Commissioning of the Anglo-Australian Telescope (AAT) is now well along and regular scheduled observing will begin at the end of June. This paper is an account of the main features of the telescope as they will appear to observers, and is intended in particular as a guide to people who are formulating programmes. First however some remarks on the telescope design.

The Hertzsprung phenomenon, in which secondary bumps in the light or velocity curves of cepheid variables occur at systematically earlier phases as the period lengthens, has been known for about 50 years. As part of his programme on non-linear computations of cepheid models, Christie has interpreted the effect as a progressive wave which originates near the surface, travels to the core and is reflected back. He found that the phase varied with period in the correct sense, but to make his models fit both the Hertzsprung and period-luminosity relations he had to assign to them masses only about a half of those expected from evolutionary theory. The discrepancy is one of the outstanding problems in the field.

This paper suggests reasons why certain forms and frequencies are found in librations of the pole. The period is about 27 years, but there is also a shorter period which was found empirically to vary with the inverse of the amplitude of the Chandler nutation. It had been found also that the pattern of the motions would sometimes become reversed.

It is well known from linear theory that a magnetic field or rotation, acting separately, have an inhibiting effect on the onset of thermal convection. This has been confirmed by corresponding studies of finite amplitude convection. On the other hand, when both magnetic field and rotation act together, the linear theory predicts that they may have conflicting tendencies. It is the purpose of the present paper to report on the investigation of these effects in the case of non-linear thermal convection.

The Division of National Mapping has received, on long term loan from NASA and in co-operation with the Smithsonian Astrophysical Observatory, a Lunar Laser Ranging instrument consisting of a gigawatt pulsed ruby laser, a 150 cm aspheric Ritchey-Chretien telescope, and associated electronic equipment. The instrument was formerly operated by the Air Force Cambridge Research Laboratories at Mount Lemmon in Arizona. Its principal use by National Mapping will be direct determination of the distance between the telescope and any of the three retro-reflector arrays placed on the Moon at Hadley’s Rille, Fra Mauro and Mare Tranquillitatis by Apollo astronauts. Full scale operation in conjunction with similar instruments well separated in latitude and longitude, in particular at Mount McDonald in Texas and Mount Haleakala in Hawaii, will permit determination of Earth rotation and polar motion, lunar ephemeris and libration, and tectonic plate movement or continental drift, which justifies its use in a geodetic environment.

Scattering of radiation from solar radio sources by in homogeneities in the electron density structure of the corona can have marked effects on the observed time profile and the brightness distribution. These effects are appreciable for any localized source emitting a brief burst of radiation and are most pronounced when the radiation propagates close to the plasma level; it is such sources, illustrated here by those of type III bursts, that will be considered in this paper.

During recent years it has become rather clear that the β-Cephei variables are in a hydrogen-buring stage of their evolution. The 8 (β-Cephei variables which have been found in the Scorpio-Centaurus association highlights this. The 6 of these variables for which Glaspey (1971) provides (β, (u – b)0) photometry all fall at the tip of the sequence in the Hertzsprung-Russell diagram. This indicates that (β-Cephei stars are in the final stages of core hydrogen burning. This is in good agreement with the position of the β-Cephei strip of Watson (1972) in the H-R diagram.

Current observations and theories of solar bursts of types I, II and III suggest that the observed radiation may be emitted at a frequency close to the (fundamental) plasma frequency or its second harmonic. Refraction in a spherically symmetric corona would prevent radiation at the plasma frequency from reaching the observer except when the source is near the centre of the solar disk. However, it is found that fundamental frequency bursts are observed from anywhere on the disk. Recent analyses by Steinberg et al. and Riddle, in which the scattering of the radiation by coronal inhomogeneities was considered (in addition to refraction in an otherwise spherically symmetric corona), show that the radiation can escape from the plasma level and be observed for sources positioned almost anywhere on the disk. In addition, these authors and Fokker showed that a point source of radiation at the plasma frequency, or its harmonic, would be observed as an extended source with dimensions comparable with those observed. One implication was that the true source size is much smaller than the observed size.

Two important problems have emerged from observations of 18 cm OH emission sources that remain substantially unanswered by existing theories. The first is the problem of the origin of the high degree of circular polarization that is frequently observed. The second is what I have called the ‘problem of variety’: is it necessary to invoke a number of different physical mechanisms to explain the variety of qualitatively different behaviours observed, or can these different behaviours be explained by variations of one basic physical process?

The study of short time scale phenomena of astronomical interest is often handicapped by the presence of wide band noise. In particular, in the study of the optical variability of stars, such noise arises from atmospheric scintillation, extinction variations, sky radiance variations and photon statistics. For example if in order to resolve rapid stellar variability we reduce the length (T) of the time intervals over which a photometric record is integrated and sampled, we find that the fractional noise fluctuations increase, eventually obscuring any intrinsic variability.

The object EX Hya is a dwarf nova with a binary period of 98.3 min (Mumford 1964, 1967). Warner (1972, 1973a) has observed two complete cycles of this star with a photoelectric time resolution of 5 sec. These observations suggested that EX Hya can be understood in terms of the model proposed by Warner and Nather (1971) in their discussion of U Gem. In this model, a white dwarf primary of a semi-detached binary system is surrounded by a disk of gas formed from matter transfered from the secondary, which is a cool dwarf star filling its Roche lobe.

Although the maser hypothesis for anomalous OH emission encounters difficulties it has not been seriously questioned because, as pointed out by Turner, no alternative has been available. The following ideas might suggest that there is an alternative hypothesis for anomalous OH emission.

Observations of the relative positions of fundamental and second harmonic bursts can be used to study refraction and scattering effects in the corona. Smerd, Wild and Sheridan have pointed out that the observed positions of type II bursts can be interpreted as favouring the backward emission of the second harmonic in a smooth corona. In this case the harmonic is seen after reflection from near the fundamental plasma level. Several other examples of type II and type III bursts have been reported which show this effect. We wish to present further evidence from a recent study of a large number of type III bursts observed with the 80 MHz heliograph and 158 MHz interferometer.

A program has been designed for Hewlett-Packard 9100 A Calculator connected by a 9102A Buffer to a 9101 A Extended Memory, a 9120 A Printer and 9125 Plotter. The program evaluates and plots one-night photoelectric measurements in 3 colours of a short period eclipsing variable and detects the exact time of the minimum. The program may take a maximum of 24 observations either of a comparison star or of the variable in each colour but it could easily be modified for a maximum of 36 observations in two colours or seventy two observations in one colour. The usual alternating succession of the type CVCVC… is supposed but up to three successive measurements both of the comparison star and of the variable is possible, e.g. CVCCCVCVCVVVCVC…

Established techniques for inverting observed solar spectra rely on the use of centre to limb observations.

In 1966 McGee, Gardner and Robinson carried out an exploratory search at 1665 MHz for OH in 26 thermal sources in the southern Milky Way and reported emission in 12 sources. Subsequent measurements by Robinson, Goss and Manchester confirmed emission from six of these and detected four additional OH emitters.

In the atmosphere of the Sun, most spectral lines are broadened by collisions between the radiating atom and neutral particles. The interaction between two atoms is usually represented by the multipole expansion of the total interaction potential. For two neutral atoms, the first non-zero term in this expansion arises from induced dipole-induced dipole forces and is proportional to r-6. Most calculations of broadening due to neutral atoms have assumed this van der Waals interaction, V(r) = —C6 r-6, between the atoms. Such an assumption is only valid if the interatomic separation is much greater than the sum of the radii of the two atoms so that no overlap occurs and only the leading term of the multipole expansion need be considered. However, in fitting the wings of solar line profiles, O’Mara found that the value of the van der Waals constant, C6, had to be increased by up to a factor of ten in order to obtain agreement between theoretical and observed profiles.

Although cosmic rays detected in interplanetary space have often been correlated with visible flares at the Sun, little is known about the transport of these particles through the corona. Lin demonstrated a good correlation between ≳20 keV electron events detected by spacecraft near the Earth and type III radio bursts at the Sun. In a detailed investigation of many of these electron events from one particular active region source, Lin proposed that the injection of electrons was characterized by a source region in the corona which extended over ~70° in longitude, such that in this region the electrons had direct access to an ‘open cone’ of propagation in interplanetary space. When the spacecraft was situated outside this open cone (by up to 15°), impulsive electron events were still recorded, but these were now modified by diffusion through the corona of the electrons from the 70° source region.

A considerable amount of observational interest in short timescale phenomena has developed in optical astronomy in recent years. This interest has stemmed primarily from search efforts directed towards optical pulsars and the study of rapid fluctuations in cataclysmic variable and flare stars. Such studies have been made feasible by the development of the necessary photon pulse-counting technology required for rapid data sampling.

Bipolar structures are discernible in three categories of solar radio emission: the slowly varying component (S-component), microwave bursts and metre-wavelength type I storms. Piddington and Minnett, discussing the S-component, showed that when coupling between the magneto-ionic modes along the ray path is ignored the emission from a regular dipolar field should have two specific polarization properties. These properties (see Figure 1) are that both feet of the bipolar structure should be similarly polarized, i.e. both left-hand (LH) or both right-hand (RH), and that this handedness should reverse on central meridian passage (CMP). (Reversal of the handedness at CMP had been suggested earlier by Martyn.) These arguments should apply also to microwave bursts and to type I storms.