CSO History


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 he heart of any telescope is, of course, the apparatus that detects the received radiation and transforms it into something that can be sensed by the astronomer. In the 1970s infrared astronomy was making great advances, and radio astronomy at long wavelengths was well established. But the region of the spectrum between the far infrared and radio waves of a few centimeters was still an unexplored territory. Detectors that could respond to the presence of millimeter to submillimeter radiation existed, but none with sufficient sesnsitivity for astronomy.

Figure 8. Thomas Phillips in the 1980s.

Among those who broke new ground in this problem was Thomas Phillips. Soon after completing his doctoral work in low-temperature physics at Oxford University in the UK, Phillips went to work for Bell Labs in 1968. He was studying the motion of electrons through crystals at low temperatures (1 to 10K). Such motions interacted with electromagnetic (em) waves in the millimeter range and carried information from one side of the target to the other. He needed to develop methods of detecting these waves. His approach was to use an InSb hot electron bolometer system operating at a temperature of 4K. Keith Jefferts was working in a lab next to his and was a collaborator in the work that Arno Penzias and Robert Wilson were doing in another part of Bell Labs. They were trying to detect the presence in space of molecules in dust clouds and not just CO and HCN. Phillips had claimed that his detectors were much better than the Schottky diode detectors available at Bell Labs, so Arno suggested he build a receiver for detecting DCN, which was predicted to be about 10^-5 of HCN. Their own receiver had a noise temperature of some 10,000K and was not able to detect the DCN transition around 90 GHz. Phillips, using his InSb hot electron technology, was able to develop a receiver with a noise temperature of around 100K. It turned out, however, that the DCN line was much stronger than expected, and so it did not require the new receiver. But with this new receiver Phillips was the first to observe the CO 2-1, 3-2, and 4-3 transition lines, as well as other molecular and atomic (carbon) lines. Thus, it was the work of Penzias and Wilson that got Phillips interested in astronomical problems requiring applications of his experience in low-temperature physics.

In the early '60s Dayem and Martin [1] discovered the phenomenon of superconducting quantum photon-assisted tunneling (SQPAT). This phenomenon was pretty much ignored for years, being overshadowed by the better-known Josephson Effect. But in 1976 it occurred to Phillips that a device employing SQPAT might make a suitable detector of photons of millimeter waves. Bell Labs had developed technology to construct a device consisting of a junction of superconducting-insulating-superconducting (SIS) materials. It was a small area lead-alloy superconducting tunnel junction whose I-V characteristic showed two states of the device - the Josephson supercurrent state and the resistive state. This was intended for use in a superconducting computer to compete with IBM, but never achieved adequate reliability. However, this device was also capable of detecting individual photons by measuring the increase in a current passing through the junction as a photon was absorbed, breaking a Cooper Pair. One of the released quasiparticles could have enough energy to be promoted above the energy gap, in analogy with a photoelectron. It was the key to the subsequent development of heterodyne receivers in the millimeter and sub-millimeter range of em radiation. But the acronym SQPAT did not catch on with the community; hence, SIS, a description of the structure rather than of the physics, became the acronym for this detector. The first working example was placed on the first 10m telescope at the Owens Valley Radio Observatory (OVRO) in 1979.

Figure 9. Phillips in his receiver development lab.

In 1974 Phillips spent a year in the UK at London, where he developed a working relationship with Martin Ryle in Cambridge. Ryle wanted Phillips to develop a proposal for a submillimeter telescope to compete with a proposal being prepared at Jodrell Bank to upgrade the surface of the MkII. Phillips' proposal was successful and ultimately became the basis for the James Clerk Maxwell telescope presently on Mauna Kea. But in the mean time, Phillips had become acquainted with Bob Leighton and was invited by Leighton to join the Caltech faculty, which Phillips did in 1979. Leighton had a grant from the NSF to construct three millimeter-wave telescopes to form an interferometer to be located at Owens Valley. Leighton wanted Phillips to use a fourth, precision telescope to build a submillimeter observatory at a suitable site, and this is what Phillips was also eager to do. However, the NSF was not happy about progress with the interferometer and didn't want to hear about the fourth telescope which they had not yet intended to fund. The NSF had given Leighton only 3/4 of his requested funding, thinking that he would build only three telescopes, but Leighton managed to squeeze out funding for the fourth from that supplied, with additional help from NASA and the Kresge Foundation. The NSF did not approve further development until the OVRO development was finished. Thus, the Division Chair, Robbie Vogt, put Phillips in charge of the OVRO project, so as to get that done before he could go to work on his own project. That took 4 years!

The three OVRO telescopes were fitted with the new SIS receivers by Phillips and David Woody [2]. These receivers were based on the heterodyne principle in which a local oscillator signal is mixed with the incoming signal to generate an intermediate frequency in the range of about 10 GHz and then detected by the SIS.

In the meantime, Phillips continued to lay the groundwork for the submillimeter telescope. The question of the optimal location for the submilllmeter telescope was investigated. Mauna Kea was found to be the best in terms of the optical quality of the site and the logistical ease of development. Arrangements were made with the University of Hawaii to lease a site, and an environmental impact study (EIS) was initiated.

So, by 1983 Phillips was ready to go to work on the Caltech Submillimeter Observatory. The EIS was successfully completed and approved. Fortunately, at this same time, funding became available through the NSF due to an unexpected increase granted to the NSF by the Reagan Administration.

[1] A.H. Dayem and R.J. Martin, "Quantum Interaction of Microwave 
    Radiation with Tunneling Between Superconductors"
    Phys. Rev. Letters 8, 246-248 (1962)
[2] T.G. Phillips & D.P. Woody "Millimeter and Submillimeter 
    Wave Receivers" An. Rev. Astron. Ap. 1982, 20, 285.

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