Clark Lake Radio Observatory: A Pictorial History

Bill Erickson and his 1952 Packard on their first visit to Clark Lake in 1958.

I [Bill Erickson] received my PhD in 1956 at the University of Minnesota under the theoretical physicist, Charles L. Critchfield. About a year before receiving my degree I had occasion to visit Washington, DC and, because I had decided that I wanted to do observational radio astronomy as a career, I visited several observational groups in that area, including the Naval Research Laboratory (NRL) and the Carnegie Institution of Washington, Department of Terrestrial Magnetism (DTM). While I was at the DTM, Bernie Burke and Ken Franklin told me about a strange, sporadic radio source that they had been observing for about two months with the 2 degree beamwidth, 22 MHz Mills Cross that they had recently completed. The source was certainly sidereal; it crossed the beam about 4 minutes earlier each night, but it was not perfectly synchronized with sidereal time. They were very excited because they had just realized that the reception of these signals fit beautifully with the times when Jupiter passed through the beam of the array. They had set the declination of the array for the Crab Nebula and, serendipitously, Jupiter was at nearly the same declination. This low frequency emission from Jupiter was one of the most spectacular early discoveries in radio astronomy. It gave me the mistaken impression that low frequency work was easy. All that you needed to do was to erect some poles, string some dipoles between them, and great discoveries would result.

About a year later I accepted a Carnegie Fellowship at DTM. By the time that I got there it had been recognized that the Cross had such high sidelobe levels that it could not be used for its primary goal, a deep survey of radio sources. It was being dismantled and my only contribution to the program was to help to pull it down. Most of my time at DTM was spent on a 21 cm survey of neutral hydrogen at high Galactic latitudes.

My thesis advisor, Charlie Critchfield, had gone to San Diego where he accepted a position as Vice President and Director of Scientific Research for Convair, an aircraft and missile manufacturer. He formed the Convair Scientific Research Laboratory (CSRL), a small, company sponsored scientific research laboratory with a staff of about two dozen PhD scientists plus support personnel. In late 1957 I joined the staff. We were given complete freedom to choose our research projects within the confines of a very limited budget. The staff were expected to act as in-house consultants for the engineering divisions of the company and to carry out basic research to increase the company’s competence and reputation in science. There were one or two staff members in most areas of science of interest to the company; theoretical physics, cosmic ray physics, radio astronomy, solid state physics, metallurgy, aeronomy, aerodynamics, chemistry, mathematics, and the like. I was the only radio astronomer. All of us were young, enthusiastic PhDs. I have never worked with a group that had as much esprit de corps. In considering what research program I could undertake, essentially on my own and very cheaply, I thought about the Jupiter discovery at DTM and decided upon low frequency work. So I made plans for the construction of a large decametric array.

My first task was a search for a large flat site in the San Diego area and I found Clark’s Dry Lake, a playa about 100 km northeast of the city. It is in the Anza-Borrego desert, a desert formed by the rain shadow of the coastal mountain range. [Above] is a 1958 photograph of my first visit to Clark Lake; it is a completely flat area 3 km across. In fact, soil has slumped in around the edges of the playa enough to almost perfectly compensate for Earth curvature across the area, making it nearly a perfect plane. It is located in a valley about 15 km by 35 km in size and it is surrounded by 1800 m (6000 ft) mountains that partly shield the area from interference.

The first equipment trailer with propane tank for AC generator in the distance.

One of the antenna construction parties at Clark Lake.

Wives of the laboratory staff working on construction.