In recent years observations of γ-ray emission from the disk of the galaxy have provided a new opportunity for research into the structure of the spiral arms of our own galaxy. In Figure 1 we show a map of the structure of the disk of the galaxy as observed for γ-rays of energy > 100 MeV by the SAS-2 satellite (Fichtel et al. 1975). The angular resolution of these measurements is ~ 3°, and besides two point sources at l = 190° and 265° several features related to the spiral structure of the galaxy are evident in the data. Most of these γ-rays are believed to arise from the decay of π° mesons produced by the nuclear interactions of cosmic rays (mostly protons) with the ambient interstellar gas. As a result, the γ-ray fluxes represent a measure of the line of sight integral of the product of the cosmic ray density N CR and the interstellar matter density N 1

A major requirement of modern radio astronomy is the attainment of the highest possible angular resolution. This high angular resolution should be matched by a high sensitivity and by spectral and polarization capabilities. Study and experience have shown that no form of telescope is more effective in this respect than the aperture synthesis type. The past few years have seen a growing realization amongst astronomers, both here and overseas, of the need for a new synthesis telescope able to observe the full southern sky. This realization has been stimulated by several factors:

At the first A.S.A. meeting in 1966 radio astronomers knew of only the 21 cm line of hydrogen, a handful of hydroxyl lines and some radio recombination lines. The number of molecular lines now known in the radio spectrum is close to 220, so radio spectroscopy has become a fully fledged subject. I shall not discuss here the recent ultraviolet studies of H2, HD and CO molecules that have been reviewed by Spitzer and Jenkins (1975).

I shall briefly outline some observational aspects of the Magellanic Stream before discussing the pros and cons of the two conflicting theories of its origin: a) that the Stream was pulled out of the Clouds by tidal forces produced by a close encounter with our Galaxy and b) that the gas clouds of the Stream are primordial and in the same orbit as theMagellanic Clouds.

In this talk I shall describe briefly some of the theoretical ideas of the origin of the Magellanic Stream and then give a more personal view of the results of model fitting.

The Magellanic Stream definitely seems to be an extragalactic feature: the apparent connection of the Stream to the Magellanic Clouds and the continuity of the radial velocity profiles make it highly implausible that the Stream is a local foreground feature.

The history and achievements of interferometric angular diameter measurements have been reviewed recently by Hanbury Brown (1974) and only a brief outline will be given here. As the title is intended to imply, the present discussion will be limited to interferometry involving baselines in excess of telescope aperture diameters.

The major programs at the Molonglo Radio Observatory are nearing completion. Much can still be done using the existing 1 mile Cross-type radio-telescope (Mills et al. 1963) but, to tackle properly many of the current problems in radio astronomy, a fundamental change is required from a transit to a trackable radio-telescope, combined with an increase in the operating frequency and the resolution.

Four years ago a Table was shown in which the sensitivity of the Fleurs Synthesis Telescope (F.S.T.) was compared with that of three other radiotelescopes of similar resolvingpower (Christiansen 1972). At that time the high sensitivities quoted for the F.S.T., like those for all but one of the comparison telescopes were only theoretical since only the eastwest array of the F.S.T. was in operation and low noise r.f. amplifiers had not been installed. Now the telescope is in operation with characteristics close to those given in the Table quoted. Sources are mapped separately with the eastwest and north-south arrays and the two maps are combined. The combination produces maps with a resolution that isapproximately the same in declination and right ascension. This is unique amongst existing rotational synthesis telescopes. Some of these maps will be shown in papers now in preparation.

Fabry-Perot instruments have been extensively used for many years for the study of spatially extended emission nebulae because they offer a potential luminosity resolution product (L × R pot) which exceeds by a high factor tha attainable by aerating spectrometer. Meaburn (1970) shows that this product is given by the relationwhere P is the number of resolution elements each containing a maximum solid angle, Ω observed simultaneously, ti is the refractive index of the medium between the plates, A is the acceptance area of the device, εo is the transmission coefficient of the associated optics and εF the transmission coefficient of the Fabry-Perot (which if coated with dielectric multilayers can be very high).

Cosmic rays produce a component of photomultiplier noise which often dominates the high amplitude region of the noise pulse height spectrum and which is not reduced by cooling the tube. The source of the noise is Cerenkov radiation produced by individual high energy cosmic rays in their passage through the glass envelope of the tube, principally in the glass faceplate on which the photocathode is deposited.

The K giants are the brightest stars in clusters of both intermediate (∽ 108 years) and old (∽ 109-10 years) age. They have strong metal lines and strong bands of CN and CH in the optical spectrum, and are the logical objects in which to measure the metal abundance of such stellar systems and trace the evolution of heavy elements in the universe.

The deflection of the light rays of a distant star by the gravitational field of a nearby star was first derived independently by Link (1936) and Einstein (1936). It was shown by Link (1936,1937) that the light of a distant star is not only deflected forming two separate images of the distant star, but also the brightness of the distant star (in bothimages) is affected.

One of the major problems in the theory of type III solar radio bursts concerns the development of the two-stream instability. On the one hand, Sturrock (1964) argued that one expects the instability to develop rapidly, and if it does it should prevent the stream from propagating through the corona, contrary to observation. On the other hand, one appears to require that the instability develop partially, in the sense that there is some significant amplified emission of Langmuir waves, in order to account for the observed emission.

SV Cen is an eclipsing variable of the Beta Lyrae type and is especially known by its rapid changes of period. The system was studied photometrically and spectroscopically by Irwin and Landolt (1972). According to the previous study by O’Connell (1951) the system exhibits a 34-year periodicity.

The strategic value of infrared sensing has prompted military development — mostly in the USA — of detectors for 8-30 μm far more sensitive than any commercially available. In 1970-72 the Air Force Cambridge Research Laboratories conducted a rocket-borne survey of the sky N. of about -35° dec at 4,11 and 20 μm. Although this survey was initially classified, a small number of preliminary versions of it were distributed to selected infrared astronomers based at U.S. institutions.

Observations of Jupiter’s decametric radio burst have established that above 20 MHz more than 90% of the emission is elliptically polarized in the RH sense, but below this frequency the proportion of the LH polarization increases (Sherrill 1965) until at 10 MHz nearly 40% of the emission is LH polarized (Dowden 1963). The best time and frequency resolutions so far obtained when examining the polarization of the spectra of Jupiter’s bursts have been 10 ms and 50 kHz respectively (Gordon and Warwick 1967, Riihimaa 1975). To study the fine structure of Jupiter’s emission as observed by the linearly polarized Llanherne low frequency radio telescope (Ellis 1972), spectrum analysers with resolutions in the ranges 0.3 — 1 msec and 2–10 kHz have been used (Ellis 1973a, 1973b, 1974, 1975). A new telescope at Llanherne, which is capable of detecting the RH and LH circular components of incident radiation, is being used in conjunction with these analysers to yield information of the hyperfine polarization structure of Jupiter’s decametric radio emission. This paper is a preliminary report of this investigation.

The fact that Jupiter’s inner magnetosphere is not axially symmetric was originally demonstrated by ground-based radio measurements. Departures from symmetry are evident in the pattern of bursts occurring at decametre wavelengths and also in the characteristics of the polarized synchrotron emission observed at decimetre wavelengths. Direct measurements of Jupiter’s magnetic field, made from the Pioneer space probes, also indicate departures from an axially symmetric configuration.

The radio emission from Jupiter in the frequency range from ∽ 50 MHz to ∽ 5 GHz is mainly synchrotron emission from electrons in the intense radiation belt which surrounds Jupiter out to several planetary radii. Information about the pitch angles of these electrons can be derived both from the radio observations and from the Pioneer space probe observations. In this communication we discuss the pitch angle distribution inferred from the radio data and the apparent conflict with the Pioneer data.

Since the discovery of highly masering OH emission, and later of H2O masers, there has been little progress made in understanding their geometry and structure. Although both OH and H2O spectra are highly complex, no one has been able to ‘decode’ the information from the many individual features. Knowles and Batchelor (1976) have recently suggested that many water vapour spectra have a symmetrical character, with one or more pairs of features equally spaced in radial velocity about a central axis. It is here proposed that a similar symmetry exists in HH-region OH masers as well as main-line OH stars such as VY CMa. We also demonstrate that features at close to the same radial velocity are often present in several different OH and H2O transitions and probably come from the same masering cloud. From this widely applicable empirical observational model we then make a few simple deductions about the possible geometry of such masering regions.

Most of the known 1.35 cm water vapour masers have been detected in the direction of known 18 cm OH masers. However, a small number have been found in the direction of HII regions where no OH maser is known. Several new examples of H20 masers with no known OH counterparts have recently been published by Kaufmann et d. (1976), who suggest that these constitute a new class of H20 maser.