Phase coherent interferometers with intercontinental baselines became possible because of the development of stable frequency standards. With sufficiently stable frequency standards, no connection is necessary between the two ends of an interferometer. The first VLBI experiments were conducted by a group at the University of Florida who used an intensity interferometer with independent tape recorders for observations of Jupiter. Later, they changed to a coherent system using crystal-controlled oscillators. Since then, several interferometer systems have been developed. A Canadian group developed a system using video tape recorders at each end of the interferometer. They recorded the data in analogue form and managed to bring the two tapes together and to synchronize them to an accuracy of better than a microsecond. After synchronization, the outputs were combined and fringes extracted. Their system has a bandwidth of about 4 MHz. No-one else has attempted a wide-band analogue system.

The adjective ‘high’ when applied to the resolving power of radiotelescopes, has a meaning that has changed very rapidly during the last twenty-five years. In the late 1940s a resolving power of one degree was considered to be ‘high’.

It is now well established that observations of interplanetary scintillation from a single station can yield useful information on the angular structure of radio sources, and on the propagation of large-scale disturbances through the interplanetary medium. However, when it is required to obtain detailed information on the plasma properties single station observations suffer from the disadvantage that, in general, it is not possible to measure the velocity and scale of the irregularities independently. For example, a change in the width of the observed temporal power spectrum of the scintillations might be caused by either a change in velocity, or a change in scale, or a combination of both.

In this paper we report that the available data on the measured pulsed fraction of X-rays from the Crab Nebula between the energies 0.7 and 300 keV indicate that it is no longer necessary to postulate an intrinsic change in the pulsar spectral index near 30 keV. Furthermore the spectral flattening which is evident at low energies can be entirely attributed, within the precision of the available statistics to the destruction of time coherence of the pulse structure by the scattering action of interstellar dust grains.

As the night sky near Canberra grows brighter each year, many of the traditional areas of investigation at Mount Stromlo are suffering, and work in these fields is shifting to Siding Spring Mountain, near Coonabarabran, N.S.W. One of several projects designed to ensure the continued productivity of the 74-inch reflector at Mount Stromlo as conditions change has been the installation of an échelle grating system in the coudé focus of the telescope. This will allow very high dispersion work on bright stars, and should be littles affected by nearby city lights.

The third UHURU catalogue of X-ray sources lists 64 sources at galactic latitudes higher than 20°, many of which have been identified with galaxies and clusters of galaxies. One of the best criteria for identifying a cluster of galaxies with an X-ray source is the extended nature of the X-ray source. Table I is a compilation of all clusters of galaxies known to be within the boundaries of X-ray error boxes. Not all have measured X-ray angular sizes (see column 3). Since the southern sky has been inspected too (Vidal 1975a), we believe that this list may represent a complete sample (down to the sensitivity of the UHURU satellite and the completeness of the Abell catalogue and the southern survey). Column 1 gives the optical identification, column 2 the 3U designation, column 3 the X-ray angular radius, column 4 the radio spectral index (see below), column 5 the redshift (corrected for galactic rotation) and column 6 the X-ray luminosities assuming the Hubble law. Numbers in parentheses give the references for the entries. When no such references are given, the data have been taken from Kellogg et al. (1973) and Bancall (1974).

The detection of formaldehyde absorption of the microwave background radiation in a region of the Southern Coalsack (Sinclair and Brooks 1972) was made at about the same time as an optical study of this region (Tapia 1972) in which six globule-like clouds of interstellar dust were found. The size (0.3 pc) and position of the formaldehyde cloud were similar to the corresponding parameters of one of the globules (No. 2 on Tapia’s list).

Cen X-3 was first discovered by Chodil et al. Since then, observations using the UHURU satellite have shown it to be an X-ray pulsar, and it must now have become one of the most searched for objects in the sky. It is both an X-ray pulsar (P~4.8 secs) and an X-ray eclipsing binary (P~2.08712 days).

A description was given in a previous paper of the first observations of the two-dimensional image of the Crab Nebula as it became broadened by the solar corona in June 1969. In this paper we describe further observations at 80 MHz during 1970 and 1971, again using the CSIRO Radioheliograph at Culgoora, N.S.W., and we discuss the derived values for radial and tangential broadening in relation to previous work at various phases of the solar cycle. Other methods of observing angular broadening have generally employed two or three interferometers at different position angles, and only the simplest model for the image could be assumed in interpreting the results. The radioheliograph, however, has the advantage of recording the complete two-dimensional image and also, simultaneously, the surrounding background.

The Molonglo cross-type radiotelescope has been in operation since 1967 and it is expected that its principal task, the fundamental sky survey, will be completed within the next few years. This by no means represents the end of the useful life of the instrument but we have been investigating modifications to extend further its usefulness. The most desirable modification appears to be a substantial increase in operating frequency to improve the resolution and positional accuracy.

Preliminary mapping of the thioformaldehyde distribution in the direction of Sgr B2 followed the detection of the 211←212 transition of interstellar thioformaldehyde by Sinclair et al. (1973). Observations were made with the Parkes 64-m telescope in conjunction with a 9 cm parametric amplifier.

Zeldovich and Guseynov suggested that neutron stars and collapsed stars (‘Black holes’) may be found as highly evolved companions of single lined spectroscopic binaries. They pointed out that the detection of X-rays or γ-rays from such a system would constitute positive evidence for the highly evolved nature of the companion. Trimble and Thorne searched through the catalogue of orbital elements of spectroscopic binaries (Batten) and found a few systems which were single line spectroscopic binaries with companions of mass M2 ≥ (1.4 M⊙, M1), where M1 is the primary mass. None of these has, however been identified with an X-ray source and other explanations such as multiple systems could not be ruled out. The alternative procedure, namely the identification of galactic X-ray sources with spectroscopic binaries is clearly more rewarding from the view point of searching for black holes and neutron stars. In particular, a high mass secondary which could be identified with a black hole is more likely to be discovered as a companion of an X-ray binary with a massive primary such as a blue supergiant.

The paraboloid primary mirror of a classical reflecting telescope gives an image that is perfectly corrected for spherical aberration and free from chromatic aberration. However, the image suffers from coma, astigmatism and field curvature. These aberrations severely limit the usable field. For example, the primary mirror of the 200 inch telescope gives an acceptable image of a field with a diameter of only ~2’ arc.

During the last few years, a new type of radio object has appeared in the literature: this is the Early Type Emission Line Star, or ETELS for short. These objects are possibly the most interesting radio object discovered since the pulsars. They are certainly more easily understood, since an explanation of their emission processes probably does not involve any ‘strange’ physics but only simple gas dynamics and thermal bremsstrahlung.

In the preceding paper, observations of the coronal broadening of the Crab Nebula during 1969-71 were described. The basic parameters, radial and tangential broadening, and their relation to previous work were discussed. Whereas previous observations have utilized two or three interferometers only, so that the degree of broadening along any particular axis could only be obtained under the assumption of a particular form (e.g. Gaussian) for the angular power spectrum, the present work has enabled complete sampling of the two-dimensional brightness distribution of the broadened source. It is therefore possible, and of considerable interest, to compare the observed distributions to those computed on the basis of various theoretical models for the scattering process.

A large proportion of the easily accessible radio astronomy spectrum lies between 50 MHz and a lower limit of about 1 MHz set by interstellar absorption. The features of the spectrum in this frequency range, from sources such as the galaxy, extragalactic sources, pulsars, the Sun and Jupiter, remain only partially explored mainly owing to the large sizes of telescopes necessary to obtain adequate angular resolution and sensitivity. In addition, below 20 MHz, interference from man-made radiation and from the ionosphere severely hinders observations. At the lowest frequencies, the effects of the ionosphere can be overcome by using earth satellite telescopes at the expense of greatly increased difficulty in attaining sufficient telescope aperture.

The 16-inch telescope on Canopus Hill (Tasmania) has been equipped with a Coudé spectrograph.

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.

The structure of small diameter radio sources can be investigated by studying the scintillation of the source due to the interplanetary medium when the line of sight approaches the Sun. Observations of radio source scintillation are currently being undertaken with the separate arms of the 1 mile radio telescope at the Molonglo Observatory. The EW arm allows successive transit observations with three fan beams, 1′.4 EW by 4°.2 NS at 408 MHz, bandwidth 2.5 MHz. Sources transit the half-power points of each beam in 6 sec δ seconds of time (δ is the source declination). The NS arm gives eleven fan beams at neighbouring declination, 1′.5 sec Z NS by 4° EW (Z is the zenith angle). Complete transit of a NS beam takes 15 sec δ minutes.

Following the detection of the 9 cm ground-state triplet of CH by Rydbeck et al. (1973), preliminary observations at the CH frequencies were made with the Parkes 64-m telescope (beamwidth ∼6′ arc) in December 1973 and January 1974. We have detected the CH lines in many galactic HII regions and in the direction of the galactic centre.