The highest frequency at which a radiotelescope can be used depends on the surface accuracy. The rms error of the surface should be less then l/16th of the shortest wavelength. This should be not only under static conditions but also when the telescope is being driven, under the influence of wind and temperature changes. With this in mind we can trace back the development of the paraboloidal dish since the time when Grote Reber constructed his antenna in Wheaton, Illinois.

Although it is well known that HII regions are present in the innermost regions of the Galaxy their kinematics are still not fully understood. In one study Pauls et al. (1976) surveyed with a beamwidth of 3′ arc the 10 GHz recombination line emission in directions within 15′ arc of the nuclear radio source Sgr A. They found that the emission velocities varied from position to position within the range -50 to + 50 km s-1but appeared to lack any overall pattern. In contrast, we have recently observed the recombination line emission from the galactic centre region with a beamwidth of 4′.5 arc, and find strong evidence of ordered motions near the galactic nucleus.

The structure of the radio source 1308-441 has been determined with the Molonglo and Fleurs radio telescopes. The nuclear component has been identified with a magnitude 15 galaxy and an optical spectrum, obtained with the Anglo-Australian Telescope shows galactic absorption features with a redshift of 0.0515 but no emission lines. The radio image shows the nuclear component linked by a well defined bridge to diffuse lobes extending for 11 arcmin west and 6 arcmin east. The asymmetry is believed to be due to projection effects. The projected size is 670 kpc (Ho = 100 km s-1 Mpc-1). The bridge contains several peaks with one strong unresolved peak 90 kpc from the galaxy.

The occultation of a star by a planet of similar brightness allows accurate determination of the planet’s radius R, mean density ρ and of the ratio T/μ, where T and μ are the planetary atmospheric temperature and mean molecular weight. These quantities are important for the formation theory of the planet and of its atmosphere (see e.g. Öpik).

The basic system of differential equations governing the non-radial adiabatic oscillations of stars are given in the review article by P. Ledoux and Th. Walraven. The numerical integration of these equations has been undertaken by P. Smeyers and by R. Van der Borght and Wan Fook Sun, in the latter case for stars of uniform composition in the mass range.

where μ is the molecular weight of the stellar material. These integrations were based on models derived by R. Van der Borght.

Electron temperatures of compact gaseous nebulae such as planetaries may be derived from radio-frequency data combined with isophotic contours accurately corrected for resolution effects, from the continuum jump at the Balmer limit and other features of the nebular continuum, and from forbidden line ratios.

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.

A departure from an isothermal mass distribution in a rich cluster is expected if the genesis involves a coherent recollapse phase. The character of this departure is calculated and looked for in the data available for the Comma Cluster. No significant deviation from an isothermal mass distribution is found for r < 360’.

Computers and the Internet have created a revolution in the way astronomy can be communicated to the public. At Sydney Observatory we make full use of these recent developments. In our lecture room a variety of sophisticated computer programs can show, with the help of a projection TV system, the appearance and motion of the sky at any place, date or time. The latest HST images obtained from the Internet can be shown, as can images taken through our own Meade 16 in telescope. This recently installed computer-controlled telescope, with its accurate pointing, is an ideal instrument for a light-polluted site such as ours.

We model a massless viscous disk using smoothed particle hydrodynamics (SPH) and note that it evolves according to the Lynden-Bell & Pringle (1974) theory until a non-axisymmetric instability develops at the inner edge of the disk. This instability may have the same origin as the instability of initially axisymmetric viscous disks discussed by Lyubarskij, Postnov & Prokhorov (1994). To clarify the evolution we evolved single and double rings of particles. It is actually inconsistent with the SPH scheme to set up a single ring as an initial condition because SPH assumes a smoothed initial state. As would be expected from an SPH simulation, the ring rapidly breaks up into a band. We analyse the stability of the ring and show that the predictions are confirmed by the simulation.

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.

This paper presents the current status of a Ph.D. project undertaken to search for extended-scale bursts of star formation (> 1 kpc) in irregular and collisionally produced ring galaxies, principally in the southern half of the sky. Results of recent 8.4 GHz radio continuum observations and UBVRI CCD imaging of some of the program galaxies are presented.

A powerful approach to the understanding of any group of stars is the study of any companion stars which can be shown to be physically connected. But RR Lyrae variables, in common with other members of Population II are deficient in physical pairs. Baize’s catalogue of variables with companions contains only one: CSV 1795 = ADS 9354 A. As far as can be determined no period or light curve has been published for this star and it remains doubtful whether it is an RR Lyrae variable.

Ina recent paper, Lockwood et al. (1991) have used IMP spacecraft and Neutron Monitor data to consider the rigidity dependence of three large Forbush Decreases over the energy range 50 MeV to 30 GeV. Some of their conclusions are extrapolated to higher energies.

In an earlier paper (Duldig, 1987a), one of us discussed the need to consider the presence of isotropic intensity waves when determining the Forbush Decrease spectrum at energies up to a few hundred GeV. Lockwood et al.’s conclusions are discussed in the light of these results.

The study of plasma instabilities described by the inclusion of a finite conductivity in the hydromagnetic equations has aroused considerable interest since the initial work of Furth, Killeen and Rosenbluth for a two-dimensional geometry. In particular, the so-called ‘tearing’ mode, which has been observed in laboratory plasmas, has been suggested as a possible source of energy for solar flares.

Observations have been made of the interstellar D-line absorption of Nal towards a loose grouping of OB stars (containing the association Ara OB1 and probably the Hα emission regions RCW 107 and 108) centred at 1 = 337°.5 and b= -0°.5. The individual absorption spectra contain several velocity components which show well-defined groupings at –37, –18 and 0 km s-1. The corresponding median optical depths are 2, 7 and 6. The evidence indicates that both the –37 and –18 km s-1 Nal clouds are located in the Carina-Sagittarius spiral arm. Several HII regions are located at similar longitudes, but most are close to zero latitude and are more distant than the arm.

The association contains the galactic cluster NGC 6193. The results suggest that the turbulence seen optically towards this cluster has been caused by the interaction between two cloud groups moving towards each other at velocities of 10 to 15 km s-1.

Results of the observations of SN 1987A from Ginga are presented. The first detection of X-rays from the SN 1987A was July, 1987. The energy spectrum is quite unnusual for any of the known classes of X-ray sources, and apparently consists of two seperate components; a soft and a hard component. The soft component is significantly time-variable, and also showed a flarelike increase in January, 1988. Whereas, the intensity of the hard component has remained fairly stationary for more than 300 days. The origins of the two components are also discussed.

Recent high resolution Ha photographs by Bray and Loughhead (unpublished) have confirmed that much of the chromosphere is quite non-uniform. Rather, it appears to consist of many distinct structures, which have a typical transverse section of several seconds of arc, i.e. about 2000 km.

The aim of the research reported in this contribution is to attempt to analyse the non-uniform chromosphere by first investigating a single structure. A simple model of a structure was chosen. From the model, line profiles of Hα and the H and K lines of Can were computed assuming a variety of physical conditions to exist in the feature.

In recent years two 408 MHz surveys of the northern sky have been made to study the large-scale, low-surface-brightness features of the galactic background such as the loops and spurs (Haslam et al. 1970; Haslam et al. 1974). These surveys were designed to make maps down to the confusion limits of the telescopes used and to provide as far as practicable accurate calibration of the temperature scale and zero level for large-scale features. This work is complementary to the work of the large array telescopes; these telescopes can provide high resolution detail but they have lower surface brightness sensitivity, and have problems in determining the low-order terms which define the large-scale structure. The Parkes 408 MHz sourthern survey continues this work using the 64-m telescope to complete the coverage of the sky to the south pole. The project is a joint venture between the Max-Planck-Institut and CSIRO. For the southern survey it was felt desirable to measure the linear polarization distribution, besides making an accurate average temperature map in left-hand circular polarization (which is insensitive to variations of ionospheric Faraday effects). The previous surveys did not include linear polarization measurements.