition, I added a motor to turn the squeezed waveguide to switch between the antenna and the reference noise source at 10 Hz. This, combined with a phase-sensitive detector I constructed, formed a "Dicke Switch" which was useful when measuring weak signals. After stabilizing the room temperature and all of the components of our system, the stability was so good that we usually just rotated the squeezed section by hand and recorded the receiver levels on a pen recorder.

Figure 4.16 shows a picture of the actual installation. The rotary joint that allowed the horn-reflector to turn while the receiver stayed stationary in the cab is at the right edge of the picture and the polarization rotator is on the left. An adjustable 0.11-dB attenuator seen at the bottom of the picture connects the cold load, which is below the picture, to the reference port of the switch. It could add well-calibrated additional increments of noise. The top of the maser is seen above the polarization coupler for the reference port and its massive magnet is hidden from view. The relatively large, strong cabin of the 20-ft horn-reflector, which does not tilt with respect to gravity, allowed us the freedom to build our receiver almost as though it were in a lab room and be with it during observations. The ease of working in the cabin undoubtedly contributed to our success. As graduate students

Fig. 4.16. Our radiometer installed in the cabin of the 20-ft horn-reflector.

Arno and I had both attached masers cooled with liquid helium to conventional antennas in which the focus tilted with elevation angle. We very much appreciated this arrangement.

Before we started making measurements with this system, there had been careful measurements of horn-reflectors with TWMs at Bell Laboratories. First, before going to the trouble of building a 20-ft horn-reflector, Dave Hogg had been asked to calculate the "sky noise" in the microwave band (Hogg 1959). To confirm his calculations the antenna and maser groups had put together a test system (De Grasse et al. 1959). They had a 6-GHz maser and a small horn-reflector antenna. They hooked the two up with a calibrating noise lamp and saw that indeed they got a system temperature of 18 K, which was very nice, but they had expected to do a little better. You see in Figure 4.17 that contrary to the expected value of less than 0.1 K for ground pickup from the antenna, they have assigned 2K to it. They assigned 2.5 K for atmosphere and 10.5 K for the temperature of the maser. The makers of the maser were not very happy with that number. They thought they had made a better maser than that. However, within the accuracy of what the whole group knew about all the components, they solved the problem of making the noise from the components add up to the measured system temperature by assigning additional noise to the antenna and maser. Arno had used this horn-reflector for his OH project and was aware of the extra 2K that had been assigned to it. One of the reasons that he built the cold load was to improve on their experiment.

Fig. 4.17. The assignments of contributions to the system noise temperature in the De Grasse et al. (1959) radiometer.
Fig. 4.18. A measurement of the radiation from the atmosphere by a tipping experiment with the 20-ft horn-reflector. The small circles and crosses are theoretical fits of the atmosphere to the measurements and show excellent agreement with the measurements.

This group had measured the atmospheric radiation (sky noise) by the same technique that Dicke had first reported on in 1946 (Dicke et al. 1946). Figure 4.18 shows a chart of such a measurement Arno and I made with the 20-ft horn-reflector. It shows the radiometer output as the antenna is scanned from the zenith (90° elevation angle) down to 10°. This is a chart with power increasing to the right, and shows what the power out of the receiver did. The circles correspond to the expected change if the zenith sky brightness is 2.2 K and the crosses to 2.4 K. You can see that the curve is a very good fit to the expected values down to at least 10° elevation. A well-shielded antenna makes an accurate measurement of the atmospheric radiation very easy.

After the 20-ft horn-reflector was built and was being used with the Echo satellite, Ed Ohm, who was a very careful experimenter, added up the noise contribution of all the components of the system and compared it to his measured total. In Figure 4.19 we see that from the sum of the components he predicted a total system temperature of 18.9K, but he found that he consistently measured 22.2, or 3.3 K more than what he had expected (Ohm 1961). However, that was within the measurement errors of his summation, so he did not take it to be significant.

Our first observations with our new system were somewhat of a disappointment because we had naturally hoped that the discrepancies I have mentioned were just errors in the experiments. Figure 4.20 is the first measurement with our receiver. At the bottom and top, the receiver is switched to the antenna and in between to the cold load. The level from the antenna at 90° elevation matched that from the cold load with 0.04 dB of attenuation (about 7.5K total radiation temperature). At the bottom I recorded measurements of the temperature-sensing diodes on the cold load.

Table II — Sources of System Temperature

Source Temperature

Table II — Sources of System Temperature

Sky {at zenith)

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