The wavefront from a celestial source is believed to arrive at the outer fringe of the Earth’s atmosphere as a truly plane wave of effectively infinite extent. After propagating through our refractively non-uniform atmosphere this wavefront becomes distorted in shape and exhibits a nonuniform amplitude distribution. At present there is little that can be done to remedy the effects of the atmosphere on wave propagation. However when the wave has been received by the entrance aperture of the observing telescope we should try to design this optical system, including its air-path sections, so as to minimize further distortion of the wavefront. ‘Seeing’ effects within an instrument which concern us in this paper are attributable to air of nonuniform temperature (and hence refractive index) in the sections between the optical components.

The Parkes interferometer consists of the 210-ft para-boloidal reflector of the ANRAO, and a 60-ft paraboloidal reflector mounted on rail tracks. These extend from 400 ft to 1400 ft from the 210-ft paraboloid and run both north-south and east-west. The baseline may be continuously varied during observation; receivers operating simultaneously at 468 and 1402 MHz enable the determination of complete visibility functions over the range 200λ to 2000λ.

We have measured the absorption spectra of the two principal lines of 018H at frequencies 1637 and 1639 MHz approximately in the directions of the sources Sgr A and Sgr B2. The use of 64 x 100 kHz filters enabled us to observe both lines simultaneously. The equipment and methods of observation and reduction are discussed by Gardner, McGee, and Sinclair.

A precedent to the recently-discovered pulsed radio sources or ‘pulsars’ exists in our own solar system. Jupiter could be thought of as a very slow ‘pulsar’ having a period of about 10 h or 35 000 s. Like pulsars, this emission period is known to a high order of accuracy (about 1 in 106). One difference is that Jupiter emission is received over an appreciable part of this period (1/4 to 1/2 or more) compared with about 1/30 of a typical pulsar period (about 40 ms in 1.3 s). Both pulsar and Jupiter bursts have a microstructure of the order of milliseconds, suggesting similar sizes of instantaneous emission regions. In both, the intensity observed varies from period to period. Emissions from both have relatively strong circular-polarization components at times.

Clusters containing cepheids, and especially clusters containing more than one cepheid, provide information on the advanced evolutionary stages of massive stars, and on the factors which influence the period-frequency relations and lifetimes of cepheids. NGC 1866, in the LMC, is already well known in this respect. NGC 2010 was found by Hodge to contain two cepheids. Two more variables have been found in this cluster which, from their magnitudes and colours, are almost certainly cepheids, though we do not yet have periods for them. We noticed also that two known cepheids, HV 2868 and HV 2870, are associated with NGC 2136.

There is a large body of evidence to suggest that the anomalous OH emission sources—particularly those which are classified as class I sources—are associated with protostars. It is also known, from the theories brought forward over the past four years, that maser action can be caused by quite a variety of non-equilibrium situations. It is argued here that one particular kind of non-equilibrium situation which can plausibly be expected to exist in protostars is very similar to that proposed previously in the electron-pumping model, and that, in this new context, it can mimic most of the observed features of class I sources.

For some years now, there have been suggestions that the nuclei of planetary nebulae are stars undergoing a final gravitational contraction to the white dwarf state. These have culminated in two important studies by O’Dell, and by Seaton and his collaborators, the results of which are indicated on the Hertzsprung-Russell diagram of Figure 1. The temperatures and the distances—and hence luminosities—of the central stars are obtained from flux observations of the stars themselves and the surrounding nebular shells, the former by the Zanstra method, and the latter by that of Shklovsky.

For the past six years astronomers have been regarding the universe through a new window—the X-ray window. In these six years this subject has experienced a ‘remarkable’ growth. In late 1966 workers from the Naval Research Laboratory (NRL) published a catalogue containing 35 X-ray sources. In my preparation for this report I have compiled a new catalogue containing only 22 sources (which I believe contains all of the presently confirmed X-ray sources). This early proliferation of ‘sources’ may be attributed to the exuberance of the experimenters or perhaps to a poor understanding of the laws of statistics. A simple calculation shows that, with this rate of decrease of ‘observable’ sources, X-ray astronomy will cease to exist as a subject in 1972!

The Molonglo fan-beam survey is a digitized survey carried out in March and April 1966, using the 1 mile E-W arm of the Molonglo cross operating at 408 MHz. Three 1´.5 fan beams separated by 1´.5 in right ascension were used. The survey covered the region in right ascension from 10h30m to 18h30m and in declination from +18° to —40°. The region common to the 4C survey has been analysed and we have obtained right ascensions for the 4C sources.

A number of strong radio sources have been examined for structure with the 3’.5 beam of the 80 MHz radioheliograph at the CSIRO Solar Observatory, Culgoora Contour maps of brightness distribution are presented here for the galactic source W44, and the extra-galactic sources Fornax A, Pictor A and 3C353; they are compared, where possible, with maps at other frequencies.

This paper presents observations of weak X-ray sources at photon energies between 20 and 100 keV which were made during a balloon flight conducted from Mildura, Australia on 29 February 1968. Results obtained from Sco XR-1 and the Sagittarius region during this flight have been presented elsewhere, and here we report additional results obtained during a search of an area of sky in which several sources have been reported from rocket experiments at photon energies below about 10 keV. The detector used was an actively collimated and shielded NaI(T1) crystal, 2 mm thick and 54.3 cm2 in area. Incident photons were sorted into 16 energy channels between 7 and 167 keV (1.76 to 0.07 Å). The collimator field of view was 8°FWHM; the telescope axis was fixed at a zenith angle of 32° and its azimuth angle was varied by rotating the observatory below the balloon. This allowed the telescope to scan back and forth in azimuth across a source, the azimuth angle at any time being indicated by magnetometers.

A search for the 126a hydrogen recombination line, with a rest frequency of 3248.713 MHz, has been made in the 19 sources listed in Table I using the Parkes telescope. The beamwidth was 6′ are and the frequency resolution 37 kHz. In the Table RCW refers to the Rodgers, Campbell and Whiteoak Atlas, W to the Westerhout catalogue, and the co-ordinate numbers to the Parkes Catalogue of Sources. In each case the ratio of the peak of the line to the continuum TL/TC, the half-intensity width Δv and the mean radial velocity to the local standard of rest, were determined. The related velocities observed in OH emission and absorption are also shown.

Over a period of one year from March 1967, the 24-inch rotatable telescope at Siding Spring Observatory was used to make polarization measurements for about 1400 stars, most of which were within 400 pc of the Sun. It was found that the E vectors of the polarization formed quite definite patterns on the sky which could only be produced if the magnetic field has a helical structure. Results of model-making showed that the magnetic lines of force form tightly wound right-handed helices of pitch angle 7°. They lie on the surface of tubes which have elliptical cross-sections of axial-ratio 3 with semi-major axes parallel to the galactic plane.

Most extra-galactic radio sources show angular structure over a wide range of size. Long-baseline interferometers and scintillation observations have shown that resolved sources extending over 10′ arc can have significant contributions from components <1″ arc. Observations at low frequency with the Molonglo cross telescope show also that a large fraction of the emission can come from halo, bridge or extended regions which may form a diffuse ridge or arc structure extending well beyond the small diameter components. However the majority of sources exhibit a basic double structure, and this has led to the adoption of two extended (Gaussian) components as an initial model for the interpretation of brightness distribution in most papers on the structure of extra-galactic sources. Parameters based on such a model will usually match the observed distribution quite well, but because of confusion, there is a danger in using intensity parameters for a model component to derive the spectral index for part of the source.

A survey of clusters of galaxies from Abell’s catalogue has been carried out using the Mk II 11 cm receiver recently installed at Parkes. For the present preliminary report only sources with flux densities down to about 0.3 f.u. will be considered, although the sensitivity limit of the receiver is about one-sixth of that value.

High-resolution surveys of the galactic radio emission show detailed structure both near the galactic plane and at high galactic latitudes. Some of this structure is describable as ‘spurs’ while the rest comprises isolated features. We describe here measurements of the spectral index of the background radiation with a resolution of 3°.5. These measurements are aimed at a detailed understanding of the high-latitude synchrotron emission.

During a recent balloon flight from Mildura, Australia, the region of the Galaxy between −10° < lII < +20°; −5° < bII< +5° was surveyed for high-energy X-rays of photon energy > 17 keV (λ < 0.7 Å) using an active collimation scintillation detector described elsewhere. The narrow opening angle of the telescope enables two sources to be resolved in this region of the sky. The position of one of these sources agrees well with the source GX3+1 observed at rocket wavelength by Bradt et al. while the other source is probably the same as that observed by Gursky et al. and designated GX−5.6. We present herein improved positional information for this latter source which hereafter will be designated GX354−5.

One of the basic tasks of the one-mile cross at the Molonglo Radio Observatory is the compilation of a catalogue of radio sources complete to a low flux density level. This level has been tentatively chosen as 0.2 f.u., which is well above the ultimate detection level which can be reached in selected areas of sky. Such a limitation is necessary if the catalogue is to be restricted to a manageable number of sources; a few tens of thousands, which will be achieved with the above limit, seems appropriate and useful at the present time.