It has been claimed that MkIII/EVN experiments do not achieve the expected level of image quality, or dynamic range. An EVN test observation of a strong and compact source, DA193, has allowed us to investigate in detail the possible systematic errors affecting EVN images. Indeed, we have identified two effects which both significantly affect image quality: instrumental polarization and an amplitude error caused by the stepwise delay tracking in XF-correlators. This paper shows how these effects influence total-intensity images and discusses the correction of these non-closing errors. Their removal increased the peak/rms dynamic range from 5000 to 19,000 in our image of DA193.
The dates for the second (ca. 2700 B.C. to within 250 years) and the third ecliptical quartets (ca. 1200 B.C. to within 400 years) evaluated earlier are considered in the context of ancient Egyptian history. The origin of the second quartet coincides with the Great Pyramids and the initiation of the Egyptian solar, or so-called “civil” calendar, the first of such a type in the world. The third quartet is concurrent with the solar conversion of Amenhotep IV (Akhenaton) and takes place at the finale of the great Sothic period of 1461 years after the initiation of the solar calendar. It is argued that the Great Pyramids seem to be monuments to the Sun-god built in honor of the Egyptians having reached an understanding of the Sun's track upon the starry background, Akhenaton's conversion being in a direct connection with the original Pyramids' ideology. This paper is the third part of a single investigation. The first two parts “On the Origin of the Zodiacal Constellations” and “Prehistory of Zodiac Dating: Three Strata of Upper Paleolithic Constellations” were published in Vistas in Astronomy in 1993 and 1995.
The EVN is currently in the midst of an ambitious upgrade programme. The hardware and software developments in progress are briefly summarized, and changes in operational practice and user support are also described.
We show how the total intensity data acquired with the European VLBI telescopes are affected by a systematic error, the physical origin of which lies in the instrumental polarization of the telescopes. The effect on the data can be modeled and therefore removed by data processing.
The observational evidence for jets and phenomena arising from rotation powered radio pulsars is reviewed, including many recent and exciting discoveries at X-ray wavelengths. The well studied jets of the Crab pulsar are summarised, including recent results from the HST. The evolutionary links between the known binary radio pulsars and jets sources in X-ray binaries are discussed.
The unique object SS433 is briefly reviewed with particular reference to the part played by the stellar wind ejected by the early type companion star. The aims of this observational campaign are described and the theory of the mass loss rate measurements explained. VLBA 1.6 GHz maps are presented as well as preliminary 23 GHz VLA and MERLIN maps. Mass loss rates of around 5 × 10−6 M⊙ yr−1 are found, consistent with earlier results and the properties of Wolf-Rayet stars.
We present a new image compression method, for very high quality lossy compression. This method caters for image data in regimes of (i) detector imperfections, which motivates a robust approach based on the median transform, and (ii) noise, which is explicitly sought and separated out, since noise is inherently non-compressible. An in-depth assessment is carried out on real data, relative to the standard JPEG compression method. Comparable visual quality is based on 260:1 compression with the new method, and 40:1 compression with JPEG. The assessment procedure, based on the astronomical images used, is an objective approach for determining very high quality visual reconstructions.
Global λ6 cm VLBI polarization observations of the intraday variable sources 0716+714, 0917+624, and 0954+658 are analyzed. Integrated VLA measurements during the VLBI observations suggest that although there were no substantial total intensity variations, the polarization for all three sources varied significantly during the 24 h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to identify corresponding rapid variability in the VLBI structure. There is evidence that the polarization variability indicated by the VLA measurements of 0917+624 is present in the VLBI data (i.e. it occurred on mas scales), but this is not the case for the other two sources.
Image fusion techniques are increasingly applied to astronomical images in order to improve sensitivity or to compare images. We discuss in this paper the ability of multiscale transforms for many applications: geometrical and photometrical registrations, co-addition and compression. A strategy for multiscale image fusion is derived.
Databases in the real world are often dynamic, incomplete, noisy and very large and most of the problems which appear in the Data Mining field are caused by these characteristics. However, it is often unnecessary to obtain results (rules, clusters, etc.) with a high degree of precision since the input data is itself imprecise. For these reasons, we propose a new approach to cope with these situations. We refer to the “Soft Computing” methodology which combines the ability of Fuzzy Logic to represent and manage imprecise data and knowledge together with the accepted capacities for learning and heuristic computation of Neural Networks and Genetic Algorithms. The aim of the paper is to present some of the possiblities offered by the use of Soft Computing.
Modelling of the point spread function of the UKIRT IRCAM3 array was conducted in order to test for any extended emission around the X-ray binary Cyg X-3. We found that the point spread function cannot be represented by a simple Gaussian, but modelling of the stars required additional functions, namely Lorentzian and exponential components. After modelling for the PSF, we found that Cyg X-3 could be represented by two stellar-type profiles, 0.56″ apart.
The concentration of information is a general problem in image processing. We illustrate this in a particularly simple case, the ISO Serendipity Survey, where a detector of 2 × 2 pixels slews through the sky and the task is to detect point sources. There is a theoretically optimal solution for information concentration: the Eigenvalue Transform. The practical shortcomings of the ET are solved by the Karhunen-Loèv Transform concept; and the application of the KLT to our particular problem yields the first-order two-dimensional Haar Transform. The above mentioned transform concentrates the information of the point sources and the glitches in different coefficients (gradient and curvature respectively). A one-dimensional (along the slew) Adaptive Filtering of the gradient is used to detect the point sources.
We present observations of the FRI radiogalaxy 3C66B during a joint EVN-MERLIN session in May 1995. The jet has been mapped from the 5 mas scale up to about 3 arcmin. The newly mapped VLBI jet shows two elongated blobs in the direction of the brightest kpc jet (NE). We interpret these two blobs in the light of a new model for VLBI jets, making use of the two-fluid concept.
In this paper we present a technique, based on multiresolution wavelet decomposition, for the merging and data fusion of two images. The images to merge are a high-resolution panchromatic image and a low-resolution multispectral image. The method consists of adding the wavelet coefficients of the high-resolution image to the multispectral (low-resolution) data. We applied the method to merge SPOT and LANDSAT (TM) images. The current data fusion methods may not be satisfying because they can distort the spectral characteristics of the multispectral data. The technique presented here is clearly better than the standard IHS and LHS mergers in preserving both spectral and spatial information. The fusion of reconstructed images and the application of the technique to astronomical images are commented on.
The method of kernel density estimation can be readily used for the purposes of classification, and an easy-to-use package (alloc80) is now in wide circulation. It is known that this method performs well (at least in relative terms) in the case of bimodal, or heavily skewed distributions.
A short account is presented of the struggle to understand and provide a theory for stellar pulsations. The observational and theoretical developments from the discovery of δ Cephei in 1784 up to Baker and Kippenhahn's key paper of 1962 are described and their significance discussed.
Observational signatures of helical jets can be found in some X-ray binaries (XRB), planetary nebulae, Herbig-Haro objects and in jets of active galactic nuclei (AGN). For example, in the XRB SS433 a kinematic model of precessing jets has been applied very successfully and yielded a determination of its distance which is independent of conventional methods. In galactic jets precession appears to be the predominant mechanism for the production of observed helical signatures. In extra-galactic jets other mechanisms seem to be similarly frequent. As a result of their strong dependence on the direction of motion with respect to the observer, special relativistic effects can be pronounced in helical jets. These have to be taken into account in AGN-jets and the newly discovered galactic sources which show apparent superluminal motion. Since the galactic superluminal jets are located in a binary system, jet precession is very likely in these sources. In this paper I review the main structural and kinematic signatures of helical jets and briefly mention the physical mechanisms behind them. I will present kinematic simulations of relativistic jets which are helically bent or have an internal helical flow field.
We present the main lines along which information fusion has evolved from the first days of data fusion up to image fusion. Then we discuss some of the reasons why image fusion cannot benefit from many of the results of data fusion. In a second part, we present the main tools used to make a fusion of images, and we discuss the fundamentals of these tools. Along the way, we show how important the role of the user is in the design of an appropriate scheme for image fusion.
We discuss the mechanism of particle diffusive acceleration by shocks developed by non-linear fluid instabilities in collimated flows as the basis of a model to interpret the synchrotron brightness distribution and spectrum of extragalactic jets. Predictions for future VLBI observations are derived.
The integration of information from various sensory systems is one of the most difficult challenges in understanding both perception and cognition. For example, the problem of auditory-visual integration is a correspondence problem between perceived auditory and visual scenes. Two main questions arise when designing data analysis systems: what is the useful information to be integrated?, and what are the integration rules? The problem of integrating information becomes relevant whenever: (a) the same kind of data are detected by spatially distributed sensors; (b) heterogeneous data are detected by different sensors; (c) heterogeneous distributed data are involved. General problems concerning the integration of experimental data are reviewed. The case of the BeppoSAX X-ray astronomical satellite is given as an example.
