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Interview of Cornell Mayer by Joan Bromberg on 1986 November 8,
Niels Bohr Library & Archives, American Institute of Physics,
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Collaboration with Charles Townes and Townes' graduate students in affixing a maser receiver to the Naval Research Laboratory telescope in the 1950s. Discussion about the telescope; Townes contacts about a maser receiver; details of some of the disadvantages maser and parametric amplifiers at that early time; briefly relates some subsequent maser work on the NRL telescope.
The Naval Research Laboratory had the largest antenna and best radiometer in the country at microwaves in the 1950s. The parabolic mirror of the telescope was the most accurate large reflector in the world at 50 feet in diameter and accuracy to 3 cm wavelength. The telescope mount was not so good; it had no altitude and azimuth readouts. You could set the azimuth and altitude angles, and it would go to them and stop. I never understood why it was designed in this way rather than with readouts to tell you the pointing position.
We began work on the telescope about 1947 or 1948. Our group was then called the Radio Frequency Research Branch and worked on centimeter wave radar components for waves shorter than 10 centimeters. Dr. Hagen was in charge. He obtained the money and got the antenna put up. The deficiencies of the mount were not his fault, because he couldn't obtain enough money for a better mount.
To design the 3 centimeter wavelength radiometer, I started with the Dicke radiometer and modified it. I replaced the chopper wheel with a switch which could switch the receiver between a measurement of the sky temperature, using a horn antenna, and the signal. This design was made possible by BTL's Hogan invention of a microwave ferrite circulator that was nonreciprocal. It allowed us to make a measurement with the antenna that was independent of the impedance. Thus we no longer had the error which resulted when we had to disconnect the antenna from the receiver and point to a noise for source calibration, and then reconnect to the antenna, and gave much improved gain stability.
What happened was that Townes telephoned me, since I was the one who had made the radiometer and had access to the 50 foot reflector, and said he had a maser, and asked if they could come down and try it out on the antenna. The reaction in our group was uniformly positive. We thought that it would be wonderful to improve the sensitivity of our readings by the order of 10 or 12. But although we confirmed our earlier measurements, we didn't discover many new radio sources. It's not possible in reasonable time to scan the whole sky or even a large part with an 8' beam. There may be a few new sources in Alsop's results on planetary nebulae. They have never been published but should be in his thesis. We looked at lots of objects; not all of them panned out.
One of the principal difficulties of masers was the effort and money involved in their use. The cost of the helium, for example, was not negligible. We paid, as I recollect, $8 to $10 per liter. The principal trouble was probably that masers were difficult to engineer. The earliest were not as stable as one would desire. Several years of intensive engineering work would have helped. There were many complications due to the use of liquid helium on a large antenna 60 or 70 feet in the air.
Parametric amplifiers were embraced by most radio astronomers because they got rid of a lot of the complications, like liquid helium. They were cheaper, though not cheap by any means. But they didn't work very well; one problem was getting good diodes and another was the instability of amplification. It was a technology that was not well in hand. Sometimes someone would luck out and get a good diode, and get good results. The Australians did. We tried a half dozen times with different parametric amplifiers and gave up.
Our work with masers didn't have any influence on the radar people. They ignored us, as did the majority of people at the laboratory. We did, ourselves, do some applied work, but I would estimate 99% of our work at the time was on radio astronomy. Ben Yaplee, who worked on moon radar, tried to put a maser in radar, as a member of our group. He got hold of a Hughes maser, which worked quite well.
We did follow up on the maser work. We didn't have the people, the time, or the expertise ourselves but worked with Prof. Townes' students. To put it another way, we had many other things to do with our radiometer. In all, we had on one or another of our telescopes the 3 cm. maser of Giordmaine et al., a 21 cm. maser of Arno Penzias, and a 9.4 cm one built by Bill Rose, now at Maryland. Each of these people did their thesis work on our radio telescopes, which served in this way as a resource for other laboratories, like a large accelerator, although in our case, the arrangements were all informal. The 21 cm. and 9.4 cm. masers were used with the 84' telescope at Maryland Point, 45 miles south down the Potomac. Yaplee's maser was eventually inherited by us, but we never bothered to fix it up because its radar frequency differed from the frequencies we were using, and we encountered radar interference at this frequency.
After Prof. Townes went from MIT to Berkeley, Al Cheung of U Cal Berkeley tried to make masers at the water line, 1.25 cm. and he brought several of them to our laboratory. There were used in conjunction with another telescope, our 85' antenna. He and our people tried hard to make them work, but not with completely gratifying results.
In about 1980, JPL spent a lot of resources on a well-engineered maser at the water line frequency, and got one that was very stable. They used it at both that frequency and, I believe, as an intermediate frequency amplifier. These had broader bands than the narrow cavity masers we used, a few hundred megahertz as against our 30-50 megahertz. JPL used them successfully on their Goldstone antenna and their installations in Spain and elsewhere to track vehicles traveling in deep space, and the National Radio Astronomy Observatory made copies which worked well, I believe.