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The recent demonstration of a real-time direct imaging radio interferometry correlator represents a new capability in radio astronomy. However wide field imaging with this method is challenging since wide-field effects and array non-coplanarity degrade image quality if not compensated for. Here we present an alternative direct imaging correlation strategy using a Direct Fourier Transform (DFT), modelled as a linear operator facilitating a matrix multiplication between the DFT matrix and a vector of the electric fields from each antenna. This offers perfect correction for wide field and non-coplanarity effects. When implemented with data from the Long Wavelength Array (LWA), it offers comparable computational performance to previously demonstrated direct imaging techniques, despite having a theoretically higher floating point cost. It also has additional benefits, such as imaging sparse arrays and control over which sky co-ordinates are imaged, allowing variable pixel placement across an image. It is in practice a highly flexible and efficient method of direct radio imaging when implemented on suitable arrays. A functioning Electric Field Direct imaging architecture using the DFT is presented, alongside an exploration of techniques for wide-field imaging similar to those in visibility based imaging, and an explanation of why they do not fit well to imaging directly with the digitized electric field data. The DFT imaging method is demonstrated on real data from the LWA telescope, alongside a detailed performance analysis, as well as an exploration of its applicability to other arrays., Comment: 12 pages, 8 figures, 2 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society

A main science goal for the Large Synoptic Survey Telescope (LSST) is to measure the cosmic shear signal from weak lensing to extreme accuracy. One difficulty, however, is that with the short exposure time ($\simeq$15 seconds) proposed, the spatial variation of the Point Spread Function (PSF) shapes may be dominated by the atmosphere, in addition to optics errors. While optics errors mainly cause the PSF to vary on angular scales similar or larger than a single CCD sensor, the atmosphere generates stochastic structures on a wide range of angular scales. It thus becomes a challenge to infer the multi-scale, complex atmospheric PSF patterns by interpolating the sparsely sampled stars in the field. In this paper we present a new method, PSFent, for interpolating the PSF shape parameters, based on reconstructing underlying shape parameter maps with a multi-scale maximum entropy algorithm. We demonstrate, using images from the LSST Photon Simulator, the performance of our approach relative to a 5th-order polynomial fit (representing the current standard) and a simple boxcar smoothing technique. Quantitatively, PSFent predicts more accurate PSF models in all scenarios and the residual PSF errors are spatially less correlated. This improvement in PSF interpolation leads to a factor of 3.5 lower systematic errors in the shear power spectrum on scales smaller than $\sim13'$, compared to polynomial fitting. We estimate that with PSFent and for stellar densities greater than $\simeq1/{\rm arcmin}^{2}$, the spurious shear correlation from PSF interpolation, after combining a complete 10-year dataset from LSST, is lower than the corresponding statistical uncertainties on the cosmic shear power spectrum, even under a conservative scenario.

Many data analysis problems in science reduce to one of model selection. For example, a materials scientist investigating the structure and properties of a new compound may wish to know: are there two peaks under these diffraction data or three? For how many layers of materials is there most evidence in these reflectivity measurements? Do these data indicate a phase transition? In this introductory presentation, we show how probability theory can be used to address such questions in a simple and straight forward manner.

We describe here the first fully three-dimensional simulations of the jets of extragalactic radio sources. Application of a steady precession to the jet leads to a variety of unsteady features in the resultant flow pattern, and gives structures which bear a striking resemblance to those observed. The simulations contain secondary hotspots which are formed in the manner of the dentist's drill model and are not properly described as splatter-spots, although material continues to flow into them after their creation. We propose explanations of the characteristics of multiple radio hotspots.

Observations and earlier models of the jet in the Crab nebula are reviewed, and a new model, in which the jet is formed by material from the supernova remnant propagating up the interior of a cooled star trail left by the progenitor, is described. This model is similar to a previous model for the jet by Blandford R. D. et al. (1983, Nature, 301), but differs in that the jet lies in the plane of the sky, and the Crab's progenitor moves supersonically rather than subsonically through the interstellar medium. Numerical simulations indicate that the model offers a successful description of the structure of the jet.

We present a method for accelerating the calculation of CMB power spectra, matter power spectra and likelihood functions for use in cosmological parameter estimation. The algorithm, called CosmoNet, is based on training a multilayer perceptron neural network and shares all the advantages of the recently released Pico algorithm of Fendt & Wandelt, but has several additional benefits in terms of simplicity, computational speed, memory requirements and ease of training. We demonstrate the capabilities of CosmoNet by computing CMB power spectra over a box in the parameter space of flat \Lambda CDM models containing the 3\sigma WMAP1 confidence region. We also use CosmoNet to compute the WMAP3 likelihood for flat \Lambda CDM models and show that marginalised posteriors on parameters derived are very similar to those obtained using CAMB and the WMAP3 code. We find that the average error in the power spectra is typically 2-3% of cosmic variance, and that CosmoNet is \sim 7 \times 10^4 faster than CAMB (for flat models) and \sim 6 \times 10^6 times faster than the official WMAP3 likelihood code. CosmoNet and an interface to CosmoMC are publically available at www.mrao.cam.ac.uk/software/cosmonet., Comment: 5 pages, 5 figures, minor changes to match version accepted by MNRAS letters

We develop the maximum-entropy weak shear mass reconstruction method presented in earlier papers by taking each background galaxy image shape as an independent estimator of the reduced shear field and incorporating an intrinsic smoothness into the reconstruction. The characteristic length scale of this smoothing is determined by Bayesian methods. Within this algorithm the uncertainties due to both the intrinsic distribution of galaxy shapes and galaxy shape estimation are carried through to the final mass reconstruction, and the mass within arbitrarily shaped apertures can be calculated with corresponding uncertainties. We apply this method to two clusters taken from n-body simulations using mock observations corresponding to Keck LRIS and mosaiced HST WFPC2 fields. We demonstrate that the Bayesian choice of smoothing length is sensible and that masses within apertures (including one on a filamentary structure) are reliable, provided the field of view is not too small. We apply the method to data taken on the cluster MS1054-03 using the Keck LRIS (Clowe et al. 2000) and HST (Hoekstra et al. 2000), finding results in agreement with this previous work; we also present reconstructions with optimal smoothing lengths, and mass estimates which do not rely on any assumptions of circular symmetry. The code used in this work (LensEnt2) is available from the web., 13 pages, 11 figures, now accepted for publication after minor revision. A version with full resolution figures is available at http://www.mrao.cam.ac.uk/projects/lensent/version2/

The application of maximum entropy to the calculation of drug absorption rates was introduced in an earlier paper. Here it is developed further, and the whole procedure is presented as a problem in scientific inference to be solved using Bayes' theorem. Blood samples do not need to be taken at equally spaced intervals, and no smoothing, interpolation, extrapolation, or other preprocessing of the data is necessary. The resulting input rate estimates are smooth and physiologically realistic, even with noisy data, and their accuracy is quantified. Derived quantities such as the proportion of the dose absorbed, and the mean and median absorption times, are also obtained, together with their error estimates. There are no arbitrarily valued parameters in the analysis, and no specific functional form, such as an exponential or polynomial, is assumed for the input rate functions.

The behaviour of charged particles at electromagnetic steps is analysed using the powerful mathematical tools provided by the SpaceTime Algebra. Currents predicted in the evanescent region of a Dirac wavefunction strongly suggest that the electron “zitterbewegung” (ZBW) represents a real circulation. At higher potentials, the Klein paradox reveals a crucial difficulty of interpretation of “positronic” wavefunctions that must be overcome before Hestenes’ ZBW model can be taken seriously. The problem of radiation reaction is still not solved. Solutions of the Lorentz-Dirac equation for a potential step show crazy teleological features: certain input velocities have no possible future output states. The prospects for realistic electron models are briefly discussed.

Le spectre d'absorption HI a ete observe avec le VLA en direction du reste de supernova G74.9+1.2 et de la radiosource compacte adjacente 2013+370. Les observations confirment la nature extragalactique de la source compacte et une distance d'environ 12 kpc pour le reste de supernova

Une methode d'estimation des distances des amas de galaxies, utilisant la methode du maximum d'entropie, est developpee a partir de la position angulaire, du diametre apparent et du redshift des galaxies. Une application numerique aux amas de la Vierge et de la Coma est realisee

We have heard so far at this meeting from the practical-minded men of radio astronomy. Faced with the problem: ‘my map has sidelobes and noise on it’, they promptly reach for the nearest available weapons – ‘cleaners, polishers and kneaders’. All these use varying amounts of subjective human judgement, as for example in ‘window cleaning’.

The maximum entropy (MAXENT) method has been used ab initio to solve a previously determined small centrosymmetric crystal structure, bis(acetylacetonato)dichlorotin, C10H14Cl2O4Sn [Miller & Schlemper (1978). Inorg. Chim. Acta, 30, 131-134; Webster & Wood (1981). J. Chem. Res. (M), pp. 0450-0456]. The resulting electron density maps are of a very high quality, comparable or even superior to the conventional maps calculated from the refined phases. The method, therefore, holds good promise for the solution of larger and more difficult structures. The addition of simple chemical and symmetry information about the heavy atoms in the structure greatly improves the reconstruction and shows the capability of MAXENT to solve structures from partial fragments.

In this paper, we present a comparison of the radio and X-ray morphology of the supernova remnant G109.1–1.0, based on recent radio observations at 6 and 20 cm and investigate the relationship of the SNR to a neighbouring molecular cloud.

On presente des observations HI obtenues avec le VLA qui montrent que 0022+6352 se situe derriere le reste de supernova de Tycho et ne lui est donc pas associee

Extragalactic radio sources display a wide range of complex structures. The weaker sources have twin jets emerging from the nucleus of the parent galaxy, which bend and twist as they interact with the intergalactic medium (IGM); extreme examples are the radio-trail sources, such as NGC1265 (ref. 1) and 3C129 (ref. 2). More powerful sources typically show double structures with lobes and hotspots, but even these (such as Cygtius A (R. A. Perley, personal communication)) contain curved jets. Although our theoretical understanding of sources in terms of twin plasma-jets has advanced, numerical simulations3,4 have concentrated on axisym-metric models, whereas the investigation of radio-trail and complex sources demands three-dimensional simulations, despite the obvious computational difficulties. It is now generally accepted that radio-trail galaxies are produced by the motion of active galactic nuclei through the IGM in clusters of galaxies5. Models were initially proposed in which the ejected material consisted of independent blobs or plasmons6–8, but most evidence9,10 now supports ejection in a quasi-continuous jet9,11. Here we adopt the jet model and study the formation of twin-tail sources by the motion of the active nucleus through intra-cluster gas, using three dimensional fluid-dynamical simulations of a supersonic jet in a cross-wind.

We present new high resolution observations of 3C10 (Tycho's SN) and 3C58, made at 2.7 GHz with the Cambridge 5 km telescope. These observations have a resolution of ∼ 4 arcsec, considerably better than previous observations, giving a detailed view of the radio morphology of these two remnants. These maps have also been processed by the Maximum Entropy Method (MEM).

G11.2-0,3 est une source radio non thermique dans le plan galactique; elle a ete proposee comme un reste de supernova de l'annee 386. On presente les observations radio de ce reste effectuees par VLA et on montre que sa structure est une version evoluee de Cassiope A

In this paper we present new observations that show 3C58 is much closer than has been assumed in the past. This necessitates a reappraisal of many of the quantitative comparisons that have been made between 3C58 and the Crab Nebula or other ‘filled-centre’ remnants.

The components of radio sources are identified with bubbles of relativistic plasma rising through the hot gas which produces the X-ray emission from clusters of galaxies. Continuous release of energy in the nucleus of a galaxy will lead to the formation of two such bubbles, moving in opposite directions along the axis of rotation.

Recent observations with sub-kpc resolution show that the hotspots in powerful extragalactic radio sources (for example, Cygnus A1, 3C3512, 3C268.43, 3C1334, 3C204, 3C1964) often exhibit internal double structure. One subcomponent is usually extremely compact, of size

Although a static lens can produce no change in the brightness of an isotropic radiation field, a moving lens produces a characteristic two-sided brightness pattern that may be used to deduce the transverse velocity of the lens. The magnitude of the effect is proportional to both the transverse velocity and the strength of the lens. We suggest here that this effect provides a method for measuring the transverse velocities of clusters of galaxies; in this case the lens is gravitational and the radiation field is the isotropic microwave background. The predicted magnitude of this effect is at the present sensitivity limits of observation but is at least as large as the expected level of primordial background fluctuations.

In this paper we discuss the use of the maximum entropy principle in tomographic reconstruction. We emphasize that entropy maximization is the only consistent regularization technique for images. A method of encoding prior information is discussed, and an example of the use of prior knowledge in the restoration of a fuel pin bundle is given.

The discovery of powerful X-ray sources associated with rich clusters of galaxies has led to the prediction that these clusters contain large amounts of hot gas. A necessary consequence of the existence of such gas is the Compton scattering of cosmic microwave background radiation by hot electrons (Sunyaev & Zeldovich 1972). As a result, some of the microwave photons are re-distributed with slightly higher energies and the effect may be observed as a small diminution in the background temperature. The effect is expected to be ≈ 0.5 mK for the gas clouds associated with cluster X-ray sources.

In this paper, we present a comparison of the radio and X-ray morphology of the supernova remnant G109.1–1.0, based on recent radio observations at 6 and 20 cm and investigate the relationship of the SNR to a neighbouring molecular cloud.

We have observed the radio remnant of Tycho’s Supernova (AD 1572), 3C10, with the Cambridge Half-Mile-Telescope (HMT). Two complete syntheses, overlapping in velocity coverage, were made in September 1979. Details of the telescope, receivers and the survey are in Table 1. In order to derive accurate HI absorption measurements towards the source, it is important to include carefully the contribution from large scale emission. Data containing this large-scale structure, physically unobtainable with the HMT, were derived from the work of Weaver & Williams (1973) and Williams (1973), and added to the synthesis maps. Continuum emission was subtracted from these ‘composite’ maps to give the final channel maps (Plate 1). In Plate 2 we show a ‘pie slice’ representation of an RA-velocity plot through the centre of the field.

We present new high resolution observations of 3C10 (Tycho’s SN) and 3C58, made at 2.7 GHz with the Cambridge 5 km telescope. These observations have a resolution of ~ 4 arcsec, considerably better than previous observations, giving a detailed view of the radio morphology of these two remnants. These maps have also been processed by the Maximum Entropy Method (MEM).

In this paper we present new observations that show 3C58 is much closer than has been assumed in the past. This necessitates a reappraisal of many of the quantitative comparisons that have been made between 3C58 and the Crab Nebula or other ‘filled-centre’ remnants.

We have heard so far at this meeting from the practical-minded men of radio astronomy. Faced with the problem: ‘my map has sidelobes and noise on it’, they promptly reach for the nearest available weapons — ‘cleaners, polishers and kneaders’. All these use varying amounts of subjective human judgement, as for example in ‘window cleaning’.

It has been shown (e.g. Woltjer, 1970) that many of the properties of a supernova remnant can be predicted from general considerations of the expansion of a massive gas cloud into the interstellar medium. However, this simple picture has been unable to explain the very powerful non-thermal radio emission from the young shell supernova remnants such as Cas A and 3C 10. In order to account for this it is necessary to invoke other mechanisms to produce magnetic field and relativistic particles.

A SMALL cooling of the microwave background towards the centre of the rich, distant cluster of galaxies Abell 2218 has been detected1–3. This decrement has been attributed to the inverse Compton scattering of photons of the background radiation by electrons in a hot intracluster plasma, as predicted by Sunyaev and Zel'dovich4. We report here new observations in several different directions towards the cluster which confirm the earlier results and provide an estimate of the extent of the gas.

An unequivocal detection of the Sunyaev–Zeldovich effect1, the cooling of the brightness temperature of the microwave background radiation towards the atmospheres of clusters of galaxies, has profound cosmological significance2–6, but has hitherto proved elusive. We present here measurements of the effect towards three rich clusters of galaxies made with the 40-m telescope of the Owens Valley Radio Observatory at 20.3 GHz. These results are three times more sensitive than other results previously reported7–9, are free from systematic errors at the 100 µK level, and provide very strong evidence for the reality of the Sunyaev–Zeldovich effect.

The discovery in the Lupus region of a new, extended soft X-ray source, H1538-32, is reported, having a distance of approximately 340 pc, and a luminosity of 1 to 2 x 10 to the 34th ergs/sec. The observed energy spectrum of the source is well fitted either by a thermal bremsstrahlung spectrum with Gaunt factor but without line emission, or by a coronal plasma model which includes the X-ray emission lines of various elements and the continuum as outlined by Raymond and Smith (1977). On the basis of the extended nature of the source and its thermal spectrum, it is suggested that H1538-32 may be an old supernova remnant.