Vistas in Astronomy最新文献

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.

The discovery and use of the Saros, a lunar cycle of 18 years and 10 or 11 days, is reviewed from its earliest origins two millennia ago to the present day, when it is known with precision and enables the accurate prediction of both time and type of solar and lunar eclipses. The theoretical basis for the Saros is discussed, along with other historically known lunar cycles. The geometry of the Sun-Moon-Earth system is found to repeat itself after one Saros, not only at eclipses but also at any phase of the cycle, indicating that the Moon moves in a nearly periodic orbit. The search for periodic orbits using the Saros has led to the discovery of a set of eight periodic orbits of period equal to one Saros whose time evolutions closely resemble that of the real Moon. Finally, the potential of the Saros in studying the dynamics and stability of the Earth-Moon system is examined and the existence of other Saros-like cycles of longer periods in the present, past and future of the Earth-Moon-Sun system is explored.

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.

By the 5th Century B.C. three great cultures were in contact in the Eastern Mediterranean area. The newest of these, the Greek, began by borrowing and improving on the discoveries of Babylonians and Egyptians. The knowledge of the gnomon is attested as one such borrowing. An overview is attempted of the state of timekeeping, with particular emphasis on the measure of shadows, in these two older cultures, revealing the uncertainties as to just what they had to offer the Greeks, and how they had come to their conclusions.

We analyse VLBI data of a sample of radio quasars in order to study the milliarcsecond scale structural properties of quasars in the widest available redshift range of 0.2 < z < 4.5. The statistical study is based on a combination of the 5 GHz Caltech-Jodrell VLBI survey and our data for a set of extremely high redshift sources. We show that the apparently less prominent milliarcsecond scale structures found in z > 3 quasars do not contradict the assumption of intrinsic similarity of radio sources at different redshifts and may be explained by a difference in spectral properties of cores and extended components (jets).

High-precision photometric analysis of images of crowded stellar fields needs sophisticated algorithms. The photometric precision is, however, a strong function of the completeness of source detection. We discuss several aspects of this problem, both with relation to single- and multi-channel applications. In single images, we separate detection into two phases, source image enhancement and actual detection. Comparative tests show a point spread function pixel fitting technique to give the best results. For the fraction of undetectable stars still affecting image statistics, we have developed a technique to extract information about their effect on the faint-end luminosity function. We give two examples of multi-channel image fusion applications: (1) a combination of low- and high-resolution images and (2) removal of undersampling effects by sub-pixel image displacements. Preliminary results show considerable potential for these techniques.

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.

We present new results from a radio mapping campaign on the X-ray binary (XRB), Cygnus X-3 using both MERLIN and the VLBA. This has produced two significant results. Firstly, observations using MERLIN at 5 GHz during a quiescent phase in June 1995 showed the presence of N and S radio lobes associated with a bright core. The position angle and separation of these lobes are consistent with the idea of a large scale jet in this source. Secondly, observations with the VLBA at 15 GHz during a period of small scale flaring have shown the source to be expanding at apparently superluminal velocities. This expansion was then followed by a period of apparent contraction with similar superluminal speeds.

With a time sampling of up to one observation per month, the analysis of geodetic VLBI data can reveal a detailed picture of the kinematics in numerous compact radio sources. The database comprises simultaneous observations obtained at two frequencies (8.4 and 2.3 GHz) which have been collected over a period of about 15 years. For our analysis of short-time structural variability in the jets of AGN we selected several sources, mainly on the basis of their flux density variability in the radio and, in case of correlated outbursts and intra-day variability, also in the optical regime. These sources are generally found to be active in the γ-ray regime as well. This fact enables us to study possible correlations between the structural properties seen at radio wavelengths and the flux-density behaviour in the γ-ray regime. The results for the γ-active quasar PKS 0528 + 134 are presented.

A short review of data fusion in remote sensing with emphasis on statistically based data fusion methods is given. The introductory part defines data fusion and image registration, and provides a historical background and some general references. Multivariate data are the necessary basis for any data fusion algorithm. The possible levels of data fusion and the frequent occurrence of various types of multivariate data in remote sensing are discussed. Finally, the paper presents a number of statistically based data fusion methods and discusses data fusion in the Bayesian framework.