Aims. We investigate the properties of astronomical images made by combining the best images selected from a sequence of short-exposure frames (Lucky Imaging); we assess the match between modelling and observation and discover what variations in seeing occur on very short timescales.Methods. Numerical simulations of a random phase-changing screen passing across a telescope aperture with ideal optics are used to determine the expected point spread function and isoplanatic properties for a range of seeing conditions for comparison with observations.Results. All the model images comprise a diffraction-limited core with Strehl ratios from 0.05-0.5 and an underlying broad disk. The isoplanatic patch sizes are large and coherence times long. The observations are a close match to the models in most respects. Large variations in seeing occur on temporal scales as short as 0.2 s and spatial scales as small as 1 m.

We present a final unified catalogue for the 7C survey at 151 MHz with resolution 70 x 70 cosec(delta) arcsec(2). This has been constructed by amalgamating the existing catalogues derived from individual fields imaged at this resolution and eliminating redundancy in regions of mutual overlap. This is a non-trivial procedure because the flux in multiple-component sources may be fitted differently on alternative images, owing, for example, to differences in local noise and beam distortion. The final catalogue lists 43683 sources over an area of about 1.7 sr. Separate final catalogues have been published for the 7C Galactic Plane survey (7CG) and the lower-resolution survey of the low-declination strip 9(h) < RA < 16(h), 20 degrees < Dec < 35 degrees.

The Mooney Mooney Fault System and Coolac ophiolite suite separate the Tumut Synclinorial Zone and Young Anticlinorial Zone in southeastern Australia. The Tumut Synclinorial Zone has been interpreted as an inverted rift basin and the Coolac ophiolite suite as obducted oceanic crust. An alternative interpretation is that the Tumut Synclinorial Zone formed by thrust-tectonic terrane amalgamation. In both cases, the principal deformation was Late Silurian. The Coolac ophiolite suite cannot simply be basin substrate. Mafic magmatism occurred in the trough-fill stratigraphy when the basin was too narrow to form typical oceanic substrate. The presently associated `ophiolitic' components were partly juxtaposed by tectonism. Because the development of rift basins can stop before typical oceanic crust is formed, the preservation in the geological record of this precursor to oceanic crust is here termed an embryonic ophiolite. The Mooney Mooney Fault System enclosed the Mooney Mooney Terrane of Basden et al. (1985, 1987). This paper discredits the terrane boundaries. establishes the trough-fill as an overlapping assemblage, and reaffirms the para-autochthonous tectonic model. Mylonite kinematics support Late Silurian westward upthrusting of the Young Anticlinorial Zone against the Coolac ophiolite suite (possibly during dextral transpression), associated with a deep-seated tectonic surge affecting the Young Granodiorite. Early Devonian tightening of the Late Silurian upright folding and less ductile reactivation of the fault systems are advocated.

ABSTRACT Radio and near-infrared observations towards the steep-spectrum Galactic plane radio source WKB 0314+57.8 are presented, in order to clarify the nature of this source. The radio observations include archival and survey data, together with new Giant Metrewave Radio Telescope observations at 617 MHz. The near-infrared observations are in the J and K bands, from the Gemini instrument on the Shane 3-m telescope. The radio observations show that WKB 0314+57.8 is extended, with a very steep spectrum (with flux density ∝ frequency−2.5 between ≈40 MHz and ≈1.5 GHz). The colour–magnitude diagram constructed from near-infrared observations of the field suggests the presence of a z≈ 0.08 galaxy cluster behind the Galactic plane, reddened by about 6 mag of visual extinction. Although the steep-spectrum source has no obvious identification, two other radio sources in the field covered by the near-infrared observations have tentative identifications with galaxies. These observations indicate that WKB 0314+57.8 is a relic source in a cluster of galaxies, not a pulsar.

We present a summary of the activity of the Cambridge Optical Aperture Synthesis Telescope (COAST) team and review progress on the astronomical and technical projects we have been working on in the period 2002--2004. Our current focus has now moved from operating COAST as an astronomical instrument towards its use as a test-bed for strategic technical development for future facility arrays. We have continued to develop a collaboration with the Magdalena Ridge Observatory Interferometer, and we summarise the programmes we expect to be working on over the next few years for that ambitious project. In parallel, we are investigating a number of areas for the European Very Large Telescope Interferometer and these are outlined briefly.

We report on a new fringe envelope tracking system installed at the Cambridge Optical Aperture Synthesis telescope (COAST). This currently uses the existing photon-counting avalanche photo diode (APD) detector system to allow real-time fringe tracking on up to 3 baselines simultaneously. This system has been recently tested on the sky and has proved to successfully track the fringe envelope on a 38m baseline. The algorithm based on an envelope method has also been implemented and tested at the Infrared-Optical Telescope Array (IOTA) interferometer.

We present details of the design and construction of a new pinhole Spatial Filtering system at the Cambridge Optical Aperture Synthesis Telescope and show the results from the first astronomical observations with the. system. We discuss the advantages of using such a system to reduce uncertainties in visibility observations with an optical/IR interferometer. We compare simultaneous observations with and without the system and show the potential to improve both the quality and quantity of scientific output of an interferometer. We also show that filtered observations are much more robust to changes in atmospheric seeing than unfiltered observations, improving the accuracy of calibrated visibility measurements while reducing the need for repeated calibration. Given the simplicity and high throughput of pinhole spatial filters we suggest that they could be a valuable addition to many current and future arrays.

We present a summary of the status of the Cambridge Optical Aperture Synthesis Telescope, and review developments at the array through the period 2000-2002. Summaries of the astronomical and technical programmes completed, together with an outline of those that are currently in progress are presented. Since our last report two years ago in 2000, there have been significant changes in the context for astronomical interferometry in the UK. We review these developments, and describe our plans for the near and intermediate term at COAST, and with colleagues in Europe at the VLTI and in the USA at the Magdalena Ridge Observatory in New Mexico.

The first-generation COAST array is now primarily operated as a tool for astrophysics, with any development work aimed at improving observing efficiency and at prototyping hardware for future arrays. In this paper we summarise the full range of astrophysical results obtained with COAST in the previous two years. Results of a programme to investigate hotspots on red supergiant stars are presented in detail.

The Cambridge DIMMWIT was developed for the site of COAST (the Cambridge Optical Aperture Synthesis Telescope), a prototype optical interferometer. Unlike other differential image motion monitors, this design is portable in order to carry out seeing campaigns at the site of any optical inteferometer. Of particular interest is the site of a second-generation interferometer proposed by the MRO (Magdalena Ridge Observatory) consortium. The DIMMWIT design has two objectives: to measure the Fried parameter r(0) and the speckle lifetime tau0, and to be easily transportable. Here, we outline the theory of differential image motion, the design of the DIMMWIT, describe how turbulence parameters can be measured with COAST, and compare measurements of the seeing conditions made simultaneously by the monitor and COAST.

We use a numerical simulation to investigate the effectiveness of pinhole spatial filters for optical/IR interferometers and to compare them with single-mode optical fibre spatial filters and interferometers without spatial filters. We show that fringe visibility measurements in interferometers containing spatial filters are much less affected by changing seeing conditions than equivalent measurements without spatial filters. This reduces visibility calibration uncertainties, and hence can reduce the need for frequent observations of separate astronomical sources for calibration of visibility measurements. We also show that spatial filters can increase the signal-to-noise ratios (SNRs) of visibility measurements and that pinhole filters give SNRs within 17 per cent of the values obtained with single-mode fibres for aperture diameters up to 3r(0). Given the simplicity of the use of pinhole filters we suggest that it represents a competitive, if not optimal, technique for spatial filtering in many current and next generation interferometers.

We present a summary of the status of the Cambridge Optical Aperture Synthesis Telescope (COAST). Since our last report we have concentrated on improving both the efficiency of use of the array and its astrophysical capabilities In particular we have achieved useful improvements in throughput, detector sensitivity and the efficiency of securing measurements of visibility amplitudes and closure phases. With five telescopes fully operational, COAST is now being used routinely for parallel programs of astrophysics and as a technical test-bed for its proposed successor, the Large Optical Array - LOA.

At the Cambridge Optical Aperture Synthesis Telescope (COAST), first-generation photon counting avalanche photodiodes (APDs) have been used as the pupil-plane fringe detectors in the optical regime. These are being replaced with EG&G's super-low k (`SliK') APDs, which have an exceptionally low dark count (fewer than 100 counts per second) and high detection efficiency (up to 70% at 700 nm). The new detectors have increased the limiting magnitude of the telescope, enabling the observation of targets previously too faint to be seen. We shall discuss the operation of these devices at COAST and present new interferometric observations of stellar objects at visible magnitudes of eight and fainter.

We present the latest astronomical results from the Cambridge Optical Aperture Synthesis Telescope (COAST). COAST is a first-generation stellar interferometer, which uses an array of small (40cm) separated telescopes to perform high-resolution imaging at visible and near-infrared wavelengths. The new science results from COAST exploit two recently-added capabilities of the COAST array, namely the ability to observe in any of the infrared J, H and K bands as well as at visible wavelengths, plus operation with five telescopes. We present contemporaneous observations of the red supergiant Betelgeuse at three wavelengths in the red and near-infrared. These data show that the apparent symmetry of the stellar disk is a strong function of wavelength, but that the bright spats seen in visible light are consistent with a convective origin. Data obtained using all five array elements on the symbiotic star CH Cygni reveal an elliptical distortion of the disk of the red giant, possibly related to mass transfer to a compact companion.

We discuss the design of the Large Optical Array, an optical/IR interferometer optimized for rapid imaging of complex astrophysical sources.

We report the direct detection of cyclic diameter variations in the Mira variable χ Cygni. Interferometric observations made between 1997 July and 1999 September, using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT), indicate periodic changes in the apparent angular diameter at a wavelength of 905 nm, with amplitude 45 per cent of the smallest value. The star appears largest at minimum light. Measurements made at a wavelength of 1.3 μm over the same period suggest much smaller size changes. This behaviour is consistent with a model in which most of the apparent diameter variation at 905 nm is caused by a large increase in the opacity of the outer atmospheric layers (which is mostly owing to titanium oxide) near minimum light, rather than by physical motions of the photosphere. The 1.3-μm waveband is relatively uncontaminated by TiO, and so much smaller size changes would be expected in this band. The latest non-linear pulsational models predict maximum physical size close to maximum light, and increases in opacity near minimum light that are too small to reproduce the diameter variation seen at 905 nm. This suggests either that the phase-dependence of the model pulsation is incorrect, or that the opacities in the models are underestimated. Future interferometric monitoring in uncontaminated near-infrared wavebands should resolve this question.

ABSTRA C T We report contemporaneous multi-wavelength interferometric imaging of the red supergiant star Betelgeuse (a Orionis), using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT), at wavelengths of 700, 905 and 1290 nm. We find a strong variation in the apparent symmetry of the stellar brightness distribution as a function of wavelength. At 700 nm the star is highly asymmetric, and can be modelled as the superposition of three bright spots on a strongly limb-darkened disc. However, at 905 nm only a single low-contrast feature is visible and at 1290 nm the star presents a featureless symmetric disc. The change in spot contrast with wavelength is consistent with a model in which the bright spots represent unobscured areas of elevated temperature, owing perhaps to convection, on a stellar disc that itself has a different appearance, i.e. geometrical extent and limb-darkening profile, at different wavelengths. The featureless centre-to-limb brightness profile seen at 1290 nm is consistent with this model and suggests that future interferometric monitoring of the star to quantify the size changes associated with radial velocity variations should be performed at similar wavelengths in the near-infrared.

We report the direct detection of cyclic diameter variations in the Mira variable χ Cygni. Interferometric observations made between 1997 July and 1998 September, using the Cambridge Optical Aperture Synthesis Telescope (COAST) indicate periodic changes in the apparent angular diameter with amplitude 45 per-cent of the smallest value.The measurements were made in a 50 nm bandpass centred on 905 nm, which is only moderately contaminated by molecular absorption features. To assess the effects of atmospheric stratification on the apparent diameter measured in this band, we have also measured near-infrared diameters for a sample of five Miras, in both the J-band (1.3 μm) and Wing's (1971) 1.04 μm band, which is expected to isolate essentially pure continuum emission. We present J-band visibility curves which indicate that the intensity profiles of the stars in the sample differ greatly from each other.