Observations using a maser radiometer at 3-cm wave length.

Abstract

Observations of radiation at 3-cm wave length from the planets Venus and Jupiter and from NGC 4486, Virgo A, have been made using a solid-state maser amplifier at the focus of the U. S. Naval Research Laboratory ~o-foot reflector. The antenna temperature due to Venus on April 18-19, 1958 at ~3.37 cm was o?45C which corresponds to a blackbody temperature about 5750K. The apparent blackbody temperature derived from the observations of Jupiter on May 25 to May 30, 1958 at X3.18 cm was 1650K. The estimated probable error of the blackbody temperatures is about 10 per cent. These results are in fair agreement with previous observations of Venus made near conjunction in 1956 (Mayer et at. I~58a) and of Jupiter made in 1957 (Mayer et at. I958b). The radio source Virgo A was observed at X3.37 cm and gave an antenna temperature of I?25C. The maser amplifier is a three-level type (Bloembergen 1956) with a 0.1 per cent concentration of Cr+3 in ruby (Makhov et at. 1958) serving as the paramagnetic medium. The ruby crystal is mounted in a doubly resonant microwave cavity in liquid helium at I ?4K, and is in a magnetic field of 3500 oersteds. The voltage gain-bandwidth product of the maser is about 12 Mc, and optimum signal-to-noise ratio for the entire system is obtained with a gain of about 20 db. The maser is mounted near the focal point of the 5o-foot reflector. * Esso Research Education Foundation Fellow, 1957- 1958. The radiometer has been described previously (Mayer et at. I958a). After passing through a ferrite chopper device the antenna signal enters the reflection type maser cavity through a three- port ferrite circulator. The amplified signal is then passed to a balanced mixer and I-F amplifier. With a 5 second output time-constant, an rms value of the output fluctuations of about o?o~K has been attained. For the detection of continuum radiation this output fluctuation level represents an improvement factor of about ten in signal-to-noise ratio over the radiometer without the maser pre-amplifier. The improvement factor is limited in part because the present maser pre-amplifier has a bandwidth about one- fifth that of the I-F amplifier alone. A new maser with a gain-bandwidth of 6o Mc has been constructed and is expected to allow a considerably greater sensitivity. These results are consistent with the expected low noise characteristics of maser amplifiers and demonstrate the improvement in radiometer performance obtainable with maser pre-amplifiers. They also show the feasibility of applying maser techniques to radio astronomical observations. The work has been carried on at Columbia University, supported by the Signal Corps, the Office of Naval Research and the Air Force Office of Scientific Research. Bloembergen, N. 1956, Phys. Rev. 104, 324. Makhov, G., Kilcuchi, C., Lambe, J. and Terhune, R. W. 1958, Phys. Rev. 109, 1399. Mayer, C. H., McCullough, T. P. and Sloanaker, R. M. 1958, Ap. J. 127, 1. 1958, ibid., p. ii. Colitmbia University, New York, N. Y., and Radio Astronomy Branch, U. S. Naval Research Laboratory, Washington, D. C.