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Millimeter-wave receivers are discussed from the point of view of applications to line astronomy. A heterodyne-bolometer system is described which has a noise temperature of 300 K at 230 GHz.

The maser and the parametric amplider form a new class of microwave amplifiers which can exploit the properties of bound electrons in a solid. These amplifiers have several common characteristics, among them being their very low-noise performance. This paper reviews the method of operation of these amplifiers, discusses the performance achieved and achievable by the various versions, and points up some of the difficulties involved in effectively utilizing the extremely low-noise figures obtainable. A bibliography is included in which an attempt has been made to include all published papers on masers and parametric amplifiers.

A paper [1] sent to the Manchester Symposium on Radio Astronomy in 1955 described the views developed earlier [2] concerning a magnetobremsstrahlung (synchrotron) origin of nonthermal cosmic radio emission. Unfortunately, for unknown reasons, it was not included in the Symposium volume [3]. At the same time the paper by Unsöld was published [3] (see also [4]), in which the nonthermal galactic radio emission is connected with radiation of a great number of cold dwarfs (radio stars). At the same time, Unsöld criticizes the theory of cosmic-ray origin, according to which the main sources of these rays are envelopes of supernovae and, perhaps, novae [2, 5, 6, 7, 8]. Therefore, we begin with some critical remarks.

Two years ago I reported at the Jodrell Bank symposium on the initiation of the Agassiz Station project in Radio Astronomy. At that time the 24-ft. antenna was under construction and the electronic equipment for observation of the 21-cm. line of neutral hydrogen was being built by Harold I. Ewen, who is the co-director of the project. The first successful observations were obtained in the autumn of 1953 and reported early in 1954 (see Bok[1]). Since then two papers have been published giving the results of the first studies by Heeschen[2] and Lilley[3]; a report on the equipment and the basic programme was given by Bok and Ewen[4], Earlier this year, at the Princeton Meeting of the American Astronomical Society, Bok[5] drew heavily upon the Agassiz Station results in a paper entitled ‘Gas and Dust in Interstellar Clouds’, in which an attempt was made to blend the results of radio and optical research. We note also at this point a joint paper by Heeschen and Lilley[6] in which attention was drawn to the important role of Gould's Belt in the distribution of neutral hydrogen in the vicinity of the sun.

All available data on the microwave spectra of carbonyl sulfide and hydrogen cyanide are critically reviewed and tabulated. Molecular data such as rotational constants, centrifugal distortion constants, dipole moments, hyperfine coupling constants, and structural parameters are also tabulated. All rotational transitions from 100 MHz to 300 GHz that are deemed likely to be of interest to radio astronomers are calculated and tabulated along with their estimated 95 percent confidence limits. Microwave measurements are tabulated for most isotopic species and for many of the lower vibrational states. For both carbonyl sulfide and hydrogen cyanide a bibliography is given which includes nearly all the spectroscopic work reported in the literature. For each molecule a bibliography of related astrophysical papers is also given.

Methods of increasing antenna resolution by data processing are of interest in radio astronomy and radiometric mapping applications. This paper deals with the fundamental limitations of such operations. Communication theory techniques using prolate spheroidal wave functions have already been applied to antenna pattern synthesis. These techniques are applied to the problem of reconstituting an observed temperature distribution. The manner in which noise limits this reconstitution process is clearly shown. A least expected mean square error reconstitution process is also detailed. An interesting analogy between data processing and supergaining is noted. Finally, the limitation set by noise on the resolution of a data processed finite aperture is concluded.

This paper describes the radio astronomy instrument flown on both Polar Orbiting Geophysical Observatories II and IV (OGO-II and OGO-IV). The instrument was designed to map the brightness distribution of cosmic noise over the sky at 2.0 and 2.5 MHz by using the theoretically predicted ionospheric focusing of an electrically short antenna. An antenna impedance bridge was included as a necessary part of the system. The system is unusual because the impedance bridge and the 2.5-MHz radiometer operate simultaneously at the same frequency. OGO-II was launched on October 14, 1965, and the instrument operated normally until the spacecraft was turned off November 1, 1967. During this period the instrument was operated for 4495 hours with many on-off cycles. OGO-IV was launched July 28, 1967, and the instrument operated properly for 18 330 hours prior to termination of spacecraft operations on October 23, 1969. Results of these experiments are published separately [5], [6].

This paper describes the results of studies on the effects of the atmosphere on radio astronomy observations at X-band. The fluctuation component of atmospheric noise radiation is shown to be strongly correlated with cyclonic conditions and frontal activity.

The subject of the 21-cm. line needs no introduction to this audience. Perhaps as an introduction I may mention one moment that belongs to the pre-history of the investigations of this line. In the spring of 1944 Oort said to me: ‘We should have a colloquium on the paper by Reber; would you like to study it? And, by the way, radio astronomy can really become very important if there were at least one line in the radio spectrum. Then we can use the method of differential galactic rotation as we do in optical astronomy.’

The RAE-1 is the first spacecraft designed exclusively for radio astronomical studies. It is a small, but relatively complex, observatory including two 229-meter antennas, several radiometer systems covering a frequency range of 0.2 to 9.2 MHz, and a variety of supporting experiments such as antenna impedance probes and TV cameras to monitor antenna shape. Since its launch in July, 1968, RAE-1 has sent back some 10 billion data bits per year on measurements of long-wavelength radio phenomena in the magnetosphere, the solar corona, and the Galaxy. In this paper we describe the design, calibration, and performance of the RAE-1 experiments in detail.

This paper reports further detailed measurements of strong radio pulses from the pulsar NP 0532 in the Crab nebula, recorded at Arecibo.

Long-baseline interferometry achieves high resolution by using two or more widely separated radio telescopes and recording video signals on magnetic tapes, which are later brought together and cross-correlated. This paper contains discussions of the coherence and timing requirements and of calibration procedures. Applications to measuring brightness distributions and to spectroscopy are reviewed briefly. Some pertinent phenomena connected with radio-wave scattering in irregular media are discussed.

Sparse aperture imaging has its roots in the work on optical interferometry by Fizeau and Michelson over one-hundred years ago, and the development of radio astronomy nearly fifty years later. In their quest for higher angular resolution at meter wavelengths radio astronomers were forced to seek alternatives to filled aperture telescopes. Radio interferometers, as these instruments are called, measure the complex visibility, which is the Fourier transform of the source brightness distribution. The earliest instruments measured only the amplitude of the visibility, and it was several years before phase measurements became routine. Both the amplitude and phase of the visibility are needed to produce images of a complex source by Fourier inversion. It wasn't until the 1970s and 1980s that the technology was available to allow modern optical stellar interferometers to be built. This paper traces the history of sparse aperture imaging from early radio measurements to the current generation of ground-based optical interferometers, and discusses their general principles of operation.

Some authors (Payne-Scott and Little (1952) [1]; Owren (1952) [2]) have mentioned a phenomenon in the enhanced solar radio emission which they call ‘non-selective fading’. The present paper is meant to call attention to another, rather peculiar, type of scintillation in the radio emission of the sun, which differs from the non-selective fading in some important respects. This scintillation has been observed since 1952 by the division ‘Ionosphere and Radio Astronomy’ of the Netherlands Telecommunications Service in the course of a continuous survey of solar radio radiation. It has been found at the radio frequencies 140, 200 and 545 Mc./s.

The paper reports attempts to detect radio emission from the x-ray source Cygnus X-2 at a wavelength of 4.5 cm.

MEASUREMENTS of radio source positions to an intended accuracy of 1 arc second or better have recently been carried out independently at the National Radio Astron-cm), although the actual methods which were use Both sets of observations were done by means of interferometry at a nominal frequency of 2,695 MHz (λ 11.1 cm), although the actual methods which were employed differ radically. The positions measured for the twenty-eight sources common to the two lists are generally in excellent agreement, as will be shown. Papers describing the methods used, and containing full lists of measured positions, will be published separately elsewhere (ref. 1 and unpublished work of Adgie et al.).

This paper presents a summary of a survey of theoretical and experimental methods employed in the analysis and synthesis of acoustic systems. The work has been conducted under the sponsorship of the Office of Naval Research. Although primary emphasis has been placed on underwater acoustics, the survey has drawn heavily from related work in the fields of radar, optics, geophysics, radio astronomy, and electromagnetic communications. The conceptual model which has guided the program involves space‐frequency‐time interrelationships existing within information “fields.” Selection of analytical and experimental methods has been based on their relative utility and efficiency in specifying space‐frequency‐time interrelationships in order to (a) improve understanding of acoustic systems and processes and (b) improve capacity to perform specific functions. Morphological analyses of instrumentation which may be employed in exploring space‐frequency‐time parameters will be discussed.